Niels Bohr - Session IV

Bohr:

In the last days I just have thought about things from those years. But I wanted just to know what you would like to learn about. The question of my father and brother, and so on — that I think has to wait a little. That we will very soon take up, but that I must think over a moment. And then also there are the early papers and so on. But now we have started with the papers in the Manchester time and. Copenhagen. And now the question is just to speak about it. The one point is to learn what actually Rutherford's attitudes to the problems were. You see, I'll do this quite informally today. And that is one thing about which you perhaps have some questions. Then the next problem is actually what was the reception of the papers. The point is first of all that it was a small group in Manchester, and most of them had very much to do; it was only later on that they got interested in it. But surely Moseley was interested as you see from his letters. But that was also later on, you see, when he has made his own work. But we really (found) out various things about Evans. But now the question is, what would you like to hear about first?

Kuhn:

All of these are first rate topics. If we are to talk about this area it might be as well to start with what is, in a sense, the first of the problems. That would really then be those first conversations with Rutherford after he had seen your paper. You tell us something about those in the Rutherford Lecture, but there must have been more. Did you make that trip to Manchester from Copenhagen simply because of those letters?

Bohr:

That is later, you see. You mean the next year?

Kuhn:

In March, I think, you have sent in the paper.

Bohr:

Then I went over. Yes, but that was clear that I really behaved very badly. He wanted it to be shorter, and then I had, in the meantime, sent him a longer one — considerably longer — than the first, and I thought I had to go-to see him and fight it out with him, or try to fight it out with him. And he was very nice in that way. I know that later on he said that he had never thought I should be so persistent — or stubborn, — perhaps. But we actually went through it, and everything which was in the paper got to stand, you see.

Kuhn:

Were there any revisions made as a result of those talks?

Bohr:

No. No, not any revisions in the meaning except for a few sentences, and so on. He only felt that it was a definite view (that I wanted to publish it at all.) And when he tried to suggest that something be left out, then I said, "No, that is quite essential for the argument." Which in some ways, you see, it also vas. And it is a terrible thing to me now. But it is really a paper with a lot of things in it. So that is really all that can be said about it.

Kuhn:

Did you get the feeling that his concern was entirely with length and form of the presentation? Did, he talk also about his feelings about the basic theoretical ideas involved?

Bohr:

No, not more, I think, than he wrote in his letter to me. And that was very interesting. It was a central point later on. The next thing that happened was — that was in '17, I think — Einstein's paper, where Einstein took exactly the same view. He took the view that spectra came from atoms in the way I did it. But he then spoke about how to treat the statistics of it so as to get Planck's formula; and that was, of course, a wonderful piece of work. This proves that he has also been troubling backwards and forwards to get it to fit. But he actually there says that in regards to putting a probability of spontaneous transitions — that was exactly the same as Rutherford has done with radioactive things. That's what I also say [in the Rutherford Lecture.] (You have made some remark that it is the second point raised which has the emphasis.) Yes, that is what I write there, but that is made completely clear. In the later years it was also done as regards (???), but this was the same thing as he had done. So, I think, (we) didn't think it was so difficult. But, of course, I couldn't tell at that time because the paper of Einstein came first five years later — or four years later.

Kuhn:

You had yourself, at that point, no fundamentally probabilistic conceptions about this?

Bohr:

No, you see, because the whole thing was different. You see, the thing was completely clear to me. It was not clear in all the points — but it was completely clear that it was right, you see. So I was just taking the view that this is obvious, you see. And I think I may have made an impression on Rutherford. Then when Moseley's work came he was more cautious, and he advised Moseley to write about it in another way. Now I'm very sorry I'm speaking so badly today. But that was the point you see. The odd thing is when one sees the paper [on the constitution of atoms]. Really that paper is a good paper — the first of it. I mean it contains something; it contains really very much because it contains all the absorption and so on. But it was obvious to me, you see. Then, for instance, the dramatic thing about these things in the first years was the helium spectrum. And that was certainly also equally obvious. But when the people wouldn't believe it, I had to find, some further points as regards the correction due to the mass of the nucleus. But there has certainly never been any trouble in my mind as regards the hydrogen and the helium spectra.

Kuhn:

Had you recognized from the start the possibility of using the mass correction as you did?

