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Oral History Transcript — Dr. John H. Van Vleck

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Interview with Dr. John H. Van Vleck
By Thomas S. Kuhn
At Harvard University
October 4, 1963

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John H. Van Vleck; October 4, 1963

ABSTRACT:This interview was conducted as part of the Archives for the History of Quantum Physics project, which includes tapes and transcripts of oral history interviews conducted with ca. 100 atomic and quantum physicists. Subjects discuss their family backgrounds, how they became interested in physics, their educations, people who influenced them, their careers including social influences on the conditions of research, and the state of atomic, nuclear, and quantum physics during the period in which they worked. Discussions of scientific matters relate to work that was done between approximately 1900 and 1930, with an emphasis on the discovery and interpretations of quantum mechanics in the 1920s. Also prominently mentioned are: Niels Henrik, David Bohr, Gregory Breit, Percy Williams Bridgman, David Mathias Dennison, Alex Ellet, Paul Darwin Foote, Ralph Fowler, Edward Lee Hill, Edwin Crawford Kemble, Earl H. Kennard, Hendrik Anthony Kramers, Ralph de Laer Kronig, Robert Sanderson Mulliken, J. Robert Oppenheimer, Erwin Schroedinger, John Clarke Slater, Edmund Clifton Stoner, John T. Tate, Webster; Conference on Magnetism (Washington), Harvard University, University of Michigan, University of Michigan Physical Optics Committee, and University of Wisconsin.

Transcript

Session I | Session II

Kuhn:

I should like to know about this whole question of the state of American science, particularly of American theoretical science; after all, you belong more or less to its first generation.

Van Vleck:

As you look back on them, they were a pretty undistinguished lot. I guess maybe it should be in the record that I was so pleased that there was a reference to Kemble in the first edition of Sommerfeldís that any American physicist should be mentioned was really something.

Kuhn:

When did you first get interested in science? Clearly in your own case, where your father was a mathematician, this is partly when did you get interested in this whole range of problems and how did the math versus the physics differentiate themselves?

Van Vleck:

I always said I was not going to be a college professor, but that was just a little of the cantankerousness of youth. I think heredity wins out. When I was an undergraduate at Wisconsin I really had no idea what I was going to concentrate in.

Kuhn:

Let me take you back further than your undergraduate years. How about school mathematics, school science, social science, and math around the home?

Van Vleck:

I donít think that I bad a great deal. I had a high school course in physics and in solid geometry. That was about it.

Kuhn:

Did it interest you particularly? Was there something you got excited about?

Van Vleck:

Perhaps moderately, but I was not emotional about it, I would say.

Kuhn:

Was there anything else you were particularly interested in high school?

Van Vleck:

I canít think of anything in particular.

Kuhn:

Did you do any extra math around the house, because of your fatherís role in it, or were you resisting that together with the idea of being a college professor?

Van Vleck:

When I was in Europe for a half year father tutored me in some course in mathematics which I had to pass off by examination. But that was just routine. I certainly never had anything that remotely could be construed as an accelerated progran in mathematics or physics in high school.

Kuhn:

It wasnít something that you sort of read on by yourself as sort of a hobby?

Van Vleck:

No!

Kuhn:

How about the extraordinary memory? That developed early, didnít it?

Van Vleck:

I never knew I had one.

Kuhn:

What about the time tables?

Van Vleck:

That was a special case.

Kuhn:

Was there anything except time tables?

Van Vleck:

I donít know.

Kuhn:

When you started in as an undergraduate, you still had no notion of what you were going to do?

Van Vleck:

My father did not ever put any pressure on me to go into science or mathematics. The only thing he insisted was that I should take mathematics through calculus, and have a course in astronomy sometime. I got around to calculus by my junior year. Even at that I got a late start because I had the Spanish influence in a student Naval Training Corps unit. I remember opening my first calculus book when I was recovering from that. Thatís my start in mathematics. I took my course in physics, I think, my freshman year. I did take a course in photography for which I wasnít very well prepared, but that I remember I wanted to do. I wasnít an especially good experimentalist.

I didn't have chemistry at the time so it was a hard course for me. I liked French literature but I never could get a decent pronunciation, so I was really in a quandary. I had one other thing I liked very much, geology. But you had to be able to draw well, be very good at roughing it, Professor [Warren Judson] Mead, when I got to the third course in geology, insisted that triangles be solved graphically. That really burned me up. That was the end of my thinking of concentrating in geology.

I liked physics, but then there was no theoretical physics, or very little. There was also a requirement of a senior thesis. That was very bad because that was an experimental thesis and Iíd have to take a course in glass blowing. That was really an appailing project. But there was a provision on the books of the university going back to the days when forensics was a very prominent undergraduate activity that if you were on the joint debate which was a big undergraduate intramural debate, you were excused from a thesis.

So I got on the Athena team, argued on the government ownership of railroads, didnít have to write a thesis, and majored in physics, with probably just about the minimum number of credits allowable for a physics major.

Kuhn:

When did you make that decision? Was it made by your junior year?

Van Vleck:

I guess about my junior year. It more or less took care of itself as time developed. junior year Prof. [L. R.] Ingersoll let me into a course in kinetic theory which I was illóprepared for, because I was taking integral calculus at the time. I sat next to a very bright advanced graduate student by the name of Warren Weaver who found so many mistakes in a textbook by [William Pingry] Boynton that was then used that the class had to be prolonged to a second term.

There was no examination, but I tried to look wise and I got an ďA". I think it was a gift by association, because I sat next to Warren Weaver who was the real star of the course. By the time I was a senior I took a course in dynamics at about the Whittaker level given by Professor [H. W.] March, for which I was by that time moderately well-prepared. That was my first real introduction to theoretical physics that I was capable of understanding. I was not sufficiently advanced that I could take any course with either [C. E.] Mendenhall or [Max] Mason while I was still an undergraduate.

I graduated in 3 1/2 years because father was a visiting professor at Harvard in the spring of 1920, and my family was anxious for me to go there with them so I graduated in 3 1/2 years. I came to the Harvard Graduate School, took three courses in physics, and one in the Business School on railway operations with Professor [Wiiliam James] Cunningham. That convinced me that I would never get along very fast in railroading, and I liked the Harvard Physics Department, so I just stayed on here and took my Ph.D. in 1922.

Kuhn:

Let me push you back a little bit. You clearly started awfully late. I take it you had a sort of survey course in your first year?

Van Vleck:

I had a course with Professor [John Ransom] Roebuck, the more advanced of the two freshman courses, but that was elementary by modern standards. We had Henry Crew's book as a textbook. [Break, because of technical difficulties. The tape failed to record part of the interview, whose more important points are repeated in the following section.]

Van Vleck:

At Wisconsin I had the more advanced first year course, but not advanced by modern standards, I had a quarter each of electricity, optics, and heat. I had a course in analytical dynamics, and the one in kinetic theory I already talked about. I had a year and a half of calculus, took trigonometry as a freshman, analytical geometry my sophomore year.

Kuhn:

I think you mentioned that you had not taken differential equations?

Van Vleck:

Thatís right. Since my father gave it, I didnít think it was quite cricket to take it from him. I think I was wrong and was grieviously punished because I had at Harvard this course from [C. L.] Bouton which was not a standard course in differential equations, no power series at all. Instead, it was all sorts of trick algebraic manipulations With much written work: I never sweated so hard on a course. At Wisconsin I found the mathematics courses enjoyable and easy. They required little written work, and I took them whenever I wanted something which was not too time-consuming, such as the period I was on the debating team.

