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Interview of Paul Peter Ewald by Charles Weiner on 1968 May 17,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
For multiple citations, "AIP" is the preferred abbreviation for the location.
This is a tape-recorded interview with Dr. Paul P. Ewald at his home in New Milford, Connecticut, and this is Charles Weiner, about to be served coffee by Mrs. Ewald. The date is Friday, the 17th of May, 1968. In earlier interviews with Tom Kuhn and in published articles you covered pretty well the early events of your life. Beginning with your home life, your work at Munich and then the two separate intervals at Goettingen, you described the people there and the general atmosphere.
You described your thesis work and its impact, the reaction to it, and the origins of the dynamical theory and its subsequent development, up to a point. And I would like to start at the point in all of this which is most obscure to me, because I've seen very little information concerning your own recollections for the period about 1921 when you were at Munich. You made the transition in 1921 to Stuttgart, and it is not at all clear how this came about, what your expectations were, why you left— that's what I'd like to start you on.
Well, let's see. After the war—I think I should start it in 1918, when there was a flood of people returning from the war—I had "habilitiert" myself during a furlough, become a lecturer in Munich, I think by the end of '17. After the armistice the University had to arrange a special summer semester, an abbreviated semester, for the people who flooded back from the army. My first job in lecturing was to give a course, alternating with Sommerfeld, on mechanics. He gave one lecture, I gave the next, and, of course, this was not always a very good lecture I gave. I remember especially making one terrible howler concerning the Carnot energy loss, though it was a good thing because later on when I had to lecture on mechanics in Belfast a good deal, and also in Stuttgart, I knew I had to be careful with the Carnot loss.
It was also during that period that in some Easter vacation I gave a series of lectures on the theory of x-ray diffraction, which of course was still relatively unknown. And rash as I was, I wrote to my friend Berliner, the editor of Naturwissenschaften, that I was giving these lectures and I thought I should write them down and would he publish them as a book? I would have the manuscript ready by about Whitsun- tide. Berliner said yes, he was very willing to publish it.
I'd written for Naturwissenschaften previously, so he knew me. I put a young student to take notes during lectures. This young student was Otto Laporte. This Otto Laporte wanted to go on a spree on one of the dates of the lectures, and so he charged someone else [with the responsibility] of taking the notes. This was Heisenberg. And so I got notes which really weren't very good, even as notes go. Of course, I was entirely wrong in my idea that I could finish up the lectures into book form within a few weeks, and it took me two or three years...Well, it finally came out in '23. This was the origin of Krystalle und Roentgenstrahlen. It took much longer, and I worked on it in Stuttgart, and I worked on it during the inflation, and I worked on it during vacations, which we took on the Isle of Ruegen together with the Schroedingers when the times were very hungry. Of course, when the book came out it was fine because I sold many copies in Sweden and abroad and got foreign currency, which I could turn into good value during the German inflation when nothing was stable.
Where did you meet the Schroedingers?
Well, I first met Schroedinger in Munich when he and his wife, a newly married couple, came through Munich and he gave a colloquium talk there. This must have been right after the war, in 1918 or 1919 [with Mrs. Ewald's confirmation]. (You see, my wife is the historian; she knows the dates. I'm all confused about dates. That's why she has to be with us.)
You know the dates, but you just don't know what happened on the dates.
All the dates, yes. Well anyway, Schroedinger was on his way, I think to Jena, where Max Wien had offered him a position. He stayed there only for a few months, then he went to Stuttgart on an Extraor- dinariat for Theoretical Physics, which had been held up to that time by Max Abraham. But Abraham, you know, was sick; he had a brain tumor, and I guess that's the reason he had to quit. In Stuttgart then, Schroedinger was his successor. At that time there was so much demand for theoretical physicists that Schroedinger didn't stay there for more than half a year, and then he was called to Breslau. And in Breslau he didn't stay for more than about half a year, and then he got a call to Zurich. And so they were really on the move. I became Schroedinger's successor in Stuttgart.
Why was there such a demand for theoretical physicists?
There weren't any, so people wanted theoretical physics; Atomic theory had grown up and developments in physics had taken place which made all of Sommerfeld's pupils go off, as the German saying is, "Wie warme Semmeln," like hot rolls.
"Like hot cakes." Well, this was a period when the universities were crowded, so there was a need for teachers.
There was a need for teachers, and it became fashionable to have extra chairs or extra Lehrauftrag for theoretical physics.
I see. And at Stuttgart, for example, there had been one man, and then he was replaced by another. Was it ever the custom at the smaller institutions to have a group of theoretical physicists?
No, except that I soon got an assistant at Stuttgart. The experimental physicist at Stuttgart was Regener. He did the cosmic ray work.
Before we get on to that, and I know I got you off the track, I want to get you back to the period in Munich, which you started to describe.
The question was why I left Munich?
That's right. You started to describe how you got interested in working on the book, which culminated much later than you had thought.
Yes. Well, I stayed on in Munich until 1921 and I gave several courses, one of which was on lattice dynamics. And there was a very intelligent young man sitting in, Wolfgang Pauli, who made a very good contribution because he found a better way of defining stress in a crystal than in Born's book. So it was really very stimulating to lecture there. Well, why I left Munich: clearly, you've heard of Privatdozentenkrankheit—everybody who is young wants to get into a position of his own, and I saw Lenz going off and Kossel being called away, and so I was waiting for my turn. And when the call to Stuttgart came, of course there was no question of my accepting it.
Do you know how it came about? Had Sommerfeld made the recommendation?
Oh, I'm sure that Sommerfeld made the recommendation. I'm sure the people knew of me through Schroedinger.Among the people who really made the policy at Stuttgart at the time, the main man was Fritz Emde. You know him from Jahnke-Emde, Funktionentafeln. Fritz Emde was the professor of electrical engineering, and he was a very interesting and very important man. I learned nearly as much from Fritz Emde as I learned from Sommerfeld.
In what way?
Well, you see Sommerfeld's lectures on Maxwell's theory, which formed one of the parts of his regular course, were beautiful and very interesting, but they were entirely from the viewpoint of a theoretical physicist. The permeability of a substance was always a constant, the dielectric constant was always a constant, and with that, of course, you could never do anything in a practical engineering application. And Emde, who I think had never been to anything more than a Technikum— a Technikum is a medium-grade institution, not of the standard of a university—was a self-made man who worked at Siemens, in the Siemenswerke, before he was called to the chair in Stuttgart. He really tackled the problems as they came up in reality. His sense of reality was much greater than Sommerfeld's.
Was this typical or atypical of people in the universities, to have this contact and prior experience with industry and to apply it then to their own work in the university? In other words, you would expect that a professor of electrical engineering would know the real world. But what about in physics?
Not in physics. You see, one of the nice things about Sommerfeld was that he was keenly interested in Hydrodynamics and the problem of turbulence, which was not solved. I don't think it's ever been solved... Well, it may be solved now by the Prandtl theory, but at that time it was quite unknown why, when you exceed the Reynolds number, suddenly turbulent flow begins. In a little play we acted in Sommerfeld's house the saying was: "Und stimmte was nicht, behauptete Keckich, der Aether ist rein, die Materie dreckig." I'll have to write that down.
Give us a rough translation anyway.
"And if something did not fit in with the theory, then I would dare to state, that the ether is clean and matter is dirty." And that was, so to say, in a nutshell, the attitude of the theoretical physicist— he liked to have a simple theory.
Did Sommerfeld actually say this, or was it just attributed to him?
Well, he said something similar; we modified it a bit, to put this little rhyme into verse.
Was this done by the students?
Yes, his students. You see, Sommerfeld quite regularly gave invitations to groups of students to his house, and we quite often acted and gave a little play or something. And I should credit this particular verse to the author, Ludwig Hopf (not the mathematician Hopf but the later professor of mechanics in Aachen and a great friend of Kármán and Sommerfeld—Ludwig Hopf).
You mentioned Fritz Emde and his particular approach. Do you think he had a say in the selection of theoretical physicists?
Fritz Emde? Oh, yes, surely.
So you think that with Schroedinger there and with Emde there and with the contact with Sommerfeld ...
Well, Schroedinger, you see, was an Extraordinary Professor which means that he had no say, except in a personal way, of course, in the choice of his successor, but he was not on the faculty board determining or proposing the next man.
Well, then, one way or another, you got the invitation and you were very pleased and ...
I was very pleased and I had a race with my wife, which she won. The race was that we expected our fourth child—a boy but we didn't know it would be a boy—to arrive, and I wanted to go to Stuttgart punctually on the first of April, 1921, because that was the date of my appointment, and so I took off from Munich on the 30th of March and paid a visit to Stuttgart and looked around and presented myself at the ministerium in order to draw my first salary check. I was very proud, of course, and when I came home late at night, I found a notice on the outer door of the apartment from my mother-in-law, saying, "quiet. Arnold sleeps in Father's bed." So my wife had won the race.
And so that delayed your permanent move to Stuttgart somewhat.
Well, I went over, and they couldn't come because there weren't any apartments to be had. This was a terrible time.
Especially not for a family with four children.
And it took more than ...
A year until I could come.
... a year until the family could move.
Then you commuted?
No, I stayed there. Luckily I had a kind of godmother or aunt in Stuttgart and I stayed with her until we found an apartment.
[to Mrs. Ewald] But you were in Munich?
No, I was really in the country with my mother-in-law.
I see. But in the country near Munich?
Near Munich, on the Ammersee. [brief interruption]
We just interrupted for a second to make sure that all contributors were represented on the tape, and we've determined that that is the case. What were your duties the first year in Stuttgart?