Bohr:

No, no. But it was so obvious. If you have two spectra and they almost together) then it is to the theoretician an obvious thing to say, "Yes, but certainly there must be some correction due to the mass of the nucleus." And that lies, of course, in that we have to do with the effective mass, you see. It's an obvious thing, and the nice thing is that it fits so well. And if it wouldn't fit then, of course, one wouldn't know what to do, but that was not the question. One was convinced before hand that the thing was right.

Kuhn:

Do you have any sense, yourself, sir, as to at what point in your thinking this terribly strong conviction developed?

Bohr:

I think it was (comparable) to just when one saw the hydrogen spectrum and also, just at that time, learned about the helium. The helium was the thing where Nicholson plays a part, but in a very indirect way. Nicholson published in the Astronomical, and Fowler did the same. Therefore, Fowler's paper, I think, appeared in the same thing as Nicholson's. So, I think, that was the reason that one learned about it so quickly. That was wonderful that Fowler had just —. But it was difficult to disentangle because Fowler used a mixture of hydrogen and helium. But the odd thing is that many years have gone by since the lines were observed in the Zeta Puppis. And just at that time — it was December '12 — I got it. I was working on it in January of '13, so it had just come out, you see. I don't know when I saw it; whether I saw the hydrogen spectrum first or whether I saw these things, but that was about the same time. But then when one knew that it could come in a mixture of hydrogen and helium, then the whole thing was obvious, you see. It may be wrong, but I considered the whole thing as completely obvious. And now I was very interested in it because if you are interested in the Rutherford atom, then, of course, the whole point is to get all the elements. And this was the beginning of getting the elements when one became so clear that two spectra from the same kind of (model), although their constants were different, could be so much alike. That was the odd thing, you see. Now it's not the meaning to speak (now), but I (told you when we were discussing it) that this was a very interesting thing that two completely different substances could have spectra which were so much alike. That, (had never been) thought before. And so that fit in with the isotopes so wonderfully, you see. But now I don't know if there is more you want to know about this special problem.

Kuhn:

Well, I really am curious about how typical the strength of Rutherford's convictions here were. Did he look at all the papers from people in his laboratory which came out? And what sort of control and power did he really have?

Bohr:

First of all, you see, one was, of course, independent. But, on the other hand, it was very (reasonable) that one just turned to Rutherford. He looked at all the papers, I think, because most of them were very simple to look at. (That was a terrible thing.) Then his book was out in the next half year, and Rutherford just was always very busy. But he was just wonderful because he gave his time. He had, these strong views with regard to (language) and so on which is clear from that. But he felt that it was of no help to interfere when a man is so convinced.

Kuhn:

Were there cases at the laboratory of people wanting to publish papers and Rutherford's telling them not to and stopping the paper?

Bohr:

You see, I think, it went differently. This was not a theoretical place. My paper, and also my first paper, was very different from the others. When it had to do with experimental work, then it was Rutherford who said, "You must publish it as soon as possible," or something like that. And there was no trouble about it. But first the point was whether it was correct or not. When it was a good piece of work then it should come out, and so on. And. Rutherford was, of course, a complete genius. Therefore, this special connection with him in relation to that paper was, of course, a bit out of his line. But he gave in and was very nice. Then he wrote to me a few months later in the autumn of '13 about the Stark effect. That he just saw because he was an honorary member of everything in the world. He got that paper from the (Breslau) Academy. And then he wrote to me about whether I thought it was the tine for me to say whether — I could explain the Zeeman and the Stark effects. And that was a very interesting thing. Now, this may seem very odd when one knows that a few years later one had the whole thing. But the point is that one just saw that there were possibilities, and really also quantitatively, to say that the orbits can lie differently with respect to the field and the nucleus. And one just went, without believing it, to the case where (it went all the way to a flat orbit.) But that gave the order of magnitude of the difference, so, therefore, it was practically the same. That was a great amount of joy because one found something which could not possibly be explained in the ordinary manner. If the hydrogen spectra were actually elastic things from a definite atom, then it should not show any Stark effect. It should not show anything at all in the first order. Because if you put such a thing in a field then you just have a displacement of the equilibrium state and you would have the same vibrations around it. And that was the one thing; another thing was that really all the typical things of the Stark effect came out. Sometimes, for instance, the lines — the big lines — were stronger or weaker, depending on the way the experiment was done. These experiments were done with the positive rays where the particles have a velocity. And it fit it; it fit it not exactly, but it fit it as exactly as one wanted it. So that was an extraordinary thing. But, you see, I never doubted it; that was the whole meaning, you see. Then, of course, at that time one couldn't explain all the lines, and so on, and. Sommerfeld at that time developed ways of fixing the stationary states. They were not right, but they were just on the same line a little better, you see. And the odd thing is that it fits exactly. That is due to the fact that the wave mechanics is so close to the ordinary thing that if you have one electron and that electron is close to the (general) state, then it gives exactly the same results. I mean the whole problem of the Planck vibrator and the hydrogen atom — they're the only systems on the classical mechanics where the variable orbit is periodic in the first approximation. And in that case then the wave mechanics gives the same, and that was the reason why one believed in it so long. But, you see, the odd thing is that those people who actually, or partly, made the wave mechanics, like Pauli and Heisenberg, they believed much stronger in orbits. Heisenberg has written the most complicated things about the anomalous Zeeman effect; Pauli has written a whole dissertation on the hydrogen molecule ion. And this doesn't fit, you see, so they learned. it the hard way that that's not possible. But that we'll come to. But now what more can we speak about? We can also speak about the Moseley work.