Kuhn:

You had earlier said about how you got to Harvard in the first place. I want to check my impression that at the time you came here first it was simply because the family was moving here, is that right? Was it clear that you wanted to go on in physics?

Van Vleck:

No, I wanted to graduate with my class in June, but my parents persuaded me to hurry up a little, and have a half year at Harvard, which was undoubtedly wise.

Kuhn:

Was there any other place you would have considered for graduate school at that time?

Van Vleck:

I never thought about it. This thing sort of moved everything up half a year. That meant I was here, and I liked the courses when I was here, and the professors. The emphasis was much more on theory I should say, than most of the courses I had at Wisconsin, and that, of course, appealed to me.

Kuhn:

You did say something of having thought of Berkeley at some point as a place to go?

Van Vleck:

Thatís because they had here on the bulletin board advertisements of the teaching assistantships there, and the idea of being a little independent appealed to me, and the West Coast sounded good, but it was just the exuberance of youth, nothing more rational.

Kuhn:

If youíd had a more leisurely course and had determined to take on physics earlier at Wisconsin, what else could you have included that you didnít in a normal undergraduate curriculum?

Van Vleck:

Well, there was this course in differential equations that I mentioned. I suppose I could have gotten a course or two under Mendenhall and Mason my senior year. I donít think Iíd have been up, though, to Masonís equivalent to Bridgman's course the senior year. I doubt if I could have gotten that far.

Kuhn:

That was the thermodynamics or the electricity course?

Van Vleck:

The electricity course.

Kuhn:

What would Mendenhall have given at that level?

Van Vleck:

Something on the photoelectric effect probably. That was his specialty.

Kuhn:

That would have already been a possible choice in the undergraduate curriculum?

Van Vleck:

It probably was a graduate course, but undergraduates can take graduate courses, just like the Harvard seniors; some of them have essentially the equivalent of an M.A. when they graduate, some people have an A.B. The reason I concentrated in physics was because it had a light concentration requirement. I didnít want to specialize heavily, and I still think most of our undergraduates at Harvard specialize far too heavily today. If a person is ever going to get a liberal education, itís during his undergraduate years.

Kuhn:

With apologies, Iíll bring you back to Harvard, and ask you again to reconstruct the curriculum, the teachers, and where possible also the books that you took, starting from the spring of 1920. You talked before about the individual influence of certain of these men, and Iíd like very much again to have that on the record.

Van Vleck:

I think that Pierce had as his book, Electric Oscillations and Electric Waves, and his lectures were pretty much a replay of the book, I should say Harvey Davis did not have a text. He gave some very fine lectures in thermodynamics. I felt that this was a very fine course. My third course was this Bouton course, the fourth one in railway operation. That was my ďlame-duckĒ half year before I started in the fall. Then I took Bridgmanís course in electricity, Kembleís course in quantum theory, Osgoodís course in mechanics, and I donít remember what the fourth one was.

Kuhn:

A mathematics course perhaps?

Van Vleck:

I think my only mathematics courses were the ones in physical subjects here. Except the second term I took a course from Bill [W. C.] Graustein. because Iíd had the first semester of function theory in the summer of 1920 in Chicago. In February of 1921 then I took the second half of Math. 13.

Kuhn:

That was functions of complex variable?

Van Vleck:

Yes. I may have been taking Bridgmanís course in elasticity that year too. I may have been taking even two courses with Bridgman that fall. I know I had his course in elasticity.

Kuhn:

When we talked before I think you suggested that Bridgman either had used, or that in this connection you had almost certainly read what?

Van Vleck:

Jeans and Abraham were the texts. Bridgmanís two year course in electricity, and Loveís Elasticity, Lambís Hydrodynamics, were not texts, but were the collateral reading things for Bridgmanís course in elasticity. The more I think of it, the more I think that I took that in the year 1920ó21 because in 1921ó22 I did not take a full course program at Harvard. in order to finish up my thesis.

Kuhn:

Do you want to say something about Bridgman as a teacher and as an influence on the development of physics here at that point?

Van Vleck:

I think Bridgman was a very fine teacher for the better students. I think that his operational philosophy has probably subconsciously influenced my outlook on theoretical physics; itís very hard to say definitely. I never had a course on the philosophy of physics With him but I donít think you can help but be influenced by the fact that he laid emphasis on agreement with experiment and that physics bad meaning only in so far as it could be interpreted in terms of experiment.

Kuhn:

That was always pretty explicit in his teaching, was it?

Van Vleck:

It was pretty implicit.

Kuhn:

Certainly electromagnetic theory leaves points at which that sort of attitude can come out quite clearly. I asked you particularly about Ted Kembleís course.

Van Vleck:

Yes. A very fine course. So were Bridgmanís.

Kuhn:

Can you tell me something about what was in Tedís course? I hope weíre going to come up with his notes on it, but

Van Vleck:

I think of it more or less as Sommerfeldís Atombau und Spektrallinien ó something at about that level, essentially, with some emphasis on the newer developments and so on. I think that book had come out by that time. When was the first edition?

Kuhn:

1919.

Van Vleck:

That was available; but it was kind of hard for me to read it in German, When did Bohrís Danish Academy papers come out?

Kuhn:

1918.

Van Vleck:

I think they were referred to in the course. When did Kramersí come out?

Kuhn:

I Ďm not quite certain, but early enough, I believe, so that it could have been referred to there. Since you mentioned that, one of the things I very much want in the record is your story about looking for a copy of that.

Van Vleck:

It was either Kramers, or Bohrís Danish Academy papers, I think probably Kramers. They were not in the Harvard Library. Whether there's been some irregularity in the delivery, whether they hadnít been ordered, or were at the bindery, I donít know. I do know the only way I was able to get them was to go to the Library of the American Academy. They had it by exchange, or got it more promptly than Harvard probably for that reason. I remember borrowing them and having to cut the pages, which, I think, was a little indicative of the interest in major papers in theoretical physics among physicists of that era, although the Boston area was one of the areas of highest talent undoubtedly in physics at that time.

Kuhn:

Would the Academy Library have been much used by physicists in the area, or would they expect to use their own university libraries?

Van Vleck:

I think normally they would use their own universityís. It was just a suggestion as the only place it might be.

Kuhn:

As you were studying here, particularly in Kembleí s course—apparently this course made a considerable impressionóthis was from the start the subject you wanted to go on with?

Van Vleck:

Yes, pretty much from the start. Probably a period of two or three months or maybe a term before; Iíd say I made up my mind the second semester of that year. 1920ó21, my whole thesis materialized much faster than Iíd ever figured on.

Kuhn:

Do you have any residual impression of what it is that excited you so much about this material? Or werenít you terribly excited? Was it just the thing you decided to do?

Van Vleck:

Well, I liked the combination of mathematics and physics. I was not an experimental physicist, and I was not an abstract mathematician by temperament, so this was it.

Kuhn:

How clear was it in this period that there were more than problems to be solved that there was a whole new way of doing physics to be developed?

Van Vleck:

I donít think I thought too much about it. I was always a good deal of an opportunist and a pragmatist in my approach to physics, and I guess that Ďs been true through the years.

Kuhn:

Am I right in thinking from the earlier conversation that there were really only three of you here at that stage of the game concerned with theseóyou, Kemble, and Slater?

Van Vleck:

I would say that was substantially right.

Kuhn:

You said you and Slater lived near each other?