Well, I had to lecture on theoretical physics, that is to say, mechanics, thermodynamics, electricity and magnetism and optics—this was the usual division of courses. Atomic physics at that time hadn't come in yet; there was too little of it. But then quite soon afterwards I gave lectures on atomic physics, especially to the chemists.
At what level in physics were the students?
The students had all passed their "Vordiplom". The Vordiplom an examination which they had taken after at least four semesters of study, two years, and then the total range of studies was another two years. So I always got fairly mature students who had had their mathematics background and their experimental physics background and were working on their Diplomarbeit, on a mainly experimental problem for the diploma.
Did you at any time in the '20s in Stuttgart have any one working with you on a higher degree level on thesis problems?
When did that start?
Oh, I think nearly straightaway. Quite soon. My first pupil was Ulrich Dehlinger who later became my successor in Stuttgart and did a lot of work on metals. This was very funny. You see, every year there was a kind of Rektoratsfest, an invitation by the rector of the Hochschule. to all the staff and to outside people, and there I met a rather short, stubby gentleman. I was rather new in Stuttgart; I didn't know who people were. He was introduced to me as the father of my Doktorand, Dehlinger. And we talked about his son, and I was quite patronizing, as I would be, you know—young professor—and said, "Oh, yes, your youngster is doing quite well," and so on. And only later I heard that he was the Minister of Finances, and all the money came from this man. [laughs]
And then shortly after you arrived, the book came out. This was 1923. You completed it then during your first years there.
And I am always astonished how I could manage to do so much work in those years because I had to build up my own lecture courses, and I wrote the book, which came out in '23, and quite shortly after that I began writing the article for the Handbuch.
Did Sommerfeld ask you to do that?
No. This was the Springer Handbuch der Physik, the blue one. There were two competing Handbucher at that time: the blue one and the green one. The green one, by the Akademische Verlagsgesellschaft, was more on the experimental side. And, of course, one of the main differences was that the Springer one made use of many young research people, many of them Jewish and very bright. The one of the Akademische Verlagsgesellschaft—i.e. Mr. Jacobi and Mr. Jolowicz, which is really Fock—I don't know whether you know there was a big Antiquariat in Leipzig, scientific antiquariat, a marvelous institution created by Jolowicz, who was also the driving man behind the Akademische Verlagsgesellschaft and the father-in-law of Jacobi. This green handbook was edited by Wien and Harms and is I think probably entirely Aryan.
Well, Springer offered a very good honorarium. So all the young people who needed money—and we all did after the inflation—fell in to writing articles for the Handbuch. And I wrote this article—in 1926 it appeared—and I wrote a second edition in 1933. I still think it's a very good book, but it suffered under being embedded in this Handbuch volume, and Springer wouldn't release it. Linus Pauling wrote me that he wanted to make it the textbook for his courses on x-ray diffraction, and could he translate it? So I wrote to Springer, and Springer said, "He can translate the entire volume of the Handbuch, but not your article." A mean thing to do. And so this article was hidden away in the thick volume, which was, of course extremely expensive, and nobody knew it.
When did Pauling make this request?
Well, this was, I think, after the first edition came out, that is to say, after 1926—I guess '28 or so when it became known.
So you have demonstrated that you were pretty busy.
And besides these, you see, I was asked by another publisher, Vieweg, to write for a new edition of Mueller-Pouillet. Now, I had a kind of liking for Mueller-Pouillet because it was the book from which I learned my first physics. This was the edition of 1856, I believe, which I found in my grandfather's library. I often read that book as a child, and so I had a kind of very friendly relation to Mueller-Pouillet. I wrote an article on the solid state which was then later, without my knowing it, translated into English by Blackie together with other chapters of Mueller-Pouillet by Prange and by Prandtl. I think you asked that question before.
I see. I wondered about that. I'm just trying to check the year.
Well, the Mueller-Pouillet volume appeared in 1929.
They took selections from there and put it into ...
Put it into the Physics of Solids and Fluids. And I didn't even know it. I heard of it just by chance so I made quite a row, saying this was not a way to deal with a publication, especially since I had written this article for Mueller-Pouillet, and they had kept the manuscript lying for about three years. And so, by the time it appeared, parts of it were already antiquated, and I was rather mad at them. And then two years later this translation was made and called Advances of Modern Physics or something—I don't know what the title is.
They have Physics of Solids and Fluids.
Yes, but somewhere ...
Probably a subtitle.
Yes, it's a subtitle.
It seems to me that there were very momentous developments during those years from '26 to '29, when they had the manuscript.
All the properties of metals were developed—well, not all of them; the slip was already known, and I gave a version of the research on slip in metals. Polanyi and Schmidt, and these people who worked in Dahlem, wrote articles. Well, the main people had been in England; there was Miss Elam, and, I think, Carpenter.
These are in the nature of review articles summing up the state of the field.
Yes, it was review articles but, you know, also presenting quite often details in a rather different way and in a different light. I mean, there were also quite a lot of original ideas involved; it's not pure review.
Well, by my definition of review article, it means surveying the literature, interpreting, criticizing, and synthesizing from it.
What about your other contributions during this period in terms of your ordinary research papers? Was most of your research centered on the interpretation that went into these Handbuch articles?
Yes, I think you can say that.
There were probably other papers that you had done, a good many of them. Ewald - 8
You see one thing that developed out of my book Krystalle und Roentgenstrahlen was the Strukturbericht. At the end of my book of '23, I gave a list of all the structures that were then known, and this was a list covering 20 pages. And I thought it might be a good idea to keep this list up to date, so I started a kind of card index of all the structures I came across in order to present them in a second edition of the book. Well, I never wrote a second edition of the book for two related reasons: I couldn't read this book anymore; it was not terse enough, you know, and not concentrated enough; it was much too wide. Now I know that that appealed to a great many people who entered the subject at that time, because I got compliments on the book, many times years later. The Russians now and then—there's a Russian who still says he has learned all his stuff from my book of '23. It must have been translated into Russian, but I've never seen it. But to me it was overtaken by the Handbuch article, which was a condensed and much better and, of course, much more advanced presentation of those subjects, so I never wrote a second edition. But I did try to keep this card index of the known structures going and, by and large, that became too much work because the number of papers increased so rapidly, you see. And so I tried to get help.
I wrote to Laue and asked him if he couldn't give me an assistant for that, could he procure that through the Notgemeinschaft. Laue, of course, was a very important man in Notgemeinschaft, in the distribution of money. And so Laue got together with Berliner, the editor of Naturwissenschaften, and they had the special fund—now this is quite interesting—which was handled by a special Elektrophysikausschuss der Notgemeinschaft, and as far as I know the main part of this money came from G.E., the General Electric. And it was during a visit of Whitney's in Germany that he contacted his friend Berliner and asked what could he do. And thereupon Berliner arranged for this Elektrophysikausschuss to be created. Whitney did not want the Notgemeinschaft to do it. And so they made it a separate section of the Notgemeinschaft— it was formally attached to the Notgemeinschaft but it was autonomous in its dealings. And from these funds I obtained Carl Hermann as an assistant to help me with the collection of structures.
Oh, he wasn't a teaching assistant; he was strictly for that special project?
For that special project. Now I told you this story just now about the G.E. I tried to confirm that when I wrote the "Fifty Years" book; I tried to confirm it with the people in G.E. who are still alive, especially Hull ...
Well, he's no longer alive.
he's no longer alive, but he was then.
Hull and Coolidge. I think Coolidge was still alive. I think I wrote to Coolidge and asked him if there was any trace of this gift because I felt it was really a great help at the time and very generous. And they searched the files of G.E. and couldn't find any mention, but I'm pretty certain that I'm not wrong in this statement, which I got from Laue and from Berliner.
There are ways of looking further. It would be interesting to check that out. Did Hull or Coolidge recall anything about it even though they weren't able to find records? Was this news to them when you told them this?
They had forgotten; they were of no help to me.
Well, even if it did occur, they might not have been aware of it at the time, And so Hermann came. What year was this?
'25. Hermann was a pupil of Born. He had gotten his doctor's degree with Born in Goettingen on the calculation of the optical activity of sodium chlorate. Sodium chlorate is a substance which rotates the plane of polarization of light, as a crystal, but not in solution, so you have a proof there that the rotation is caused by the arrangement of the molecules in the crystal—the molecule itself is not optically active.
And so this was a thing that you could calculate, really with the methods of my thesis, and Hermann did that calculation. Unfortunately, he got mixed up with the electrical units and left out several factors, Pi and so on, and later work in that field has been done by Hylleraas who calculated the optical activity of quartz. Well, then after his thesis, Her- mann came, I think, to Dahlem and worked with Mark, who was at the time in Dahlem, but only for half a year or so, and then he got this job with me. And he stayed with me ...
No, '30. Five years I think he stayed with you.
No, he stayed until the Nazis ...
No, then he was already in Ludwigshafen, at the I.G. Farben.
Look, he had the offer from Ecuador ...
But I think he surely was for five years... You can find it ...
Well, all right, if you say so, it's the law.
Well, I think so; I wouldn't swear to it, but that's my feeling.
Well, I have to find out anyway quite soon because I've been asked to write a biography of Hermann, for this biographical enterprise here.
For the Dictionary of Scientific Biography?
Yes, Dictionary of Scientific Biography. And I'm planning to go to Marburg and get some dates from his wife.
Were his duties during this period that he stayed with you consistently on this structure catalog, or did he get involved in other things?
Oh, he got involved in other things, too, yes.
But he was paid, basically, for this task?