Kuhn:

I'm curious about the role of the Philosophical Magazine as compared with other journals in which one might have published. What determined, for example, whether a paper was submitted to the Philosophical Magazine or whether it was given to the Royal Society?

Bohr:

These are odd questions because they are partly, I think, economic. The Royal Society existed, of course, also before, but probably they had not the money ready or something like that. For then, later on, all Rutherford's things came into the Royal Society. But until those days all his papers were published in the Philosophical Magazine.

Kuhn:

The Geiger paper, and I now forget which one, actually did come out in the Transactions.

Bohr:

Did it? It may be very interesting to see. In the early days one heard a lot of the Royal Society; my first paper was published in the Royal Society, but then the Philosophical Magazinewas such a handy paper. Then it simply disappeared almost, and I think there has been in later years some attempt at reviving it.

Petersen:

In Manchester how was your relation to Darwin?

Bohr:

That was very nice indeed. I think they didn't take an interest in me at all to begin with — (they might have, I suppose). But then after this paper about the stopping power, as far as I know, Darwin was most objective about it and thought that that was simply better than his.

Petersen:

Did he take interest, too, in your own ideas on the atomic constitution?

Bohr:

Yes, but all this came later, you see. Because actually it was me who brought it up to Moseley and. Darwin. They wanted to (speak) to me, you see. And then I explained also to them what my view was on the periodic table. I had, of course, thought about it before and had talked to Rutherford. And then, as far as I remember, Moseley said, "All, right, we will see." And he did the experiments in a very, very short time.

Kuhn:

That was in the summer of '13 then that you talked to Moseley and Darwin?

Bohr:

Yes.

Kuhn:

Really you think you had not talked to then in 1912?

Bohr:

You see, in '12 we talked about my paper on the stopping power.

Kuhn:

But clearly you were already in '12 thinking about the Rutherford atom.

Bohr:

Yes; I also spoke about it. They knew that I was interested; therefore, they came to me. And, they knew that I had views about the beta and alpha particles, and so on. But I do not know whether they believed it to begin with — (that's something else).

Petersen:

How was Darwin's place in the group there? He was a theoretician; did he also do experiments?

Bohr:

Oh, but he did experiments. Yes, he did experiments with Moseley you see. That was not in that year; that was after Laue's discovery was known, and after the work of the old Bragg and then his son.

Petersen:

But when you came there, had Darwin been aware of the problems about the nucleus or the stability of the atom?

Bohr:

That I don't know because Darwin was, first of all, in the work about the nucleus. Darwin was helping Rutherford. Rutherford did the calculations himself, but then he [Darwin] also made some calculations about scattering in regard to the weight of the nucleus. It is only in the heavy substances that you can neglect it. It's good that he didn't do something with the light nuclei, because then (he) would have found that it didn't fit; and that would have been very, very odd. One first knew the reason for it, and then one found that the experiments of (Chadwick and ???) were quite different from the others. But that was first ten years later and so on. And that developed all on a theoretical basis. They were really very lively years. Mott was here you see, but that was after the quantum mechanics.

Petersen:

When you came home you say you talked to Hansen, and then he may have called your attention to the spectra. And then when you had found it out, you said you had talked to him again and told him how nice it was. Did you talk to other people then about it too? For instance, did you talk to your brother?

Bohr:

You see, first of all, I'm not sure whether my brother was at that time in Göttingen. But the whole thing was completely clear to me. And really when I came back, then I spoke with Bjerrum. I didn't hear anything about his work. I think it was not ready in '12. That's the next thing - I think he went to Nernst in '12, but that we will also find out.