Van Vleck:

We both lived in Conant HaIl. I think he lived across the hall.

Kuhn:

You talked a good deal about problems in this area?

Van Vleck:

Yes.

Kuhn:

Do you remember any things that were particularly subjects for conversations between you or particularly puzzled you?

Van Vleck:

Some of these Danish Academy papers. There was a third number of Bohrís papers in the Danish Academy, that came up quite a bit later than the others. I remember Slater remarking to me that he didnít think that that bad developed the subject very much further.

Kuhn:

In addition to reading Sommerfeld, and presumably about this period reading the Bohr papers, and probably also the Kramers paper, were there other things on quantum theory- that you read at this time, or did you begin systematically to follow the journals on this?

Van Vleck:

I remember I had to write a course thesis for Bridgman on magnetism. That may have started my whole interest in the subject, which I certainly should have mentioned earlier. I gave a colloquium report on, I think, the Weiss theory and Hondaís theory. Looking back on it, Hondaís theory was a rather incorrect application of quantum theory, but Weiss, of course, was good phenomenological theory. My recollections may be wrong.

Kuhn:

Iíve got very little notion whether people took it as a sort of official professional responsibility to read the journals with regularity or not.

Van Vleck:

I donít know. I think we just wanted to pass off our courses and get our degree. I did do a lot of reading, I remember reading PauliĎs attempts at a theory of the magnetism of NO while I was at Harvard. Probably the year I was here as annual instructor, for a year after I took my Ph.D.

Kuhn:

But you wouldn't make a point of at least making sure what was in every issue of the Zeitschrift, for example?

Van Vleck:

I certainly did when I got to the University of Minnesota, when I was a little bit more mature and had more time. My aunt gave me some money for subscription of Zeitschrift fuer Physik. Life was simple in those days, because if you wanted to be up on theoretical atomic physics, you just had to read each issue as it came out. Now Iím lucky if I read even the table of contents of the different journals.

Kuhn:

You talked a bit before, and I think youíd want to put it in the record again, a little bit about Kembleís quite special influence here.

Van Vleck:

Kemble was almost unique in being one of the very few people in the country who understood quantum theory and the spirit of it, and the mathematics of it. I suppose Epstein at Cal. Tech, undoubtedly did, but I didnít know about him. I never thought of that as a place of graduate study. He was about the only other person you could single out. There were some people of predominantly mathematical interest who understood the formalism of quantum theory. [Edwin P.] Adams I suppose, at Princeton; Phillips of the M.I.T. math dept. gave a paper at Toronto in 1920, Theoretical physicists as distinct from theoretical mathematicians were rare creatures.

Kuhn:

What do you think it is that really changed that situation there?

Van Vleck:

I suppose mainly the fact that quantum mechanics worked, and red people who were theoretical physicists rather than purely formal mathematicians.

Kuhn:

Kemble said to me at one point that even with respect to the formal mathematicians—I suppose he was thinking of the ones who taught in the physics department rather than in the math departmentsóthat the theoretical physics courses tended to be handed over to somebody as a chore, very often to somebody who wasn Ďt doing much research. The research activity was experimental, and there were few bread-and-butter courses in theoretical subjects. Does that check with your own impressions?

Van Vleck:

Was this in the mathematics department?

Kuhn:

No, this would be the subjects in theoretical physics, at least except to the extent that they were handled in the mathematics department. They were often handled by a figure who had not turned out to be much good in. experimental work, who worked these subjects and gave them up, but who was really not much of a theoretician in terms of any notion of doing research in that area.

Van Vleck:

I think it was true that we bad very few theoretical physicists so the experimental physicists bad to give the courses, but I donít know of much evidence that the theoretical courses were intentionally relegated to the second-class experimentalists. Certainly courses in theoretical subjects were given by Michelson and Millikan at Chicago.

Kuhn:

when you were here, there were a number of things going on both here and elsewhere, but particularly elsewhere, in quantum theory. I take it from what you said before that you really have no recollections in particular of the Stern and Gerlach-ó?

Van Vleck:

I donít really remember when that broke. I have vague recollections but my memory gets fuzzy. It was an interesting experiment, I have no doubt it was reported on at a colloquium at Harvard.

Kuhn:

How about the Compton effect?

Van Vleck:

Of course, because of Harvard loyalty, I wanted to see Duane right. But I remember one time Slater coming from some sort of a meeting somewhere and saying ďWell, the Compton effect is so.Ē I remember his precise words.

Kuhn:

Was that hard to take? Do you remember, in your course with Kemble, was the photon taken at all seriously?

Van Vleck:

I donít recall. It certainly was mentioned as a possibility. I donít remember any particular plea for or against it in the course. It was a very good course, and, I think, quite objective. I think he had good judgment in selecting the proper modern material. I had a course under Millikan in Chicago in 1920 where he had various students report on different theories of the atom. In retrospect, Iím sure that some of them were pretty primitive, whereas I was given the job of reporting, I believe, on the Sommerfeld paper on fine structure. I surely didnĎt understand it too well when I reported on it, I can assure you of that—because I didnít read German well.

Kuhn:

How closely did people here follow the really detailed developments that were by now going on? You refer to action and angle variables from a fairly early point. There was a lot of very detailed workóóthe fine structure work, although that Ďs older, would be a part of it, the multiply periodic systems, which you clearly know very well later, but which was making quite a lot of progress in this period. Would that have been part of the sort of thing you were learning yet, or did that wait until—?

Van Vleck:

I donít think that was very intensively covered in Kembleís course, as I remember. It was commented uponó-the question of the separation of variables and so on. Iím sure action and angle variables were mentiŗned. It was not a drill course in that. I think youíve got to remember that the caliber of the graduate students at that time was not an especially highópowered one, so the tempo of the courses was probably correspondingly influenced.

Kuhn:

What about the correspondence principle as a sort of guiding principle to use in trying to work, out quantum mechanical problems?

Van Vleck:

I always loved Sommerfeldís statement in Atombau: that the correspondence principle is a magic one which enables one to borrow the results of classical theory for quantum theory.

Kuhn:

Iím not sure where he says that. As you know, thereís a good deal of opposition between the Bohr school and the Sommerfeld school. Sommerfeld was very skeptical about the correspondence principle for some time.

Van Vleck:

I think that is a quotation from Sommerfeld.

Kuhn:

It may very well be. If itís a quotation from the first edition of Sommerfeld, which I havenít been through, itís a negative quotation. This may be magic, but it is magic. In general Sommerfeld tried for some time to hold this off. He treated this very much as solved. He tried with some consistency to go just as far as he could with the Rubinowicz approach to selection rules in particular. Then heíll say, yes, Bohr is getting results by another way, but I want to try this way.

Van Vleck:

Of course, you remember that when he got the helium atom he quoted Faust in one of the editionsó-íHere Iím stuck, and canít go furtherí.

Kuhn:

Would you think that Sommerfeld had been a considerably more important influence than Bohr in this period? There was an opposition between these two approaches.

Van Vleck:

I never really thought about that. I wouldnít know.

Kuhn:

Let Ďs come now to the helium atom paper. You told me before, and I wish youíd say again, in as much detail as you can, how youíd gotten involved with that problem.

Van Vleck:

Kemble came up with this crossed orbit model, because coóplanar orbits didnít seem to work, although thatís really what Ďs indicated by the diamagnetic behavior of helium if youĎre going to use any kind of old fashioned mechanics. He suggested that I calculate its energy for my doctorís thesis. I tried to do it to the best of my ability. I had to learn some of these astronomical techniques and so on.