Well, he was first paid by the Elektrophysikausschutz. That went on for a year or maybe two years. And after that he was paid by the Zeitschrift für Krystallographie, that is to say, by the publishers of the Zeitschrift, for writing the Strukturbericht.
Had you gotten involved with the Zeitschrift as early as 1924?
Yes. Before I answer that, may I tell you ... You asked whether Hermann had to do other things.
It's quite amusing. The first job I gave to Hermann was to discuss with me a method for turning the intensities of the diffracted rays into information about the structure. You see, if you had the amplitudes of these rays, including the phases, then you could just make a Fourier transform and you'd get the structure. But the complex part of the amplitudes and phases are not observable, and so you have to deal only with the absolute values of the amplitudes squared, which are more or less proportional to the intensity. And I worked that out and found what is now known as the Patterson diagram, Patterson method, and tried to resolve the Patterson diagram but didn't get anywhere. And so when Hermann came, who was a very good mathematician, we both tried hard to do it and didn't succeed. It hasn't been done even today, in a proper way, not in a straightforward way as we tried to do it.
So I never published anything about it, but this was the Patterson method long before Patterson's paper—I think it appeared about six years later—but he never published it. But one thing came out of it, that is, that Hermann brought in the idea of symmetry. I don't like symmetry myself and I always want to do things in the general case without much appeal to Symmetry, but Hermann was a man who saw relations of symmetry naturally; he never looked up coordinates of atoms in a crystal structure but he wrote them down by inspection just as quick as looking at the tables. He was phenomenal in that. So Hermann brought in this idea of Symmetry, and that led to his introducing what he called "Kennvektoren", which was a term he coined. This led to the nomenclature which is now used all over the world in crystal structure determination, the so-called Hermann-Mauguin symbolism.
Mauguin in Paris had invented something very similar. So, instead of the Patterson method, which I should have published, this came out of it at least. The reason I didn't publish anything at the time was that the whole idea of applying Fourier methods to the crystal structure determination had not yet been mentioned. You see, this really goes back to a paper by Sir William Bragg, but it became a practical proposition only with the advent of the Taylor-Lipson strips, which was a convenient method for summing Fourier series and a very important one in the history of crystal structure determination. It enabled people to do Fouriers in a reasonable time, whereas formerly if you wanted to look up your log tables and look up all the cosines and so on it would take you years. But here the values were printed on strips which you laid in a certain order beneath one another, and you could just sum the figures standing in the columns, and that gave you the Fourier sum.
Now, did you say that when you had done this work, there did not seem to be enough in the Fourier work to publish it? You felt that you knew enough about it, didn't you?
No, no. I probably wasn't aware of it, although in 1921 I wrote a Paper in Zeitschrift fur Krystallographie on the reciprocal lattice, and if I had been more practically minded, I think I might have turned that into the Fourier method.
I see. but as of this time when you were working with Hermann, you hadn't?
I had not.
Wasn't this discussion on intensities and the general problem of intensities the thing that led you into convening the conference?
Oh, the conference was later than the 1921-paper; the conference was in '25 or '26.
'25. That came later.
When did Hermann come?
Also in '25, but he came in the fall, and this conference was in the summer.
Hermann came only after the conference?
That's my feeling.
No, but he was at the conference from the very beginning. Yes, surely.
You listed somewhere the people who you recalled were at the conference and he is not listed here.
He's not listed?
But you said who those present were, and you emphasized Laue and Mark, Ott, Herzfeld and yourself from Germany.
Oh, there were more. Well, Darwin must be among them.
I just mentioned the ones from Germany. There were Bragg, Darwin, Wyckoff and James, Brillouin...
Right. And Waller.
But I think that Hermann was there. I have a slide where I can prove it.
Good. That's evidence.
If you can find it.
Yes, that of course is a different matter.
The reason I brought it up is to get into that other subject of the concern with intensities. Now the way you described it in this article, way , you said it was a problem and that you used the occasion of Wyckoff's visit to Europe to call together this group. It seems quite spontaneous and quite bold to call a conference, and I just wondered if you had ...
It was. [laughter]
You just got letters that the people would come to Germany at this time, all single.
No. Not all single. I mean, Wyckoff was to come over and I don't know of anyone else who would have come to Germany.
I thought there were some more.
No, no. You see, crystal structure really was at a dead end. To a certain extent you can determine a structure by geometrical means only, by measuring the angles of diffraction, because that gives you the size of the cell. And therefore you know the number of molecules and of atoms which are in the cell and quite often these atoms have to be placed on symmetry elements, because otherwise there would be too many. If you place an atom on a symmetry element of course, it's not duplicated by the symmetry, whereas if you place it next to it, it would be multiplied. The number of atoms restricts the positions of the atoms on symmetry elements, but that is possible only with rather simple structures. And now as structures became more and more complicated, you had to take the intensities into account for determining the position of the atoms. But there was no theory for reducing, or for making use of the intensities.
Bragg's method was to make every crystal, perfect as it might come into his hands, into a Darwin type mosaic crystal by grinding it up until it was an ideally imperfect crystal and then to apply Darwin's theory for the imperfect crystal and to discuss the intensities in that way. But, first of all, this was a very crude way of doing it and, secondly, it was not a very reliable way either. And so the whole structure determinations had reached a dead end. You could deal with structures where there was only one parameter, or two parameters or even three parameters to determine, but that limits the range of structures you can investigate very heavily, and the ideal was to get on to structures with eight or ten parameters, and this seemed quite hopeless. So when I heard that Wyckoff was coming I thought this is a good moment to discuss it because Wyckoff was really the leading man here in America and he did excellent work of high precision.
The Braggs work was much more inspired; they saw things in three dimensions. Old Sir William Bragg didn't want to know anything of group theory, of space groups or so; he wanted to see things before him. He spoke of the halving of planes by other atoms, and so on, and he constructed each case on its own merits, and that's the beauty of his work. There's hardly a more beautiful book than the presentation the two braggs gave in their early book, X-Rays and Crystal Structure, which is really a summary of the papers they had published mainly in the Proceedings of the Royal Society. The argument is so simple and direct and straightforward that it's a lovely book to read. But it doesn't lead very far, if you get to more complicated things. And so I thought we must do something about it. Besides, there was the hiatus between the predictions concerning intensity given by the dynamical theory and by the theory of imperfect crystals, which were miles apart, the intensity formulae, while the actual crystals gave something in between.
Did the other people in the field seem to identify this as the one important problem?
I think they all felt this was the important problem, and so I wrote to Bragg and asked him if he wouldn't come over and have a discussion with Wyckoff, and with Mark, who at that time was very active in Dahlem on structure determinations, and a few more people.
Any funds involved, any sponsorship?
Nothing. No. I suggested they meet on the Ammersee in my mother's house. My mother was a painter and she had a big studio, and I thought that would be quite a good place to meet. We certainly wouldn't be disturbed. It was during or after the inflation, so there was very little money.
Someone gave you money for the conference.
Oh, yes, I got some money, but that was a trifle only.
It was a trifle, but one didn't offer money in those times.
I think the Gesellschaft der Freunde der Technischen Hochschule.
Just a trifle to give some coffee and cake or something like that.
Nothing for transportation or anything like that?
Oh, no, they had to do that themselves. And I think we fed them.
No, only the in-betweens.
Wasn't the main meal quite often given by Mother? Well, anyway, you see there was one inn in this place, and I lodged them there. And there was a school in the nearby village, Utting, and I borrowed a blackboard from there—my mother had easels so that was not difficult to put up. And I wrote to bremen and got two boxes of cigars sent, and that was about it.
And I remember you planned with your mother what kind of cake you would give them—there was quite a good Konditorei [pastry shop] in Utting, and that was about all that was given to the people for the coffee break.
And you shouldn't forget the show that was performed, which Bragg remembered; I didn't remember it.
By the children, yes, a little play.
By this girl [pointing to a picture], Rose Bethe and this boy ...
But at that time she was Rose Ewald. And the oldest was eleven, the youngest was four, so it was really a trifle.
But it was the spirit of the time.
They all were really more or less personal friends. I did not know Bragg,but I knew his Father. And I did not know James, but I knew Fokker and Brillouin and Mark, of course, and Laue and Ott.
Had you met Darwin? He was at this conference?
I think I knew Darwin; yes, I knew Darwin.
When you say know, do you mean to know personally? Had you met him?
yes, I met him. You see, when we were newly married, my whole desire was—this was in 1913—to see southern France, and so I had planned a nice honeymoon trip to southern France on the border of Spain, San Juan de Luce. And then I saw a number of Nature, and they gave the program for the British Association assembly in Birmingham, and there was Professor William Bragg, W.H. Bragg, to talk on the structure of diamonds. All plans for France were cancelled, and we went to England to the British Association, and there I met all these people. I made the acquaintance of Sir William Bragg, who was not yet Sir at the time. Young Bragg was not there, but Darwin was there and Bohr was there and Moseley was there. So it was quite a rewarding experience. Marie Curie was there; she got an honorary degree and the newspapers said that she had come over just for the ceremony, and after receiving the degree, "the silent woman rushed off to France"—that was the newspaper [description].
It was making a mystery of her. In later years, in 1925, when you convened this group, had you been in touch with them before by letter?
I had been in touch with them before by letter. I corresponded with Bragg, even during the War. I received a postcard from Bragg with great delay by the censors, but it finally came through, and he told me, "I'm now on the structure of pyrites (FeS2), but it is terribly complicated."
yes, this is one of the quite simple crystals but one with a parameter, and it was the first one he came across where—you know all cubic crystals have four trigonal axes, the cube diagonals are trigonal axes in all cubic space groups—and this was the first one where they don't meet at the center of the cube but they go past one another, and so he had to invent that.