Kuhn:

I think that paper appeared in '12, if you mean the angular momentum paper.

Bohr:

Yes, but you see the question is what time in '12 it was.

Kuhn:

I don't have a date, but it came out in the Nernst Festschrift in 1912.

Rüdinger:

It was signed March of '12.

Bohr:

I don't think there is anything in that paper — that is just the distribution. When you have it, then you say the distribution is something, and then it fits. It was the work of Eva von Bahr which really brought out the single structure of the single lines, and therefore, it's not this [paper of '12].

Kuhn:

Well, there is nothing in that paper on the structure of the single lines, but there is a suggestion of the rotational contributions to bands and that there ought to be a line structure. Of course, it still is not done in the way you will do it; it's not a transition process.

Bohr:

And, therefore, to begin with, it was entirely different because my work was with the question of how to get a spectrum from a system which would not give a spectrum in the ordinary way. The wonderful thing was that that was the Rutherford discovery; we felt that now we have a system very different from anything which could give a spectrum, and yet it still has [one]. And, therefore, one had, to do something. Now, I'm not too definite in my views because one could have also started differently, but first of all (it was this.) Then in Bjerrum's work (he) never came into the (problem) before my paper came out. But how actually to get the band spectrum? [Bohr demonstrates at the blackboard]. ... Now, that was a splendid piece of work; one could have started from that, but how could one do it? Therefore, I just came into it another way.

Kuhn:

Do you think you told Bjerrum about your own work in these early months?

Bohr:

You see, I'm not sure because I'm not sure (that even Bjerrum was —). (It was when I came in for my wedding); then I told Bjerrum about the isotopes, and the transition (problems) and the displacement laws. And he was most interested, and (it often came back) that he had heard about that very early. (Now he doesn't live any longer). We must, if possible, get hold of Hevesy. He has not been well, but perhaps he is better. I'm not sure he is in Stockholm. I wonder whether you would like to go up to Stockholm to see him?

Kuhn:

I would be delighted to go to Stockholm, or to invite him here. It seems that the nicest thing of all, at this stage of the game, might be to have the two of you talk together, and see what comes of that.

Bohr:

That is very interesting you see; we will have very different views about things, but that is something else.

Kuhn:

In connection with Rutherford and the papers, what happened when you sent in Parts II and III? Did he then forward those immediately, or were there the same sorts of discussions?

Bohr:

No, I do not think there were. That is a thing (I've also wondered) about. You see, first of all, I came over with them. That was very important, because then I met Moseley and, had these discussions. And, I think, also that (he) looked at them, but there was not that point as in the first paper. That was all in the time before the meeting in Birmingham. That time was very interesting. My wife and I went over. Oh, I had a whole long correspondence with Rutherford about that, because he wanted me to come, and I said I couldn't. I went at the last moment, and it was very odd; I slept on the billiard, table, I think, the first night. But then Hevesy, who is so very good at arranging, he got me into the Girls' College where he had (bunked). And that was very, very practical and wonderful — the girls were away. Yes, you see, that was one of the points; it was in September. Hevesy is just wonderful, you see; he really is a gentleman. My wife and I were married in '12, and we were over in '13, and we met Hevesy and. Paneth and made some discussions with them. And that was probably the time when I met Moseley. But I had so much to do you see; it was just terrible — (you would think it's probably not too good). But I was living for these things, you see. There was, at that time, no knowledge that anyone was working with it, and, therefore, I thought I couldn't leave. But then I still left. And in '14 I came over to Rutherford; and that I have written about; have you read that?

Kuhn:

Yes.

Bohr:

That was, of course, interesting because that was the time when the papers of Franck came out. I reacted against them. I thought it was just marvelous, but I thought they were interpreted wrongly you see. You had better ask something. I'm (doing) very badly today.

Kuhn:

I wonder whether there's more that you can tell us about early reactions?

Rüdinger:

Do you remember the attitudes of other people in Manchester? I think of Evans, for instance, and Moseley.

Bohr:

Evans was such a very modest man. I do not think I knew Evans. I asked Rutherford if it could be done, and Rutherford turned to Evans. But then, of course, I have a whole correspondence with Evans. Then Evans was very interested, but Evans was not interested from the point of view of theory.

Rüdinger:

You don't think so? Because he seems to be very convinced of your theory.