That was a problem that Kramers, and Kronig also, independently investigated. We all got substantially got the same value of the ionization potential. As I said before, Kramers, when be published his work, which was a little after my own, criticized it as being only equivalent to his first order calculation.

I'm sure that criticism is erroneous because he did not realize that I was starting from a different system of orbits, as a starting point of the perturbation calculation than the one that he assumed that I had. I canít discuss that without getting technical if you want to know the details, my reply to him is found in a footnote in my book on Quantum Principles and Line Spectra [National Research Council Bulletin No. 5, Washington, D.C., 1926] as I remember.

Kuhn:

Iande had also done a crossed orbit atom model in 1919, if my dates are right. He tries them both.

Van Vleck:

Did he do it right though?

Kuhn:

No, he did not do it right.

Van Vleck:

One of them, Iím sure, he did wrong, I thought that was a planar one. I wrote a little paper for the Physical Society pointing out that power series of development in a parameter was of a different form than he assumed.

Kuhn:

I know that little note of yours to the Physical Society and I was curious about it because his model is a crossed orbit model. On the other hand, the interior orbit is very much smaller. Itís got orbits of drastically different size. In your own helium paper you really seem to take it almost as given—or you point out that there still be room for doubt here—that these orbits have to have substantially the same radius.

Van Vleck:

That was the rules of the game that I was supposed to work with.

Kuhn:

You didnít necessarily hesitate to quarrel with rules of the game. You lean very heavily on symmetry considerations. One of the things thatís striking to me about this paper is that as compared with European work in the same period youíre paying a good deal more attention to things like Langmuirís model, to chemical considerations and you donít think thereís going to be a valence electron in helium because of its low reactivity. This seems to play a larger role in your thinking about it, so that you really pretty well exclude models in which thereíll be an interior and an exterior electron.

Van Vleck:

I was probably pretty much influenced by Kembleís earlier paper and this was just the attempt to evaluate the ionization potential of the Kemble model.

Kuhn:

But you did more than that. For example, youíve got a section in that in which you discuss ways out of the dilemmaó-other force laws, Langnmirís suggestion of statical atom with the dipoles, which would, explain various effects.

Van Vleck:

Iíd probably blush if I read those paragraphs now.

Kuhn:

. . . Variations of the quantum condition. Hereís where I have my greatest difficulty with physicists. Youíre not alone in this. The attitudes toward this sort of thing for us are terribly important. Youíre trying to remember what in retrospect, and by retrospect your standard seems to be the solid part thatís left because it was the computation. You were clearly more deeply engaged than that with the whole problem.

Van Vleck:

Yes, I was. I wanted to know how in the world we explained that ionization potential of helium. Of course, I did. All you could do was speculate. It wasnít theoretical physics, it was speculation physics.

Kuhn:

But donít you suppose speculation physics played a major role in some of these developments?

Van Vleck:

In still earlier stages, but I have a feeling that by the time thatóó. Heisenberg refined the correspondence principle; that was more in the speculation physics.

Kuhn:

How did you feel about Sister? Sister in this period was obviously considerably more speculatively-minded than you were.

Van Vleck:

Iíd never thought of that. May be so. It was after he went to Denmark.

Kuhn:

Excuse me. Iím not sure where that starts, but clearly the origins with him of the Bohr-Kramers-Slater paper were older than his arrival in Denmark. At least he had talked to [R. H.] Fowler at Cambridge before he got to Denmark. I would suppose that turn of mind must have showed already when he was here.

Van Vleck:

Well, he was certainly very physical in his approach in general. He wrote an experimental thesis with Bridgman. Probably it was a little more speculative and imaginative than I was, it may well be.

Kuhn:

Did this ever lead the two of you into arguments or discussions?

Van Vleck:

No. I remember our both agreeing that certain papers that got into the Zeitschrift, which had no refereeing, were a bunch of rubbish. I remember certain ones we dismissed.

Kuhn:

Do you remember any of those in particular?

Van Vleck:

I can find it. But itís not by a known physicist. I remember I found some errors and turned it over to Sister, and said it didnít look right. He looked too and found some more. Itís rather unbelievable how the Zeitschrift fuer Physik would really hit the peak in the finest papers, but I think that the depths were pretty low. You could pick out a paper or two that should never have gotten printed.

Kuhn:

What sort of reactions were there to the helium atom?

Van Vleck:

Well, I was disappointed, of course. I was afraid that it would knock out my doctorís thesis but, of course, the department took a much more rational view than that.

Kuhn:

That what would knock your doctorís thesis?

Van Vleck:

The fact that it didnít agree with the experiment.

Kuhn:

There was no question in your mind that the discrepancies were larger than could be accounted for by either the approximations or the accuracy of the experiment?

Van Vleck:

I think thatís correct. I donít think many physicists in the U.S. were very interested or emotional about things like that. Oh, I think theyíd be interested if this crossed orbit model of helium doesnít work and so on, but to get people really emotional about that, youíve got to have a school of theoretical physicists such as there wasnít in the U.S. at that time. There were isolated theoretical physicists in various places. There was one other man, of course, at Harvard I should have included, although my contacts with him were limited because they took place much more when I was in Minnesota, That was Gregory Breit. I owe a great deal to him, But I think most of my discussion of physics.

Van Vleck:

(cont.) with him was in the Minnesota period rather than the Harvard period. I think heíd been at Leiden and then was here the year I was an instructor and probably pretty well tied-up with teaching freshmen and sophomores.

Kuhn:

After you published that paper as a paper in a European journal?

Van Vleck:

Yes. Well, PHILOSOPHICAL MAGAZINE was in the top echelons. I was terribly pleased to get it published there. PHYSICAL REVIEW was considered sort of (mine) run experimental papers. It didnít have the stature of the Phil. Iíd say that the Phil. stood in the same relation to the Review then that the Review does to the Phil. now.

Kuhn:

What determined whether a paper as to be sent to the PHIL. MAG. or the PHYSICAL REVIEW, or—?

Van Vleck:

Naturally, Kemble and I wanted it published in the best possible place, and that was the I guess the chances of it being published were greater if it was communicated by a man of high standing, and, of course, Lyman was the outstanding physicist at Harvard at that time. Bridgman was a younger man who had not yet reached the peak of his reputation, although he was at the Solvay Conference as early as 1931. Still Lyman was older, and more of a figure—the Iyman series and so onóand a little closer to spectroscopy.

Kuhn:

The question of the lack of any discussion really, except for the note thatís added in proof, of any reference to spectroscopic considerations in the helium paper interests me and itĎs also rather true of KembleĎs paper. I think I told you before that he expressed himself now as having been greatly embarrassed when Saunders came to him and said, ďYou should have talked to me about that paper. Series identification prevents that absolutely from being the ground state.Ē

Van Vleck:

That I still donít understand because that belongs to the same family of states as well, I say, the excited singlet states then called the ĎParí states. Now how did one know that one was crossed and the other was uncrossed? I donít think that we knew that much.

Kuhn:

Iíd have to know Kembleís paper better. I glanced at itóó.

Van Vleck:

It seems to me that our whole understanding of the helium atom was so fuzzy that to try to exclude any model of the ground state on the basis of the combinatory principles with the excited states, which we probably understood even more poorly, would not be quite appropriate.