Complicated system, in that period.
How about Wyckoff—had you corresponded with him?
Yes, I corresponded with Wyckoff. You see, as editor of the Zeitschrift, I had a lot of correspondence with Pauling and Wyckoff and other Americans, and altogether an international correspondence with authors.
This is a hard question to answer, but how large a group was it, do you think, who were concerned, either as their main research interest or as one of their major research interests, with crystal structure?
At what time?
At the time of this conference in 1925?
'25? I would guess about a hundred, if that many.
Throughout the world?
Throughout the world.
And this is based on your experience with the Zeitschrift, as well as your own reading?
Then the group that was there represented a good proportion of the total group. They were pretty much among the leaders, weren't they?
Yes, they were the leaders, certainly; they were the people who wanted to make something exact out of x-ray diffraction. But the greater number of people of these hundred I quoted—the actual number might be only 70 or so—the majority of these took powder diagrams. They were chemists or physical chemists and investigated reactions and took powder diagrams to compare whether substances were the same or not.
Their motivations then were different. Would you say that this group was primarily concerned with the theme that seems to have guided your work of the interaction of radiation with matter?
Yes, that is quite true.
In the physics sense of it.
In the physics of it.
And so the people concerned with the intensities would have represented that group. It's not necessarily so, because you can be concerned with intensities without having this broader interest in the ...
In the structure determination.
No, but they were mainly ... You see among the people I had got hold of ... Brillouin was not interested in structure determination; he was interested in the optics. And Fokker was not interested; he was interested in optics. Debye was there; he was not particularly interested; ne came only for a day or two. There were a number of people who came just for a short period, actually people whom I had not invited because I didn't think they could contribute very much. Debye I probably had invited.
How did the ones that weren't invited know where to come to the conference?
Oh, well, if they came to Munich they heard by word of mouth where these people were on the Ammersee so they just flocked out to see - their friends or colleagues.
But who else came just for a day? I can't remember ...
Well, Herzfeld wasn't there for the whole time.
Fajans, yes. Well, Fajans had nothing to contribute to that.
You have him listed as "an occasional visitor."
Occasional visitor—that's probably correct.
About how many would you say were in the total group? We can add up these names but it may be incomplete.
The real gang were perhaps ten.
Let me count them on my fingers.
You have eleven and yourself—twelve—as regular plus a couple of occasional visitors.
Was there a regular procedure of someone making a presentation?
but not a prepared paper, was it?
No, not a prepared paper. But, you see, we were worried about Darwin's papers. Darwin's papers were done quite early, before the first World War in 1913 and '14, and really they were very beautiful papers where he got the gist of the physics in a marvelous way and in a very simple way, so that even now people prefer to deal with Darwin's theory. Warren is just writing a book and he prefers elaborating Darwin's theory rather than the farther-going dynamical theory. On the other hand, there were contradictions in Darwin's papers which we didn't understand and so we asked Darwin to present his paper. Well, he took a reprint, I think, and more or less went along the lines he had there. And when we asked him questions, he got entirely confused and didn't know how to answer; he hadn't looked them up, and his standard answer was, "Oh,yes, yes, this I only wrote down, you know, for the print; I did it in a different way." And I don't know where it is that Bragg says—I think in this Festschrift contribution—that he was so annoyed.
In the Acta Crystallographica in '68, he said, " To our consternation we found that Darwin had typically omitted to refresh his memory of his own theories and was unable to explain them when he got up to make his contribution."
I don't know what his expression was, something from sports, that their main champion failed. ["I felt we were bringing along the British seeded player to represent our country at the meeting."]
I see. So he was called on spontaneously to explain ...
Yes. Then I gave a presentation of my dynamical theory, and, well, there were lively discussions all the time. It was absolutely informal.
How long did this go on? Did you have a morning session, and an afternoon session?
Yes, morning and afternoon sessions.
And evening as well?
This I don't remember. I don't know what else we would have done in the evening, so we probably still discussed in the evening.
You had a play one night, and ...
Well, but that was quite short.
And this lasted for, what, five days?
I should say for about four or five days.
I see. Then the people just dispersed and went on their ways?
Then some of the people went away and with some of the people we made an excursion through the beautiful woods in the hinterland of the Ammersee; we went to ... oh, what's the name of the oldest German monu ment .. I must ask Ella ... there's an old monastery, hidden away in the woods, and then to the Peissenberg which is a nice mountain in front of the main range of the Bavarian apls, and it was very dramatic because it turned out that we were a bit late. We had to catch a little train that went, at say five o'clock in the afternoon, and at four o'clock we were still up on the Peissenberg. And Darwin got very nervous and said he had to catch the train because if he didn't make that train, he couldn't get back to Munich and he couldn't get back to his steamer, his boat from Harwich and get back to England. And he didn't tell us that the next day was his wedding! So he very nearly missed his own wedding. But then at the top he and Bragg said, "Well, let's run it," and they began chasing down the mountainside, and the others followed at a bit slower rate.
And they made it?
We all made it, finally, yes.
Were any notes taken at the conference?
No. But as a result of the conference Bragg and James and Darwin overhauled the whole theory and they published a paper, I guess in Phil. Mag. And then Bragg got hold of Waller, who was one of the people at the conference, and he got Waller to come to Manchester, and they worked on the theory together. And so it really had a great effect on ... [Mrs. Ewald returns; he addresses her] Ella, Wie heisst das Kloster? Das alte Kloster ... Wessobrunn. The oldest German literary monument is the Wessobrunner Gebet ... that's the monastery there.
There's only a stone.
Yes, there's only a stone, and the monastery is no longer existing.
But you went to that spot.
It's very beautiful.
You mentioned the outcome of the conference. Was there another outcome in terms of the feeling that this was a good thing and that perhaps it should be continued—the idea of meeting together?
Well, I don't know. No. We met on this problem, which was a problem we had in common, and I don't think we thought of any further meeting. But, as a consequence of this, Mark and Ehrenberg and I started work on the intensity of diamond. This work was being done in Dahlem where Mark was in Herzog's Kaiser Wilhelm Institute, and that is an outcome of the conference.
Was there any precedent for this kind of a meeting? Had a group with this special interest—not necessarily the special problem but the overall interest—ever convened before? You mentioned a British Association meeting, but that was not specialized.
Oh, well, that was a very big meeting. No, I don't think so.
This was the first specialized group.
This was the first.
Well, certainly it was international. Was there any group that met on that subject within Germany, prior to that time? By that, I mean, you know, exclusively German.
No, not on x-ray diffraction certainly. You see, that's the queer thing about this whole x-ray diffraction: by far the main part of the work was done in England, all the successful work of structure determination was done in England. The original work was Laue, and if you look at the various methods which are used for x-ray diffraction, let's say the Laue method, the rotation diagrams, the oscillation diagrams, the Weissenberg diagrams, then several other wide-angle diagrams—they were all German inventions, but no structure work. All the really successful work was done in England, in the two schools of father and son Bragg.
And the only other work in competition with that was Wyckoff's work here— Wyckoff and a few other people. It started out in Pasadena with dickinson, and Pauling, and I don't know who else—all that is in the "Fifty Years." Hendricks, for instance, who's in Washington, was one of the early people.
Was Epstein interested in this at all? Paul Epstein.
No, except that Epstein rigged up quantum theory of x-ray diffraction , with momentum transfer to the crystal. Epstein and—who was the other man who worked on that?—I've forgotten just now. [Ehrenfest ?]
What was the relation of this to studies of the solid state? You use this word—or someone uses it—in the title of something you published in the late '20s, but is there any concept of this being a separate kind of physics concerned with separate problems?
No. Well, of course, an entirely new world was opened up by the possibility of finding the locations of the atoms in a solid, and so one was keen to apply that to calculation of physical properties. And, of course, in the same period Born developed his dynamics of crystal lattices, and he showed in a formal way how the various properties of crystals, i.e. of solid matter, could be put in relation to the structure. The first of these properties was the specific heat.
That was Debye's work, who, however, smeared out the crystal to a continuum and so was lmt very much in this same line. Whereas Born and Kärmän started the work, and Born continued the work, by assuming the real crystal with a periodic base of atoms periodically repeated in each cell. And so in a formal way the various physical properties were explained. The main properties of thermal conductivity and electrical conductivity were those which couldn't be explained at the time. Also thermal expansion was not explained, although everybody seemed to be aware of the fact that you had to introduce non-linear terms in the forces in order to do it. But how to do it was quite difficult.
Then the other application of the new knowledge came in the theory of atomic radii, which Bragg was one of the first to establish and to use for the determination of structures. It really came out of the ideas of Cambridge chemists Barlow and Pope. Pope, I think, was the professor of chemistry who had this concept of atoms packed together like spheres, spheres of various sizes, and this was taken over by Bragg and proved to be very successful. Then in Germany it was Kossel who also took up this idea and llnked it to the Bohr atomic model. He and Fajans, both in Munich, and often in collaboration, followed up this idea of atomic radii and of completed shells, which are similar to those of the inert gases and which lead to the chemical bonding.
And these developments that you've been describing occurred over how long a time?
Well, Kossel's work is quite early. That was, let's say, 1919 to, I don't know, '23. And Fajans introduced the concept of deformed ions, and this always seemed to me a very vague concept.
Let me get back for a minute to the general activities at Stuttgart. We talked about the conference. What about visitors in the normal course of things? I know, for example, Warren came over. I am sure there were others.