Bohr:

Yes, but the facts were so clear you see. ... Moseley was quite different. Moseley really thought that there was something in it, and he wrote to me — you have seen this letter — and said, ("I am) finished with these things." And I wrote to him pointing out something where, if he really thought about such (things), there was not the energy for it. But then the whole solution came through this very beautiful work of Kossel. It was very early. And that was another thing — where was Kossel? Sommerfeld knew him, ... but he had no connection with Sommerfeld. I could now also speak about the connection with Sommerfeld. Sommerfeld wrote me when my paper was out — and that you have seen; it was a card. He said that he was interested in it, but he didn't know really what to think about atom models. Then Göttingen has just decided against it. But Debye was, of course, a very considerate man. And I came to Göttingen in the summer of '14.

Kuhn:

Do you know who was responsible for that invitation?

Bohr:

I just came; there was no invitation, you see. I came and I gave a talk, and there was very, very great enthusiasm, you see. Then I went to Munich, and there was also great enthusiasm. At all these places I gave a lecture of that kind which is in the Danish Physical Society, where I really made it clean. And that was before there was any work in Göttingen or Munich. So they had a first hand exposition of it.

Kuhn:

In neither of these cases then had you been asked to come and give a talk?

Bohr:

No. Well, (I came with my brother); we were on a tour. You see it was that time when the war broke out. We were going on a walking tour in Switzerland at the (???). Then we felt that we had to go out, and we came out of Switzerland (just right) so that we could pass through Germany as Danes. And we came to Munich and Berlin, and it was very, very curious. But this was before, you see. (That was no physicist...). But on the way down we went to Göttingen; (that was) a place where my brother knew them all, you see. And they were delighted to have him, and so when I came to Göttingen, they asked me to give a talk to them. And I did it without any preparation. They gave a great lunch before the talk, and I was afraid to drink all this wine. But they said it helped. So it was really very nice, you see, and that was the way the (???) went.

Kuhn:

Who there was particularly interested? Was it Debye you think who was the ring leader?

Bohr:

Debye in Göttingen and Sommerfeld in Munich. But they were all interested, you see. A man who was certainly very interested was Ewald. But there was no question — they couldn't doubt at that time. Then, later on, Sommerfeld started work with it. Then they had this paper about the (further) quantization.

Kuhn:

By the summer of '14 was Runge now convinced? Was he there do you remember?

Bohr:

I'm not sure, but I think he was. You see, Courant was such a close friend, and Courant was his son-in-law. This thing about Runge must not be told. The point is that when Runge [sic] heard about this very critical attitude in '13, then he thought that that was so, and that it really was a deplorable accident that the literature should be contaminated with such things. Because he felt that in some way or other the knowledge of the atom should come out of the spectra. And it is very nice that it did. But it was such that one first had this preparation, that is one first had a system which couldn't give anything — really completely nothing. Then one felt that one could, by using the quantum theory in some way, get something out of it. But it changed the whole thing around, you see, when one saw the spectrum. I'm sorry for the trouble I've made by making it difficult to understand, but that was the point. And then we were very taken up with work and everything. And then when I came back, Sommerfeld came — in '16, I think, perhaps '17 but I think '16 — to Lund. And that was very interesting. I also think he came to Copenhagen, but, I think, it was not on that occasion.

Rüdinger:

Yes he also came to Copenhagen in connection with this trip to Lund.

Kuhn:

There was a problem for him traveling at this time?

Bohr:

No, because that we arranged for him, you see. Certainly the whole thing, was difficult with the Germans; they had no money of any kind — and especially later. But he was invited to Lund, and we invited him to Copenhagen. That is another thing, you see, because that is a point where we should go into it very closely. But before that it was really the Stark effect which I made in '13. And. I wonder whether you could find the paper of '13 on the Stark effect? It maybe that therein I also say this thing about —. [Rüdinger locates and hands Prof. Bohr a paper]. This is March, Ilk; here I am speaking about a theory of Sommerfeld. I can't find, it now, but perhaps we can find in one of the later papers what I wrote about the fine structure of the hydrogen lines. That was quite (a victory) for Sommerfeld, you see. I was alone, but when he started working on it, then he understood that I was right. That is that we could not (quantize) the hydrogen atom further as long as it was degenerate. And then if you disturb it in some way, then how to quantize it depends on the way you disturb it.

Kuhn:

I think I have what you are looking for here. It's in the Phil. Mag. It's a letter to the Phil. Mag.in connection with a paper of Curtis'.