Kuhn:

Just one other question on that paper in particular. In Part II you work up your perturbation techniques with really no literature references except generally to astronomical techniques in this area. Were you really working those up for yourself from scratch?

Van Vleck:

Yes.

Kuhn:

There was some European perburbation literature of this kind?

Van Vleck:

Yes. Well, I read the papers on action and angle variables but I didnít quite see how I could make the calculations work for this helium system and this seemed to me the way I figured out of doing it. Now why I didnít use action and angle variables I donít know.

Kuhn:

Do you remember what astronomy books you used as background for this? You do record this as a standard— . You refer to two sorts of standard astronomical techniques of which you pick one and proceed to go ahead with it.

Van Vleck:

It seems to me I dug a little into charlierís Mechanik des Himmels or something like that, and I did a little browsing in the libraries to see if I could find something that would be just an answer to my prayers. There was no such thing; but this was a sort of a variant of some of the techniques of the astronomers that seemed to work.

Kuhn:

Did the publication of the paper in the Phil. Mag. bring you any letters from Europeans? You talk about why there was as little reaction as there was here. But in Europe that was an important problem.

Van Vleck:

I got a letter from Kramers, I remember his saying that he applied the quantum theory or principlesó-quantum conditionsóostensibly in a different way, but saying that any rational interpretation of them would also agree with my formulation of the quantum condition. I was a little worried whether Iíd done them right because the system was nonseparable. I remember that I was pleased that Kramers said that he thought that my system of quantization was all right.

Kuhn:

When Slater finished his thesis, he immediately went off to Europe for a year. Did you have a chance to do anything of that sort?

Van Vleck:

At one time Slater and I were talking about both going to Copenhagen. I was going to apply for a National Research Council fellowship or something. But then this offer from the University of Minnesota came, which was quite unusual for those days because it consisted of exclusively graduate teaching. At that time Swann had left Minnesota for Yale, taking with him Ernie Lawrence, his prize graduate student.

They decided they would take on two young men in the capacity more or less of theoretical physicists. Of course Swami did some experimental work, but I think youíd say that Breit and I were dominantly theoretical, especially me. I think Breit did do some experimental work. The two of us went out there in the fall of 1923. I remember people saying to me at dinner parties, not knowing quite what we were doing. Wasnít it too bad that Minnesota lost Swann; it took two men to replace him?

Kuhn:

Could you at that point have stayed here, do you know?

Van Vleck:

I could have continued as annual instructor, but there was not room for both Slater and me, and Slater was given a faculty instructorship after be came back from Copenhagen.

Kuhn:

This tradition of Americans spending a year in Europe which so far as I can see starts about with Slater, and then there gets to be from that point on an increasing number of people; I donít know if be is the first oneó?

Van Vleck:

Urey and Hoyt were over about that time, werenít they?

Kuhn:

Yes. Hoyt may actually have been over a year before Slater. I rather suspect he was. I wondered a little at your not having participated in that. You told me now why you didnít. I wonder how badly you felt about it? Did you have the sense of a lost opportunity?

Van Vleck:

No. It would be very wearing to do research and nothing else, but I didnít mind the three-ring circus of academic life too much.

Kuhn:

To what extent did Americans, American theorists in particular, have some sense of backwardness and that a year in Europe might enable them to pick things up at the source which were notóó.

Van Vleck:

This was our feeling, certainly. I thought I was going to get to Europe a year earlier than I did. I had a sabbatical coming to at Minnesota, but that was delayed because I went to the University of Wisconsin. I had a half year off instead by teaching in summer schools.

Kuhn:

Before you actually went to Minnesota, you brought out several other papers, in particular the one you did with Kemble on ďThe Theory of the Temperature Variation of the Specific Heat of HydrogenĒ.

Van Vleck:

That I did with Kemble. I also tried doing some experimental work right after I got my Ph.D., but that was of very short duration because I had tonsillitis, my tonsils were misbehaving, and this was a dark damp room without any windows, so I got discouraged with that. It wasnít the proper thing for me to try at that particular time quite irrespective of the fact that Iím surely better as a theorist than as an experimentalist.

Kuhn:

Was this experimental work in conjunction with this paper?

Van Vleck:

Some kind of infra-red business; whether that was in connection with hydrogen or something else in the band spectra, I donít know.

Kuhn:

How did you feel at that point about the state of the specific heat problem for hydrogen? Was this a case in which agreement seemed quite adequate, and you were relatively well satisfied that this was the right answer to the problem?

Van Vleck:

I felt I was better off on that than I was on the helium business. It seems to me Epstein or somebody said. They thought it was a contribution to the development of the subject which I'm afraid subsequent events have shown was not the case. Still it pleased me at the time.

Kuhn:

I cannot get most physicists to believe that something can have been a contribution if later work under a different theory shows that the phenomena is to be explained differently.

Van Vleck:

I know what you mean. In that sense, of course, it probably helped.

Kuhn:

If you carry it out just a step further you find yourself saying that the Bohr atom was no contribution

Van Vleck:

I donít think thatís the case. That at least got some right answers. These other things never quite did.

Kuhn:

The specific heat thing got some right answers.

Van Vleck:

Not too far off. Never too good an agreement. Dennison got better agreement.

Kuhn:

Youíve said a word at least on the subject of this paper in which you criticized Lande. I wonder really that you went back to that paper and tried to do the computations over again.

Van Vleck:

Had I gone back? I thought that the paper had come out about that time, but I may have gone back, I don Ďt know.

Kuhn:

Well, the Lande paper is 1920 and your paper is 1923, so youíre going back at least a little.

Van Vleck:

I was trying to scrape around and find something that would make that miserable helium atom come out right.

Kuhn:

You think you did go on after your thesis still trying to find some way to do the helium atom?

Van Vleck:

Yes. Not intensively, but all the time in the back of my head I would wonder bow these things were ever going to be cracked.

Kuhn:

Thereís also a little note in the PHYSICAL REVIEW, two notes on quantum conditions. How did you get into that? That Ďs a curious sort of paper.

Van Vleck:

Actually, itís that conclusion. I read that paper by Trkal and I noticed that you could interpret that variational principle of his a little bit more broadly. Iíd forgotten even what the second note is.

Kuhn:

That actually is the second one. The first one is the criterion for the choice of a set, pi, qi, to give the correct energy levels under the Sommerfeld conditions.

Van Vleck:

That Iíd forgotten. I remember the Trkal part, which is very formal and is an example of the tendency of young men to develop the formal things. But, of course, sometimes out of those speculations something does come out. Nothing came out of that.

Kuhn:

Do you think of that as a tendency of young men?

Van Vleck:

Yes.

Kuhn:

Can you think of other people who did that sort of thing in their youth? This is an interesting idea to me and you may be exactly right; it simply had not occurred to me that one would look at this—that young. men tend to formalism.

Van Vleck:

Well, they like mathematics, and later on they get impressed on them the hard reality that something must agree with experiment. People donít always go that way. Sometimes people go the other way. Certainly Einstein was much more of a formalist in his later years than in his younger years. I agree that you should take that statement of mine with a grain of salt. Itís true of my own work, anyway.

Kuhn:

Are there other people you can think of in that connection, who having started relatively formalistic, and then became more physical?

Van Vleck:

I donít know. There have been people who have shifted fields like Carl Eckart who vent into hydrodynamics where heís probably more closely tied up with experiment. It was kind of hard to find material for papers in those days because we didnít have any real quantum mechanics, we didnít know the rules of the game, so about all anybody could do was flounder if they wanted to write a paper and be a theoretical physicist. I suppose thatís one reason why theoretical physics was perhaps not in a little higher repute than it was in those days, at least quantum theoretical physics. In certain quarters it certainly was not in repute.