Oh, we had a good number of visitors.
Would they come on a fellowship or on a tour or for some specific purpose?
M ainly they were visiting laboratories in Germany and came to Stuttgart as one of them. And then there were a few who settled and worked in Stuttgart for longer periods, such as Warren and Nitta, the Japanese, and where they worked d either with me or with Glocker in Stuttgart. Also Konoboyevsky, a Russian, who I think is still quite active in metals, and I don't know whether it was Uspensky also who was there. Well, I mean there were a number who were there.
This is the '20s you're talking about.
This is the '20s. This was actually after I had gotten my little institute.
The institute was 1930, I thought.
That may be. These people may be earlier.
When did you meet Lonsdale?
Kathleen Lonsdale? Oh, I came over to England quite often and each time I visited Sir William Bragg. Sometimes I was invited to stay with him, and Kathleen Lonsdale was one of Bragg's co-workers. So were Astbury and Bernal.
I know Bernal visited Stuttgart on one occasion.
I know you said you went to England often. Did you make other trips to other places in Europe?
If I went to England, I also usually, or several times, went to Paris.
What was the attraction there, other than the life of the city?
Oh, it was Mauguin. You see, Mauguin's and my book were the first books, except for the Braggs', which was really a resume of their own papers. But both Mauguin and I tried to give a balanced account of the whole field, and the two books appeared about the same time. I think mine may have been a bit earlier than Mauguin's or it may have been the other way around—I don't know. We didn't know one another, or of one another, at the time, but then soon after I took an occasion to go to Paris and to meet him. The first who went to Paris on an explorer's trip, so to say, was Mark and a group of his co-workers in Dahlem, a very jolly group. Herman Mark and his wife Mimi and an Indian, Bose, who worked with Mark. Let me see, is this the "Einstein" Bose or not?
I was going to ask you that.
I think it is the "Einstein" Bose; I'm pretty sure. And there was an Englishman, Green-something [probably Greenwood]; I think he was at the British Museum—Francis Bitter was with them and then Ehrenberg. And you see, this was one of the very, very nice parts of Mark. Ehrenberg was a cripple; he had had infantile paralysis and could walk only in a very limited way. But they took him along on this trip to Paris, to go sightseeing in Paris, and visiting the people, especially de Broglie— that was not Louis de Broglie but Maurice de Broglie, the elder brother— who had his laboratory in his own private hôtel, in the Rue Lord Byron, and they visited Mauguin, of course. And on the way back they made a stop in Stuttgart and all stayed overnight with us.
Some of them came on the way back, and I was, of course, very keen to hear all about Paris—I hadn't been to postwar Paris at that time; this must have been in '25, shortly before the conference—we could find out from our guest book the exact date—and so the description I got from Mark of the laboratory of the Duc de Broglie (yes ,Duke, the younger one is the Prince) was, "Oh, it's wonderful. It's a big hôtel with high rooms and there are Gobelins everywhere on the walls of the rooms. And two rooms are there for the x-ray equipment—they have the diffraction thing and the x-ray tube in one room, and they have the high-tension in the other room, and then they have cut a hole in the Gobelin and in the wall and have the high-tension lead passing through the wall." And the people who did the work at the time were Thibault and Trillat and it was very interesting work.
And then I 'asked him, "What kind of a chap is Mauguin?" And Mark said, "Look, how shall I describe him? He looks like a bricklayer." You see, Mauguin lived in the country outside Paris and was one of the most charming figures I ever met, ruddy-faced with a moustache somehow, and a very straightforward little French beard—a charming personality. He was very unfortunate because his wife became blind and for the last ten years at least, or fifteen years, he mostly had to live for the care of his wife. He put her in order in the morning, then he came into the lab in Paris, and then he went out in the early afternoon back to his wife because she needed him then and he looked after her.
Now, did you visit Munich often? Did you ever go back and participate in the Sommerfeld seminars?
Oh, yes. I visited Munich quite often. You see, since my mother lived not far from Munich, of course, many of our vacations were spent there on the Ammersee.
Did you go to Sommerfeld's seminars at all?
Well, I was in Bavaria mainly during the academic vacations. Oh, I went to visit Sommerfeld, or the Sommerfelds came out to visit us.
But the people who subsequently went different ways. into different branches of theoretical physics—did you have the sense that you were separated from them in a way?
Looking back on my own life I am really astonished how one-sided and strictly limited my work has been. I always thought I was fairly interested in a wide field, and indeed I think I was. But it doesn't come out in any of my work. What I've been doing is working on my thesis all my life. I'm still at it. I mean, this latest work in Acta '68 is really a continuation of my thesis work. It's a problem which has held me.
A theme. And I don't think it's finished yet. I see another part I would so like to achieve, but it's very hard to do, so I'm not certain that I shall. I've tried it now repeatedly these last years and never gotten anywhere. It's the problem of how big an optical activity there is for x-rays, and I'd like still to do that. Then my life's work would really be fairly completed, but I doubt that I'll have the force to do it.
Well, if you solve that, you'd find another problem of equal interest.
But you see I became interested in this whole subject really through a sense of duty, collecting all these structures. And that at times took really all the force I had besides my professional duties, and then the editing of all these papers, that took a lot of my work.
Well, I was amazed in looking over these descriptions of these major pieces of work, the Handbuch and the 1923 book and the chapters that were subsequently reprinted, that also you still have a group of individual research papers that you seem to have published every year.
Well, that should be the case, and other people have done much more in that respect.
No, but I'm saying besides the teaching, and the keeping up of the structure work, you have papers—starting in 1921—on Bragg's reflection law, on the reflection formula for x-rays, the reciprocal lattice in 1921; optical-electrical lattice potential, and "News from the Field of x-Rays,"—that last is an interesting title.
Well, I mean, this is sheer ...
... a note, probably a note.
This is a resume of something. I don't know. You know, this list of papers contains good and bad stuff. You may have noticed that I started with some articles I wrote when I was in high school, for the local newspaper, so this is just as complete a list as I could put together. It doesn't tell anything about the quality of the work.
It shows that you were productive. There are a number of things that go on through the period. So let me ask you how you would characterize this part of the work since these papers are reports intended for rapid publication, which implies that they are new findings. How would you characterize this work in the '20s?
You mean papers with the character of abstracts? Like those you have now for the APS meetings?
Right. Many of these I'm sure are in that category.
No, I don't think so.
I see. Then I'm talking about these. I meant these.
No. You see in the early days I had a wonderful key to all the problems; I saw clearer than most people how to deal with these problems because the construction in reciprocal space was so close to my heart, and so many of these papers deal with constructions which are not really very difficult, results which could be obtained fairly easily from a knowledge of the dynamical theory and of the what we nowadays call the Fourier transform technique.
So it was the application of these things which you had developed, it was the application of these techniques and these approaches in turn, to one problem after another. But was your main interest in trying to develop the dynamical theory, let's say, and then finding out that in doing this you had run into some problems along the way, and that you had to solve these problems in order to refine the theory further? Or was it the other way around: you felt that you had the theory and were looking for applications for the theory?
Well, at first there were no applications whatsoever. Then, if I remember correctly, the first application came when the high precision people in Siegbahn's laboratory in Uppsala measuring x-ray spectra found that they got deviations in the wave lengths according to whether they took first or second or third or fourth order reflections. That was easily explained by the dynamical theory. Actually, it's simpler to explain it; you don't need all the dynamical theory for that, but just the idea of a refractive index would do. Then the next group of confirmations came from the work of Bergen Davis at Columbia University. Now Bergen Davis is someone you should try to get into your system of notes because he really did very excellent and quite important work. Bergen Davis and von Nardroff published together. Some years ago I wrote to von Nardroff and asked him if he couldn't give me some details or write something about Bergen Davis. I never got an answer from him. Now I think he died.
Yes, von Nardroff died; about a year ago. I think.
About a year ago, yes, and it's a great pity that he died without giving any information about this work with Bergen Davis and about Bergen Davis' personality, etc. On my first visit to America in '36, I tried to visit Bergen Davis and I think I may have seen him, but he was a very sick man at the time and I have not much recollection other than just having been there.
Well, he published some things which I could discuss on the dynamical theory, and that turned out fine. Then the next were so-called Aufhellungslinien, that is to say, white lines crossing x-ray spectra or rather background, between the line spectra of x-rays. There were white lines—I would now in English call them deficiency lines—and I explained that at once. This was quite dramatic because Siegbahn had come for a visit to Munich (we were still living in Munich; it must have been before 1921) and he was one of the first foreign visitors we had in Munich after the war. Sommerfeld gave a dinner for Siegbahn at his home and invited the people who were doing x-ray work.
Among them there was Ernst Wagner, the assistant of Roentgen who was a very good experimentalist and his theoretical understanding was quite deep but his formal mathematics didn't go beyond cos and sin; I think that tan (tangent) was already too difficult for his understanding; he had to decompose it into cos and sin. And Ernst Wagner showed Siegbahn—he was a man, like all the Roentgen people, of great secrecy; this was traditional in Roentgen's institute, no one was to know what the other one did—the films he had taken with the deficiency lines in them. And Siegbahn saw them and, well, he didn't speak very much anyway and didn't say anything.
I happened to look into this group of people standing together and I saw these and I said, "Why, of course, this is when there's a third reflection occurring, which takes away the intensity." Wagner, at first, did not want to believe this, but finally he came round, he made further experiments and finally he was convinced that this was the case. So that was something which would not have been explainable by Laue's kinematic theory because that doesn't take account of intensity. So, by and by, but very slowly, some things accumulated which showed that this theory was perhaps quite useful.