Bohr:

Yes, but you are right. [general conversation] Sommerfeld (just made) a point of that. Sommerfeld was very very fair, and studied everything I had written about it. Just talking generally this whole problem of the relativity corrections of the spectral lines has been a very complicated one. And it came so that Sommerfeld did some of the very beautiful work on (correlating) things, but the distinction between relativity corrections and screening corrections was not understood and couldn't be understood. [Bohr demonstrates at the blackboard]. Here you have an orbit — which goes that way. Then if you consider the relativity correction, it will be infinitely small and impossible to separate from the big fields which make the other things. (It's not to see.) And that I understood, you see; that I have gone into in a paper with Coster about the periodic table. But the thing was that it all came out from the spin. Then it was clear. ...

Kuhn:

I was very much struck to see this discussion of relativity correction at as early a date as you do it. How long had it been on your mind? Had you worried about relativity before this paper of Curtis' and before the recognition of doublets in the hydrogen spectrum?

Bohr:

No, because, I think, all these things were very simple; they were obvious. You see the whole thing is odd, but I was completely sure that it was right. I did not know how to get the things properly and how to arrange them, but, otherwise, I felt that there was a possibility which was surely right. But the point is I had so much to do in Manchester and also in Copenhagen with all these many different things carried on at the same time — for example, the pedagogical things in Manchester. They were all gone except some very few. So I really (struck) these days when I (have) written (too much on) Manchester. Of course, it could have been done quite differently. This paper of Wilson first came when I left, I think. And there was at first nothing to do with it. And Sommerfeld felt, of course, that he could do something with it. Then he felt it was wrong and that it was due to the motion (of the ellipse).

Kuhn:

Had you known Wilson in England?

Bohr:

Yes, yes. That is another thing. He was one of the helpers of Richardson in London. He was at Somerset House at Kings' College. And that was a very, very nice paper. But it was such that it couldn't be used for anything at once.

Rüdinger:

Do you remember if you have yourself thought of how to go from periodic to non-periodic systems? Because I think it's quite interesting later on it turns out that it's the non-periodic systems which prove to be the simplest systems.

Bohr:

Yes, you see, but in some ways not, you see. The hydrogen is so simple and so rich because when one has a perturbation then one can separate it. [Bohr returns to the blackboard to indicate the coordinates for separation of the equations] And I did quite a lot of work to clear that up in my paper in Copenhagen in '18. And perhaps it doesn't contain so much, but I really worked out there the effect on the spectra of a magnetic field and the other fields in a direct way, where one actually needs no calculations of any kind.

Rüdinger:

It seems difficult to treat non-periodic systems unless you have the separate quantum conditions.

Bohr:

Oh yes, and therefore, one actually only said that they are not periodic; therefore, we must be prepared for something more.

Kuhn:

This raises a whole series of questions that I had wanted to ask you about. When you developed these subjects in 1918, you were much more explicit about the manner in which you are working out orbits and energy levels than you are in a number of your earlier papers where you tend to say that a straight forward calculation will give the result. Clearly one of the things that has happened, that is clear by 1918, is that you are using Hamilton-Jacobi, techniques and are by then using contact transformations and are referring to Charlier and Poincaré and others for the existing techniques of astro-physics. When did you come to know of those techniques?

Bohr:

Yes. That was (rather recent). (Probably) Kramers came to Copenhagen, and. I worked with Kramers — (he was one of my assistants). But next I read —

Kuhn:

Well, now had Kramers known of these techniques when he arrived?

Bohr:

He knew something. But I knew a lot.

Bohr:

I think at that time, at the moment, I was the only one who knew anything about it because I read in Boltzmann. Boltzmann had written a book about the mechanical principles. There he goes very accurately into these things. And I'm sure that Sommerfeld knew about it somewhat. But it was not of interest to (them). And the interest is, for instance, that if you have an orbit like this, [the one on the board], then in the field you have an electric center. Electric center is just 3/4 of the major axis. And that electric center has to move then in a plane perpendicular to the electric field. And that was what one learned from it. [Bohr goes on to indicate that the perturbations involved in treating the hydrogen atom differ from those considered earlier and to say that he was simply glad to use the techniques and to be able to speak about the perturbation as such."] I'm saying most things badly, but I have some meaning with it. I think perhaps we will stop now, but we will soon go on again. Rosenfeld will soon be back and my son will also be back. My son will be very interested, and he knows a lot because he was with me in the war time, and so on. We traveled together.