Kuhn:

Youíve told me something about the transfer to Minnesota. Iíd be very much interested to know in what ways you found the department there different than the Harvard department. How would you compare the state of physics at the two institutions?

Van Vleck:

Breit was one of the young Turks, one of the few people really up with these developments in the quantum theory. [John T.] Jack Tate bad worked with Franek or somebody for his Ph.D.óómay have been Hertz. He was a little comparable to Mendenball at Madison. He was in the erperi mental aspects of atomic physics.

Kuhn:

He was in any case thoroughly aware of the quantum theory at this point?

Van Vleck:

Surely. And I think he was sympathetic. He was certainly not anta- gonistic to it at all. The person I had most contact with was Breit. I donít think that anybody at Minnesota properly appreciated Valasekís discovery of ferro-electricity a year or two earlier, but thatís something else again. Thatís perhaps a little irrelevant, but I think his work and discovery has been over the years rather undervalued. He was a young assistant professor when I came there. He had certainly made a very important experimental discovery, the importance of which was not realized until many years later. [L. F] Miller, [A] Zeleny, and [H. A.] Erikson, the chairman of the department, were older men, who, I would say, were primarily teachers.

Kuhn:

It was a fairly big department for those days, was it not?

Van Vleck:

Minnesota was a large school. Probably some other people Iíve forgotten. but those are the ones I think of at the moment.

Kuhn:

Really their invitation to you and Breit together bad been a quite deliberate attempt to keep up and extend work in contemporary theoretical physics.

Van Vleck:

Thatís right.

Kuhn:

At least there was definitely no opposition there except perhaps among a few individuals.

Van Vleck:

There was no opposition. It was a quite unheardóofóexperiment at the time for a state university to start hiring two men for purely graduate instruction. Nowadays, of course, itís nothing unusual, but for those days it was. I owe a great deal to Minnesota for the time for my development. It was stimulating to give graduate lectures, quite different from doing all elementary teaching.

Kuhn:

Would you have said that this was in many ways a stronger department than the Harvard department?

Van Vleck:

No. They had no Nobel prize winner like Bridgman. Lyman was certainly a man of great standing. Kemble Iíve already discussed. Duane infringed on the medical school but was certainly a leading authority on X-rays. Certainly the echelon of the Harvard department was far greater than the Minnesota one, no question about it.

Kuhn:

Was there any stimulus from that here that you missed when you got out there?

Van Vleck:

Well, I certainly missed Kemble ama Slater. I had Breit. I would say also that I perhaps missed a little bit the fact that there were older people of high distinction.

Kuhn:

Was there a regular colloquium out there as I take it there was here?

Van Vleck:

Surely.

Kuhn:

Was it important? Did the colloquium have the same sort of role at Harvard and at Minnesota that it often had in European universities? I think the Munich colloquium, the Goettingen colloquium were really terribly important vehicles of keeping people abreast.

Van Vleck:

I donít think the American colloquium went quite as far, because they tried to cover the waterfront more generally, including more routine experimental physics. I think that was to a certain extent true at Harvard and probably a little bit more at Minnesota. Each professor tended to report a little bit on his own work.

Kuhn:

Was there any attempt to bring in people from outside?

Van Vleck:

Surely. We had Schroedinger up for a lecture, for instance. I used to go down to Iowa to lecture for G. W. Stewart about every year there for a while, We had Kramers up for a lecture. Surely, we had colloquium lectures. Whether they were colloquium lectures or visiting lectures I couldnít say, butóó.

Kuhn:

Again in the earlier years that you were at Minnesota, staying away still from the new quantum theory, there are a number of things that go on that may have led. to a good deal of discussion there. Weíve said

Kuhn:

(cont.) something about the Compton effect which presumably by the end of that argument you were already at Minnesota. What about the de Broglie thesis, which came out while you were there? Do you think anybodyóó?

Van Vleck:

I certainly never read it, never heard about it, I don Ďt believe, until I heard of the Schroedinger things. You see, I was pretty sold on the matrix approach, I had seen Max Born, who had been a visiting lecturer at Wisconsin. I was much more en rapport with the Goettingen-Copenhagen group, I should say, than the Zuerich. What few contacts I had. One thing I should mention, that in the summer of 1923 my father and mother went to Europe. I went with them, but left them for a while and spent some time in Copenhagen. There I met Niels Bohr, I believe, for the first time. I was a house guest of his several times later, but this was my first acquaintance with him. He suggested that I should get in touch with Kramers. I remember I went to Apeldoorn where he [Bohr] said Kramers was with his aunt. I got there and discovered that he was somewhere in the dunes of northern Holland, north of (Aalsmeer). I Ďd had some exchange of letters with him, and when I finally found him, I found him looking for me. He said the postman had said there was somebody inquiring who didnít speak any Dutch and he figured it was I. We took a long walk in the dunes and discussed some of these problems of quantum theory. That was my first acquaintance with Kramers. Those were the principal scientific contacts that I made on that trip.

Kuhn:

Do you have any recollection still of what sort of problems you did talk about with Kramers? What seemed the pressing things for discussion at that point?

Van Vleck:

I guess that miserable helium atom, probably.

Kuhn:

I think itís already in 1923, and perhaps already by this time, there had been a Born-Heisenberg paper which they say proves that the helium atom canít be solved. They say they examine exhaustively all of the various sorts of models you might use, and that none of them are going to give the right answers.

Van Vleck:

I donít think Iíd take an issue with that.

Kuhn:

But you donít remember that paper?

Van Vleck:

I donít remember that paper at all.

Kuhn:

How about the work of Land6 in this period? This is the period of the vector model and the g-factor.

Van Vleck:

Seems to me I remenber the Lande gófactor in Kembleís course. Is that right or not?

Kuhn:

Not really the gófactor. You get some early multiplicity rules, not the g-factor.

Van Vleck:

I think I had that in my courses in Minnesota, IĎm pretty sure I did.

Kuhn:

There are some earlier rules for calculating doublet separation.

Van Vleck:

What I learned at that period was pretty much that, and my Bulletin on Princinples and Line Spectra; if you want to know what I knew at that time, that was it.

Kuhn:

You were at Minnesota at least two years before you finished writing the Bulletin, late in 1925. When did you actually start on that?

Van Vleck:

I donít know. I was appointed to this little committee in physical optics. What it was supposed to do I donít know.

Kuhn:

Iím sorry you donít because that was something I particularly wanted to know.

Van Vleck:

The person you should talk to on that is Paul Foote, who was chairman. He was exceedingly nice to me as a young man, he gave me more or less free rein to write this bulletin. It seemed like an opportunity for a young man. I donít think the committee ever met very often, but they told me I could write this little report which ballooned to a much bigger size perhaps than they anticipated. Silberstein occasionally came to those meetings as I remember and. more or less broke them up.

Kuhn:

That issue of the Bulletin of the National Research Council really stands as an extension of your contributions that are in the individual reports of the Physical Optics Committee?

Van Vleck:

No. I mean that was it. I donít think there were any individual reports.

Kuhn:

Yes, indeed there are. In fact you sent them to me in Copenhagen.

Van Vleck:

Oh, that was something else. Was that the same committee or not? I think it was a committee on ionization potentials and related subjects and the other was a report on the committee of physical optics. I think maybe they were different committees. One was headed by Paul Foote and the other was headed by Birge—was it?—or somebody.