Now, during the '20s you were developing the theory, applying it where it could be applied, and at the same time reviewing the whole state of the field. What about the developments in quantum mechanics, the new approaches—did this make much of a rumble in Stuttgart?
Well, did it make much of a rumble? My assistants worked in that field. The assistant were, first, Fues, and then [to Mrs. Ewald] Ella, could you help out, what was the sequence of assistants? Fues ...
Then Bethe ...
Fues was there from '21 onwards. From '25 to '27 he went to Schroedinger, then to Bohr. And then in the meantime Hermann had arrived. I remember now definitely that Hermann is on the picture of the conference in the atelier. And in the fall of '25 there was this changeover from Fues to London, and Fritz London came to Stuttgart as Paul's assistant, for two years. And in '27 Fues came back from Copenhagen and Zurich, and London went to Berlin with Schroedinger. Schroedinger had in the meantime, in '28 or '27 accepted to go from Zurich to Berlin as the successor of Planck, who had reached the age limit of 70. And so Fues was with Paul until '28 when he was made—'28 or '29—professor of the Technische Hochschule in Hannover. And then Renninger came ...
Then Hönl came.
As theoretical assistant. By that time I had two assistants.
Yes, Hönl came as theoretical and then soon after, then you got your institute after '28, you know, you had the call to Hannover and you got your institute built. In '29 or '30 the son, you know, of our friend from England came ...
Yes, Ruhemann came to be your experimental assistant.
And after him, Renninger.
Ruhemann was the first, and then when he went to Charkov, Renninger came, in `32, I think.
I think that was the sequence, and Hönl was with you until you left.
Well, good. Thank you. That sets the background.
Fues worked on wave mechanics. You see, he came to Zurich just in the year when Schroedinger formulated the wave mechanics, so he worked on wave mechanics. And London worked on quantum theory in general, quantum mechanics, really, at the time.
Now by this time ... The original assistant you had was paid for by the special fund from the Notgemeinschaft, and was concerned with the structure catalog. But these other assistants were more in the nature of the traditional type of assistants.
They were paid by the Hochschule.
And so they had teaching responsibilities?
Not necessarily, but they could become, and they did become Privatdozenten.
I don't think that London became Privatdozent with me. I don't know whether Renninger ever did.
If London and Fues were working on the new developments in quantum mechanics, were either of them teaching it at Stuttgart? Did they introduce any courses in it?
Well, Fues was "habilitiert" and was teaching. You see, you couldn't teach if you weren't habilitiert.
But did he teach that? I was curious to know if he introduced the new quantum mechanics into the curriculum.
Oh, surely. The system of German universities was this: You had the so-called Kursus vorlesung, the regular course which consisted either of four or six sections, each lasting one semester. In Munich it consisted of six, which were mechanics, thermodynamics, electrodynamics, optics, and then mechanics of continua, and mathematical methods.
But these last two I don't think we had in Stuttgart because at a Technische Hochschule the course of studies is more regulated, and the regular course was only for four semesters after the first diploma, the Vordiplom. But then there were small special lectures, which were on quantum mechanics and wave mechanics and so on.
It wasn't taken into the big course, but it was additional material which was presented. For instance, I didn't give these courses; I gave courses on geometrical optics or on the physics of surfaces—one or more actual layers—or I don't know what. And then there was, as I told you before, the big course on atomic theory, which at that time meant that you discussed spectra and then term values and hydrogen-like atoms—that's as far as you could go—and the chemical implications—this kind of thing.
That course would have been taught somewhat differently after 1926, I would think, than before 1926.
Yes. You see, after ... Well, I guess after 1930 I really hadn't too much time because I was, first, head of the department, Abteilungs- vorstand, and then I became Rector of the Hochschule.
Oh, I see.
You know that this rotates; this is changing. And at that time nobody wanted to take over the rectorship because of the Nazis, and so I said I wasn't afraid of them and took over. And had a very interesting time in 1932-33, just during the revolution, der "Umbruch". But, of course, it kept me away from my institute quite a bit.
I want to talk about that, but let's go back to the institute. How did that get established?
Well, the German system for promotions is that you get a call from another university; then you go to your Ministry of Education and tell them, "I have this call to the other university and I'm much in favor of accepting it and I've made a date to visit the university, and the place; I've then to go (in my case, to the Prussian Kult-Minister) and to discuss conditions with him." Well, then the people in Wuerttemberg would say, "Well, you go ahead. You just discuss it but don't tie yourself down. We'll see what we can do for you." And so I had to call to Hannover ... [bell sounds].
The ringing of the telephone also served as the dinner bell, and we've since been to luncheon and are resuming now after a long, pleasant interval. At the time you were talking about the general procedure for getting a promotion.
Well, in my case I had a call to the Technische Hochschule in Hannover, and so I inspected the place. It was in a big castle in Hannover, and they showed me the basements where I was to establish an institute, all cluttered with hot water and steam pipes for the heating— and it wasn't attractive at all, but of course I didn't tell them. And I went to the Minister in Berlin and told him, "We'll, I don't know. What is it that you would give me to run the institute, etc., etc." And he made me an offer, and I said, "Well, let me consider it." And I went back and told the offer to the Wuerttemberg minister, and he said, Well, we can build an institute here, if you like, and if you stay here, I think you won't be sorry." So I said, "Fine, then I'll write off Hannover."
Now as to this institute, I really had the idea that I didn't want to have much space. I wanted perhaps two rooms to work in, just to set up the few tables and do some experiments and maybe a very small workshop, but I didn't want to have any institute which was comparable to the big physics institute of Regener, very well equipped, in Stuttgart, or the very well equipped x-ray institute of Glocker, also in Stuttgart.
And so "Well, said, Well, we need perhaps two or three rooms for office work, for my assistants, and then two or three experimental rooms." So they made a plan accordingly. And the workmen began to excavate, and the people from the Bauabteilung, or Bau ... what was it? Well, you see, all this is under the state, of course—who did the digging came and said "Now Now look here, why not dig out the whole institute to have all that floor space instead of digging out only one of the rooms?" ( I had made it a condition to have one room in the ground so that it had a constant temperature.)
"We might just as well dig out the whole thing—it doesn't cost much more and then we have room to put in central heating, and we can have room for storing the fuel for the central heating, the coke, and so on." Well , as it turned-out finally, I had a lovely little institute with four rooms in the basement and about six rooms on the ground floor and a lovely workshop, and it was really a joy to work in it. And from then on I had quite a series of experimental research going on.
The first thing we did—I did with Martin Ruhemann as my assistant; Martin Ruhemann had been trained by Franz Simon, (later Sir Francis Simon at Oxford) so he knew low temperature work—we investigated the structure of solid nitrogen, trying to grow single crystals of solid nitrogen.
We didn't believe in a structure which Vegard in Oslo had proposed for nitrogen. That had been done on powder diagrams, and we thought it was not very good work so we tried to grow single crystals and were successful in that we filled the capillary tube with two crystals, growing side by side, so you could disentangle the diagrams. And later there were Dr. Achille Papapetrou who worked on crystal growth and Renninger—who was Ruhemann's successor—who discovered the influence of multiple or simultaneous reflection of the intensity measurements of x-rays and did other very good work which he is still pursuing in Marburg; and Kochendörfer who was the only man thus far, or up to nearly this year, who investigated the slip properties of an organic crystal; he grew naphthalene single crystals. We built a special apparatus for testing them for slip, and we found that the laws of slip are quite similar to the laws in metals. This was an astonishing thing because, after all, a metal consists of single metal crystals, built up from single atoms, whereas a naphthalene crystal is built up of large molecules.
You would expect that the glide and slide, slip properties of these large molecules are different from those of atoms. Well, there are still a number of smaller papers which were published, and altogether I was quite satisfied with the output of the institute, especially since I had very little time to look after it, being lnvolved in the Rektorat of the Hochschule.
When did that start?
Well, first came the Abteilungsleiter, which is a kind of preparatory and lasted usually for two years, but I had made it a condition that it be cut down to one year only. And after that one year I became the rector, in 1932, at a time when there were student demonstrations and great difficulties in keeping the students down in most German universities. I never had very great difficulties. On one occasion I had to become quite energetic and tell the students that I just would not do what they demanded; on other occasions I thought, well, a demand, for instance of hoisting a red Nazi flag on the Technische Hochschule, that really isn't worth quarreling about. And so I had quite a jolly time as a rector. And it gave me much insight into the ways the Nazis took over. And when the laws against the Jewish professors came out, there was a conference of all the rectors of all the German universities, called, I think, in Wiesbaden, and the Nazi Minister of Culture and Education just treated us in a most dirty way, not giving us sufficient information to really appreciate what the law was about. And when I came back, I wrote a letter to the Minister that I could no longer perform the duties of a rector, and didn't want to continue.
You wrote to Rust?
No, I wrote to the local Wuerttemberg man, and he, of course, had to report to Rust. And so they accepted that.
It would be interesting to know how this was presented to a group of rectors, and academic people. Wasn't this a new role for the state in general? Wasn't this an extraordinary thing, to call people together on a national basis?
He didn't call it, no. There was a regular Rektorenkonferenz every year, and we had had one already in Danzig. And then these laws hadn't appeared yet in print, though one knew that they were being prepared, and so a second Rektorenkonferenz, an emergency conference, was called at Wiesbaden. The opening of that conference was delayed because we expected the appearance of the Rector of Berlin University, who was Professor Kohlrausch of the Law Faculty, and when he appeared, quite haggard-looking, he explained that he had been in the antechamber of the Minister for I think four or five or six hours, whereas all these young Nazi-dressed-up people had access to the minister. Finally he was called in and was given a few details about this law but not much more. And so we debated what we knew about it and came to the conclusion that this was unacceptable.
The Berlin rector himself said, "Why, in my own Faculty of Law in Berlin the most prominent people and my very best personal friends are Jewish, and I just can't dismiss them. This is impossible." But he said in private afterwards, "What can I do? I have five children and I have to live and I can't break with the Nazis." And the difficulty for us to rig up any protest against the laws lay in the fact that among the rectors there were perhaps five or six who were Nazi Party members and who, of course, said, "Well, if you protest, we shall report to the minister." And in that case the consequence would have been that before we returned to our respective universities, we would have been deposed and some Nazi put in instead. So it finally ended that we did not do anything, but each of us went home, very sad at heart, except for the young Nazis. I think the Goettingen rector was one of them who was very vociferous for the Nazi rule.
The consequence of not protesting and not being turned out by the minister himself, as far as Stuttgart is concerned, was that in Stuttgart we managed to get a successor to me who, of course, was a strongly conservative German nationalist—that was inevitable—but a very decent man and at heart very anti-Nazi. He belonged to the Department of Architecture and he had a little dog, a fox terrier, and when I paid him visits in his Rektorat room, he would close the door carefully, call his dog Hexe, and tell her, "Hexe, Mach 'Sieg-Heil.'" And Hexe would sit up on her hind quarters and have one paw hanging down and the other one lifted up.
Which was a caricature ...
This rector Wentzel was rector for another year and a half, and then a successor had to be chosen. And as a successor we managed to get an historian, Goering, and this Goering was a man who had been imposed on the Hochschule; the faculty had not chosen him, had not even proposed him among the three names which usually were forwarded to the ministry for the choice of the ministry; but the minister, Bazille, had insisted on putting him in a few years earlier against the wish of the faculty. Well, now it came in very handy because Goering was a cousin of the big Goering in Berlin, Hermann Goering. At heart he was not a Nazi, and in private, he told you, "Well, I know very well what a scoundrel my cousin is." And so when there was any political difficulty threatened for the Hochschule, Goering would just slip in a word and say, "Well, I must discuss that with my cousin. I am going to Berlin next week and we'll wait; I would like to see what my cousin says to that." So he had a very beneficial influence on the Hochschule, and that means that the entire Hochschule stayed without serious external interference for two and a half or three years longer.
When was your term as rector up? It started in '32.
It started in '32 and I guess in the early spring of '33 was this conference in Wiesbaden, and then after that I stepped down in April. But I retained my professorship.
And you still had your institute?
I still had my institute, yes.
What happened at the university as a consequence of the new laws? Did this, in fact, decimate the faculty somewhat?
It decimated the faculties, yes, and it gave much heartbreak to a great many people on both sides, both the passive and the active side, but vou couldn't do anything.
Was there any reaction in the German professional societies of scientists?
It was all entirely dominated by the Hitler gang. You see I had hoped that my, of course carefully worded,letter to the minister would be published in the press, and |'m pretty sure that several others of the rectors assembled at that conference resigned. Not a word of it came into the press.
This is the time when Einstein, for example, resigned from the Prussian Academy and, I was never clear whether he resigned or was expelled from the Bavarian Academy, but either way it was the same. Do you remember if this was a public event, if people were aware of it?
I can tell you where to find out about whether he resigned or not, and that's in the Correspondence: Einstein-Sommerfeld, a copy of which I have upstairs if you are interested in it. From the Sommerfeld-Einstein correspondence I gather that the Bavarian Academy (Secretary at the time v. Dyck) asked Einstein in 1933 to declare his resignation as a member of the Academy. Sommerfeld can't establish the exact wording, as the files are burnt but he read a translation of the letter in 1937 (in Geneva) in Einsteins booklet "Comme Je Vois la Vie." Einstein refused to resume the Fellowship.
Oh, yes, I would like to see that because there is a letter— I've seen the published version—that he wrote to the Bavarian Academy in response to their letter asking him, in view of his recent actions in regard to the Prussian Academy, what did he think his relationship should be to the Bavarian Academy. And in his letter he essentially explains that he couldn't be part of any group that stood by. But yet his biographer, Phillip Frank, said that he was expelled from the Bavarian Academy, so I'm not clear—that's all.
Well, I can't tell you but I have the Proceedings of the Bavarian Academy here and I've got this part of the correspondence of Einstein and Sommerfeld—there may be something in it.
Now, where did that Einstein-Sommerfeld correspondence come from?
From the Einstein Institute and partly from Ernest Sommerfeld.
Do you have copies of it?
Well, that would be important to see. What about the effect on the student population—was there a great deal of instability in terms of political upheaval in the classes themselves?
No, not in the classes. No. The only way it made itself felt was in the student assemblies, Allgemeine Studenten Versammlung. And you see the students had a governing board, the so-called Asta, Allgemeiner Studenten Auschuss, and there I think .... The only difficulties I had was that in one of these student assemblies they had proposed to vote or to demonstrate against the presence of the librarian of the Technische Hochschule, Professor Marx who was also an historian and Jewish. And I told them that they were not to demonstrate against Marx, who had great merits for the Technische Hochschule because he had really done a wonderful job in getting all the foreign journals during the inflation. Since Marx had great assets for the library, I told the students that if they made any demonstration against Marx, I would not stand it. I do not know how I could have resisted; this was really a dramatic moment where I confronted some 2,000 students, I guess—most of them in their student uniforms, their fraternity, their corps uniforms and others in their Nazi uniforms—and told them I would not stand any demonstrations against Marx. They finally gave in. It was quite a fight and quite exciting.
Was this '33?
It was maybe '33—I don't know.
Right. But it was before you stepped down as rector?
Yes, before I stepped down.
Then what about the effect on the number of students entering the university? Did enrollments in this period decrease? Did you not any change? I think there was a general decrease.
Well, already at high school marks didn't play any role What played a role was whether the student was good in gymnastics and good in sports and good in party membership.
This is even before Hitler, before 1933?
Before 1933 when my children went to school.
If you can, when would you date the beginnings of that change? How far back? Would it go into the '20s?
This question I can't answer. You see, I've no historical min [Ella says it started only after the take-over of 1933.]
What I'm trying to get at is the general state of science and of learning and university life in Germany from the late '20s on, even before Hitler, because this would be interesting.
Well, I can tell you of two experiences of my children at school One was that of my daughter, now in London, who was at the Maedchen Gymnasium and took Greek. The girls performed a play—I don't remember which—in Greek, and she should have played the leading role according to her classmates, but then the Ministry of Education found it impossiple that she, not being pure Aryan, should play this role. So somebody else had to play it. That was one experience. The other experience is quite funny. It was what my eldest son experienced in the Real Gymnasium.
In all classes they had to give lec- tures on human races, and the teachers had certain books, prescribed books, and they taught according to these books. The teacher in my son's class explained that "mixed races were very bad and you saw that in the fact that it is well known that mixed people are not fertile."
And so my son after a while asked quite timidly, "Oh, what about Brazil? I've always read that the main part of the population in Brazil is a mixture of Portuguese and Indians and Negroes and so on. Are they all infertile?" So the teacher blushed a bit and said he couldn't answer that; he hadn't the latest edition of this book at hand so he would have to look it up.
That is already in the Nazi period, beyond the '20s?
That is in the Nazi period, yes, so you can imagine that the boys who came to the Technische Hochschule were not well prepared.
Now you mention that your daughter was denied the part in this play...
I think Antigone but I'm not sure.
Well, my wife is Jewish and I am half.
Oh, I didn't know that. Did that complicate your situation as rector? After all, you were chosen as rector in '32.
No, it didn't. Officially I was a quarter Jewish. You see, my wife and my mother came from the same family, although they hardly knew one another, but it's the same family. And my grandfather, the father of my mother, was Jewish. I think he was baptized later on but he was born a Jew, and so I was a quarter Jewish.
By Nazi definition.
Yes, by Nazi definition.
I see. Now, under the laws, though, that was enough.
Yes, they wouldn't harm me—in the first period. Later on they would have. If we had stayed in Germany, I'm sure we would have ended in a gas chamber. [Slight pause.] But it was not really the anti- Jewish attitude that drove me out of Germany; it was more the fear, or the certainty, really, the expectation that Germany would get into a war with England and other countries and that if I were caught in Germany I would have to work for Hitler. And that idea was too much.
Well, what about science in Germany prior to the time you left? Was there any noticeable change in the pressure on physics and its relationship to the state, for example?
Well, of course, there was a strong party of physicists in Germany who were Nazis. Lenard and Stark were definitely Nazis. Lenard with his Deutsche Physik—I don't know whether you've ever seen that?
I've just finished going though it very carefully, different editions of it, too, as a matter of fact.
And you know that Lenard at the entrance of his institute had a notice that members of the German Physical Society were not admitted, because he had broken with the Deutsche Physikalische Gesellschaft because this Gesellschaft was not strongly national or Nazi enough.
But did this affect the teaching of physics at the University Was there any interference in the curriculum or ...
No, it would affect more the arts, history teaching, for instance
And biology, of course, as well.
Biology—maybe literature. As a matter of fact the last time went to a meeting of what we called Senate, the whole professional teaching staff of the Hochschule, I met the professor of literature, Pongs; we had been very friendly; I had been quite essential in getting him to Stuttgart. And I said, "Well, Herr Pongs, how are you? I hay seen you for a long time "Well", he said, " how should I be? Who course," he says, "we have to relearn our entire field. We have to look upon everything "unter dem voelkischen Gesichtspunkt."(under the racial or voelkische point of view). So I said, "I really pity you."