Kuhn:

Youíre quite right, the quantum principles and line spectra is part of the report of the National Research Council committee on ionization potentials and related subjects. The things I was thinking of were in fact much smaller things, summaries of major events.

Van Vleck:

This was just an annual report by our committee on just what was going on in new things. Perhaps this was an attempt of Americans to educate themselves. Things donít often get lost in the mail, but apparently as I read back, one of my reports got lost and got published a little bit divorced from the rest of the reports of the committee.

Kuhn:

The physical optics thing. Can you tell me a little more about this notion about Americans attempting to educate themselves? I donít know of any European parallel for the various committees of the National Research Council and things like the physical optics committee.

Van Vleck:

I donít know too much on what motivated the older people to form these committees, I was just asked to serve on them and that was it. Iím afraid I donít know too much of the background, but I think there was a feeling among the more sophisticated of the American physicists that we were behind in knowing what was going on in theoretical physics in Europe.

Kuhn:

Was the way to take care of that by putting out American books or by reading European books and journals?

Van Vleck:

Well, by reading European books perhaps and then getting out some books in English that went over the thing with a little easier reading. I donít think there was any explicit strategy on the subject.

Kuhn:

Do you have any notion as to when you started really working out the material for quantum principles and line spectra? You said before that that represents what you knew in this period, but that as its end product is an immensely erudite piece of work with a really thorough knowledge of a very large range of literature. It Ďs pretty clear, I think, that as of the time you left Harvard to go to Minnesota, you bad nothing resembling that as a command of the literature.

Van Vleck:

Probably so, although some of the literature came out since then. I know that I was already writing some chapters on that on rainy days in Switzerland in 1924. I would say I started writing that perhaps beginning in the spring of 1924, and finished it in late 1925. I worked on it very hard that summer. My father got after me for my very poor style of scientific exposition. I feel I owe a great deal to him for his splitting up my sentences into shorter sentences, avoiding dangling participlesó i.e., tightening up my prose style—- the same kind of drill I try to give my own graduate students now.

Kuhn:

Canít you remember more about how that enterprise really came into being and the stages through which it developed?

Van Vleck:

This committee headed by Paul FooteóI was sort of a ďrara avisĒ at that time. I was a young theoretical physicist presumably with a little more energy than commitments than the older people interested in these subjects, so they asked me if Iíd write this thing. I think it was by invitation rather than by my suggestion.

Kuhn:

Itís not clear to you how much of that you were able to write out of work you had really already done and things you had already read, and to what extent you bad to go out and do a lot of new reading in the literature and reformulations and so on?

Van Vleck:

I think all the things as far as the helium atomóthe first few chapters of that I had really done in connection with writing my Ph.D. thesis and the action and angle variables. I tried to read the articles in quantum mechanics that came along.

Kuhn:

Let me ask you about reactions to that.

Van Vleck:

The book reviews were favorable, but of course, the new quantum mechanics broke before that was off the press so, in a certain sense, that was an interesting historical document as a review of the subject up till about the time that we really had quantum mechanics. A little like volume I of Bornís atom mechanics.

Kuhn:

But volume I of Bornís atom mechanics, although its period of life may have been fairly brief, had a very active life in that period.

Van Vleck:

Iím sure it had much more influence than my book because it was in German. and the physicists were in Germany in that time, and they gave much more mathematical detail than my book did.

Kuhn:

Do you have any notion of how many copies of that were distributed or anything of that sort that would indicate the sort of use it might have gotten?

Van Vleck:

I know it at least bad one photographic reprint, maybe two.

Kuhn:

At the time, or more recently?

Van Vleck:

Not too far removed [from that time].

Kuhn:

Let me bring you back now to what intervenes between the papers we talked about before and this Bulletin. This is the absorption paper.

Van Vleck:

I think that was one of my better papers.

Kuhn:

How did you get into that?

Van Vleck:

Through a misunderstanding of something Gregory Breit told me. He said that the net absorption was the difference between the fluctuations up and fluctuations down, referred to some paper of—I think it was (Kretschmann)—but that was an entirely different thing. It was concerned with the fact that under certain phase relations the light did work on the atom and under certain phase relations the atom did work on the light wave. It was dealing essentially with statistical fluctuations. I misunderstood his remark and proceeded to try and get the differential effect between the absorption up from a given stationary state and adsorption going down. I got the term in partial rho with respect to nu. Iím very proud of the fact that I picked that one up, although Iíd really got that out of Planck in a certain sense, Slater, at Kramersí suggestion I guess, made a completely parallel calculation in Copenhagen which be never published.

Kuhn:

How did you happen to hear of his work on that? Did you correspond with Slater when he was in Copenhagen?

Van Vleck:

With Slater or Kramers, probably Slater. I sent him a draft, or told him about it, or something. I may have written Kramers about it and I think he wasnít sure whether Slater had determined the partial rho with respect to nu, but actually he did. There was a little episode at that time. I immediately saw that a similar correspondence would apply to the refractive index, at the time that Kramers wrote his little note to Nature on the Kramersí dispersion formula, It wasn Ďt clear to Breit and me whether he had shown the asymptotic correspondence of that with the general refractive effect of a multiperiodic system or whether heíd only done it for special cases. I guess my preliminary note in THE PROCEEDINGS OF THE OPTICAL SOCIETY stirred Kramers up to write another note to Nature where he explained that heíd done this for the general multiply periodic system.

Kuhn:

I think it was something of Breitís that stimulated Kramersí second note, Iíve never looked at the Breit paper, but that second note of Kramers starts out by referring to something of Breitís that apparently suggests an alternate form, and that itís critical of the Kramers form that Ďs occurred in between.

Van Vleck:

I may be wrong. Breit and I were in very close contact at that time.

Kuhn:

You speak of this remark of Breit. To what extent were you already trying at this point to work on a correspondence principle for absorp tion?

Van Vleck:

Frankly I donít know. It may have occurred to me that there was a correspondence principle for emission, maybe there should be one for absorption. I may have had that thought, perhaps I did. I donít know how I could have got to think about it otherwise, do you?

Kuhn:

That seems very likely.

Van Vleck:

Hard to think of any other motivation.

Kuhn:

You speak, in that paper, or at least in the longer version in the PHYSICAL REVIEW [ďThe Absorption of Radiation by Multiply Periodic Orbits, and its Relation to the Correspondence Principle and the Rayleigh-Jeans Law: I. Some Extensions of the Correspondence Principle; II. Calculation of Absorption by Multiply Periodic Orbits,Ē Phys. Rev. 24 (1924), 330-346; 347-365] of what you called the ďabridged radiation forceĒ. This is a way of using the correspondence principle in which you will take simply the force due to a single term in the multiple Fourier expansion and utilize this, as you have to in the correspondence principle. Iíve never run into that term or that sort of discussion before, and I wondered if this was yours, and if so, how you felt about it?

Van Vleck:

Iíve forgotten about it entirely. Iíd have to reread my paper to answer that one. Part III, as I told you earlier, which was to deal with the equilibrium of multiply periodic orbits classically under the Rayleighó Jeans radiation field, I never got around to writing because quantum mechanics broke, and more exciting things came along.

Kuhn:

The ďabridged radiation forceĒ comes right at the beginning of the second section of the paper.

Van Vleck:

Part III was supposed to deal with the radiation.