What about the students coming into physics—was there a decline? Well, first of all, you said that in general students were ill prepared. Did you find this at the higher levels in physics?
I couldn't tell you. You see I had no contact with the stu- dents. I lectured to them, but they didn't come to me and there was hardly any contact these last years. There had been before.
Did you have any Doktorands then?
Yes, I had Doktorands. I had Schmid and Bausch who finished in 1935.
But their work would be pretty much the same; they would work on specific problems.
That didn't ... You see, they were all not Nazis. None of the people with whom I was in contact was a Nazi.
Yes. Just a few more things I want to get to fill in this picture. What about colleagues in German science—did you know of many cases of people and of their difficulties in getting new positions or getting out, or anything going on in an organized way to assist them?
Yes. I think there was a large group of people helping those scientists who had to leave Germany to get positions somewhere else.
No, I mean within Germany. Was there any way that this could be...
Within Germany there was the possibility of going into industry, of leaving the academic positions and going into industry, which would have done no good in the long run. But a number of people did it. You see, the nasty thing was you had been friendly with your colleagues; I always was on very good terms with practically all of my colleagues and I knew practically all because I'd been rector so I had to have contacts with most of them. And then it became the official demand that you shouldn't say "Good day," you should say "Heil, Hitler." And on the street if you met them, you shouldn't take off your hat but you should lift your hand, heil Hitler. And you saw how these colleagues first ridiculed these things the first few weeks and then gradually they began making a very sloppy Hitler salute, and gradually it became more and more formal. And then when you met on the same side of the street or were about to meet, you saw that they crossed over to the other side and they avoided you, and it was really a shameful experience.
You must have felt increasing isolation then from the rest of the community.
Yes. Of course, there were exceptions. For instance, a new professor of electrical engineering was appointed to Stuttgart, a man called Feldkeller, and as far as I know Emde had told me that he had been one of the earliest party members of the Nazis. And after his appointment he came to pay me a visit in my institute, and I must say I found that ... I don't think I met him; I happened not to be in, but I thought very highly of this act which few others would have done.
During this time were there any scientific meetings that you attended or was this somewhat curtailed? You mentioned that you used to move pretty freely to Paris and to England and that there'd be scientific meetings that you'd attend here and there.
For these scientific meetings, of course it was now necessary to belong to a German delegation. And the delegation was headed by a Fuehrer, appointed by the Reichscultsminister, and they had to act unisono. And my only experience in that respect was that in 1936 I got my first invitation to come to the United States for the summer school in Ann Arbor. And just before I was about to leave, I got an invitation from Harvard to attend the Harvard Tercentenary meeting. Now you see I was in a queer position. Of course I wanted to go there but on the other hand I had to return to Germany. And so I wrote a letter to the Reichs- Kultminister, telling him I obtained this invitation and asking him for permission to attend the meeting. I thought that was more prudent to do. I didn't get an answer while in Germany, but many weeks after I arrived in Ann Arbor—it was actually toward the end of the term there end near the beginning of the Harvard Tercentenary—there came a letter from the Reichskultministerium not permitting me to attend the Harvard meeting. So when I went to the Harvard meeting I stayed a bit incognito. And if anyone had questioned me, from the German delegation, for instance, then " I would have said, "Well I'm visiting M.I.T."
Did you request permission to go to Ann Arbor or did you feel that that was not necessary?
Oh, yes, I had to ... Oh, that is quite amusing. You see, the year before, in 1935, I had an invitation to lecture in Madrid, and I'd gone there, lectured for one month or two months in Madrid and really hoped to get a job in Madrid, but it was very lucky I didn't because the revolution broke out quite soon, the civil war. And so I had to go to the rector in Stuttgart. He was the first Nazi rector-and the stupidest and dullest of the colleagues far and wide, but he had been nominated by the Reichskultminister to become rector. The faculty had told the Reichskultminister, "According to our statutes this is not possible. This man is not a full professor; he is an assistant professor." So he was made full professor the next day and became rector. Well, I came to him and told him I had an invitation to Ann Arbor and would like to go. So he looked very puzzled and important and finally he said, "Well, isn't it right that you've already been to Spain last year?" And I said, "Yes." Then he thought again for a long while. "Well, it might be even more disadvantageous for Germany to prevent you from going. You go."
That's a nice approach.
Isn't it nice.
Who invited you? Do you know how the Ann Arbor invitation came about? Ewald - 35
Oh, I don't know the details, but there was Laporte whom I knew quite well, and he may have had a hand in it.
Who paid you to get there? You, or did they provide funds?
I really don't remember. I got a salary there. I got paid in Ann Arbor.
You were one of the lecturers?
I was one of the lecturers, yes.
Before we get on to that, I want to get back, because that is a little story in itself, to the period in 1929 when you went to the Faraday conference in London. We also left something else out, too, and that is that one of your assistants was Hans Bethe, and I wanted to get your version of how that came about and what your impression was of him, if that was your first meeting.
Hans Bethe, well ... You see, Sommerfeld took his first trip to America in—oh, here my wife should come in again for what year it was. [in 1929]
Bethe's first trip to America?
Sommerfeld's first trip to America. And Sommerfeld had just hired Bethe as an assistant—or maybe Bethe had already been there for one semester—but anyway Bethe was free, and I had lost Fues who had received a call to Hannover, so I asked Sommerfeld if I could have Bethe. And that's how Bethe came to me. [Apart from a colloquium on his theory which he gave in Stuttgart on my invitation a year before.]
Had you heard of him by reputation?
Well, not only had I heard of him but you see his thesis is very very close to my work. This was really a translation of my theory of x-ray diffraction into the theory of electron diffraction. So from the point of view of interest we were very close. And besides, Hans Bethe's father was a great friend and formerly the assistant of an uncle of mine who was professor of Physiology in Strassburg.
His father was a physiologist.
Yes, his father was a physiologist and he was assistant and friend of Onkel Richard, my uncle Richard, so it was an old family relation, which we had cultivated; I did not know Bethe before he came, but we grew friendly very quickly. His father has written the nicest and most intimate obituary of my uncle. And his father was described to me, by the way, by Walter Gerlach who was professor in Frankfurt and therefore a colleague of Albrecht Bethe, as one of the most "geist- voll" —spirited—people he ever met. And Gerlach met a good many quite spirited people.
And so when Bethe came in '29, you put him to work on a related problem?
I put him to work on a related problem, yes, and the problem was really a very good one. It is so good—or, rather the paper which Hans made of it—that a few years ago it was translated and reprinted here in America, and this was the question I put to him: in atomic theory we deal with the influence of an electric field on an atom, which is called the Stark effect. Now we always assume the atom put into a homogeneous field. If an atom is in a crystal, it is not in a homogeneous field but the field has the symmetry of the particular position in the crystal and is quite anisotropic. What is the Stark effect? And instead of answering this question in a modest and straightforward way as I had expected, Bethe made a group theoretical study of it. And this is still today the foundation of the splitting of energy levels in crystals which chemists and spectroscopists and all use, so this was really a very beautiful paper. And Hans always says that this was one of the best times of his production, in Stuttgart, which I am very glad he appreciates.
He said that was a very happy time for him. That's when he first met your daughter, on that trip.
Yes, but with no great interest.
But he was close by later on, though, when he was in Tuebingen in '33.
yes, but I don't think they ever thought of falling in love or the like at that time.
It's hard for a father to say.
And it's not my business either. As a matter of fact, I think I tried quite hard to get him interested in looking up Rose when we were together in Ann Arbor in '36 and Rose was with the Courants in New Rochelle. And Hans went back to Europe, and I told him, "Won't you look up Rose?" but he had other interests at that time.
Well, then a little later, in '29, there was the Faraday conference in London, and it was there apparently that the idea of further international development in the field of crystallography got started.
Yes, I went to several Faraday conferences. I can't quite remember in what years. One was certainly in 1931 because that was the big centenary celebration of Faradays's discovery of electromagnetic induction. That was beautifully arranged by Professor William Bragg. And there was a conference following in Cambridge which was probably one of the "Unclear conferences on nuclear physics, which was misprinted as Unclear Physics" on the program.
The same year? Was that '31?
Yes, just a few days after. I think just the weekend separated the two conferences. Well, I should tell you systematically because that's a story in itself of my experiences with the international unions, but maybe we should leave it until later.
yes, that perhaps is a full story for next time. Do you want to go on any more today, or do you think this is a good time to break? I can tell you the other things that I had wanted to talk about, and we can decide whether it's better for next time. The reason that I asked about the Faraday conference is that it is the beginning of the story that leads to the international involvement. Then I wanted to ask about the 1934 conference in London, the conference on theoretical physics. You had a paper on the program, in the solid state section—there were two sections: one was nuclear physics, one was solid state—
That was '31.
No, there was one in '34.
'34? What paper is that?
I'll tell you ...
Is it the one on optical activity?
No, "Optical Activity" was 1930; that was the Faraday thing. Here, "Mosaic Texture of Rock Salt" with Renninger.
That was the 1934 meeting. I wanted to ask you about that and then to start with the trip to the U. S. in '36, talk about Ann Arbor, about Harvard, about other experiences and impressions of this country; and then from there to take you back to Germany and to the transition from there to Cambridge, and so forth; and then, of course,the rest of the story. So, we can stop now and then pick it up at that point. This little review will serve as a reminder to me, too.