Kuhn:

But the ďabridged radiation forceĒ goes right in from the beinning, in your discussion of the correspondence principle for emission, and then you use it for absorption, so it would beó. [Long pause, while Van .Vleck reads the paper.]

Van Vleck:

Well, it docrnít say very rieh, trying to average over initial and final states taking one particuIar path. This is again the example of some of these gyrations one went through to try and doctor up classical theory to give something corresponding to the quantum-mechanical true expectation value. [Another pause]

Kuhn:

I asked about that particularly. You said something to me about that paper once before, I think in Philadelphia at the last meeting of the committee. .... [text cut off.]

Van Vleck:

That's true. Perhaps I should say considerably more perceptive.

Kuhn:

It isnít really that question I mean to be asking. I really raised this question about the ďabridged radiation forceĒ because itís really there most precisely that you sort of isolate the single term which in the Heisenberg theory will be a collection of those single terms that gets plugged back into the classical equations. This suggested to me that if you had missed something that might have been done, it was really right at the point of isolating that instead of treating a collection of them.

Van Vleck:

I donít think so, because all I was just trying to do was to tie up an average over an intermediate path with one particular line. The only way you can do that is to take some sort of hypothetical average. I think perhaps youíre trying to read a little bit more into that than I had in mind.

Kuhn:

What do you know about reactions to this paper? Hereís one Iím curious to know about what responses from others you may have had.

Van Vleck:

Well, letís see. I was terribly pleased when the PHYSICAL REVIEW accepted it without condensation because it was a terribly long paper, especially for the PHYSICAL REVIEW. [Gordon Scott] Fulcher was a fairly hard-boiled editor [from 1923 to 1925]. I was over in Switzerland with my family in 1924 when I got a letter saying it had been accepted IN TOTO, that Tolman had refereed it and said that be thoroughly enjoyed reading it. That Ďs the only reaction that I can remember to that paper. I certainly think Bohr and Kramers noticed it, but of course, they had Slater doing the same things.

Kuhn:

Jordan tells me that he and Born were quite excited about this paper. He doesnít remember much about what they did with it but be said they were working quite hard in an attempt to reformulate it, and bad been multiplying Fourier co-efficients together, just at the time they got the Heisenberg paper that was going to be matrix mechanics. Did you ever hear anything from them about this?

Van Vleck:

I canĎt remember. Born might have said something nice to me about it when I met him. No, I guess thatĎs wrong, because when I first met him quantum mechanics bad already come on. I donĎt know as I ever met Born until the fall of 1925. People arenĎt too addicted to writing letters so I donít know any particular occasion for correspondence. Physics was different in those days. I donít think I had many reprint requests for that, I still have plenty of reprints left in the file for that, and I donít think I ordered a very big batch at Minnesota. I think thatís a partial answer to that question; thatís a reprint of which I still have a fair number of copies. I probably mailed out reprints to people like Epstein and Kemble who I felt were interested in that sort of thing. It was a limited clientele.

Kuhn:

By that time, I take it, you were well aware of Epsteinís being at Cal. Tech?

Van Vleck:

I think so.

Kuhn:

Was Epstein much of a force in the development of theoretical physics in this country? He turned out some students who were.

Van Vleck:

Iíd prefer that youíd ask Oppenheimer about that because he was on the West Coast, and we didnít have jet planes in those days, so that he may have come East for an occasional meeting but it was physically impossible in those days, people didnít have the money, time and so on. It would be judged by what students he produced out there at Cal. Tech., I should say.

Kuhn:

Thereís one other paper in the old quantum mechanics period, on the polarization of fluorescent radiation in a magnetic field. The paper was relatively straightforward and the results all come out.

Van Vleck:

Yes. The curious thing is that people have written some papers on that subject using wrong kinds of intensity formulas, and it immediately occurred to me that the way to do it was a way that bad been indicated by sum rules of Heisenberg and so on; I forget the exact details. Paul Foote said that he had the same formulas but somebody talked him out of publishing them or something, he was scared of them.

Kuhn:

You refer in that paper to an unpublished manuscript of Heisenberg on sum rules which extends the OrnsteinóBurger work. Do you have any notion what thatís a reference to?

Van Vleck:

No, I didnít even know I had. such a reference.

Kuhn:

IĎve no notion of what, in HeisenbergĎs bibliography at least, that would refer to. In this same period, what about the Bohr-Kramers-Slater paper?

Van Vleck:

That was considered almast a Gospel at a certain time, I think. The prestige of those three people writing it; it was the best attempt at the thing in that direction. I looked. at it with great reverence, I should. say. I never quite understood certain aspects of it, and Iím not sure that anybody ever has. It isnít —.

Kuhn:

Were you all then much disappointed at the results of the Compton-Simon experiments which seemed to throw it out?

Van Vleck:

I never thought too much about it. I was more interested in getting my own builetin on quantum principles and line spectra published. I didnĎt have too much time for all these things, teaching two new graduate courses a term.

Kuhn:

What were you going over with the graduate students? Do you remember what those courses in Minnesota were on and how they developed?

Van Vleck:

I think I gave one in atomic structure, one in partial differential equations which, I think in retrospect, was far too formal. It didnít develop the things that were really needed by modern theoretical physi cists. When Breit left, that left a course in electrodynamics for me to give. My main teaching was simultaneously, of course, in quantum theory and electrodynamics. Those courses were distributed between the two of us the year he was there.

Kuhn:

He was there only for one year.

Van Vleck:

Yes. He went to the Bureau of Terrestial Magnetism.

Kuhn:

Why did he do that?

Van Vleck:

Well, there was an attractive offer thereóó. I guess that was it.

Kuhn:

How about the Krainersí formula? Was it clear to you pretty much from the very start that this was what people were looking for, this dispersion problem. How clear, for that matter, was it that dispersion was a dreadful problem?

Van Vleck:

Oh, I think it was clear that dispersion was a problem, of course. The classic example was McLennan in Toronto saying he had a graduate student who was no good. The student was ďno goodĒ because McLennan said, ďHereís this problem of dispersion, do something about itĒ, and the student didnít. Which is, of course, nonsense. But I think the people realized that understanding dispersion might lead to some interesting things.

Kuhn:

Again, you see, in Europe for certain people, that problem was like the helium problem, a problem that simply showed you that the whole old quantum mechanics was dreadfully askew. I wondered whether you or others in America had taken that point of view?

Van Vleck:

Of course, I felt that I liked Kramersí dispersion formula. It seemed to me to make sense because his correspondence principle things checked.

Kuhn:

One more question before we get into the beginnings of the new quantum theory. It goes back to this little paper on the polarization of fluorescent radiation. You also in that discuss a paper of Heisenberg on polarization in the absence of a magnetic field. That paper was one that particularly interested me, and I struck gold with it when I talked to him because apparently it caused a great argument between himself on the one hand and Bohr and Kramers on the other in Copenhagen. He uses a field which he lets go to zero to line up the oscillators in the absence of a field.

Van Vleck:

That takes you into all this weak and strong spatial quantization and spectroscopic stability and so on. There was a lot of nonsense written in that period by very able people, and elements of truth in what they said too.

Kuhn:

How did you like that paper?

Van Vleck:

I liked it.

Kuhn:

In spite of the fact that you found the agreement with experiment to be quite bad?

Van Vleck:

Of course, that worried me. I canít remember the details. It did agree badly, did it? I canít even remember. It seemed to me it was a cleanó cut thing somehow, like the Kramers dispersion formula.

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