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Interview of Harry Plaskett by David DeVorkin on 1978 March 29, Niels Bohr Library & Archives, American Institute of Physics, College Park, MD USA, www.aip.org/history-programs/niels-bohr-library/oral-histories/4827
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Discussion of astronomers of greatest significance in recent history. Personal recollections of his father's (John Plaskett) associates: Karl Schwarzschild, Edward A. Milne, Arthur S. Eddington, Ejnar Hertzsprung, Joseph N. Lockyer, Alfred Fowler, Meghnad N. Saha, Henry N. Russell, Frank Schlesinger, Jan H. Oort, Edwin Hubble, Albrecht Unsöld, Bengt Strömgren, Harlow Shapley, Walter Baade, George E. Hale, William H. Wright, Vesto M. Slipher. Also prominently mentioned are: Subrahmanyan Chandrasekhar, Paul Adrien Maurice Dirac, Theodore Dunham, Jr., Jacobus C. Kapteyn, Norman Lockyer, Edward Arthur Milne, Edward Charles Pickering, Henry A. Rowland; Harvard College Observatory, Lick Observatory, and University of London Imperial College.
As we just discussed, you would like to talk about your Top 11 in astronomy and that will work us into the interview.
Yes. Karl Schwarzchild, it seems to me, is the only astronomer who is in any sense comparable in stature with the great physicists of his own and earlier times — people like Rayleigh, Kelvin, and Sommerfeld. Schwarzschild’s great and outstanding achievement, from an astronomical point of view, was his realization of the importance of the transmission of energy by purely radiative processes. Not only that, of which he worked out most of the details, but also his introduction of the idea of stability, and the importance of that, for example, in the question of the solar granulation. He was also outstanding as an observational astronomer. Well, then, I've coupled with him, not in question of historical time but just as a convenience, A.S. Eddington and E.A. Milne.
Before we go on, could you also relate as we go down this either your personal recollections or stories that you know about them. Specifically you mentioned the 1910 Solar Union Conference.
Yes, well, of course I only have second hand through my father, and the only story I recall is that the International Union, on the way out to Mt. Wilson, stopped at the Grand Canyon, and Schwarzchild, in what I would suppose to be a fairly characteristic fashion, was determined to walk down to the foot of the canyon. And this was beyond his strength, and he fainted. This was regarded, I think, with a certain amount of amusement by the other astronomers. I don't think any of them really appreciated what Schwarzschild was and what he was doing. You will find recollections of the 1910 meeting of the International Solar Union in the "Oxford Note Book" written by H.H. Turner for each issue of the Observatory Magazine. Not available to me at home but Turner's account will be scientifically accurate with a judicious selection of gossip. You will probably find some mention of Schwarzschild’s descent of the Grand Canyon in Turner’s Note Book.
Did your father talk about other things that happened?
Oh, nothing that I can recall at the moment. So perhaps we could go ahead. So, Eddington and Milne, there's no need to say anything about them. Really Eddington, on the basis of Schwarzschild's work, gave a really clear idea about the physical structure of stars. His INTERNAL CONSTITUTION OF THE STARS is a classic.
You certainly must have had direct contact with Milne.
Oh yes. Not so much with Eddington, but a great deal with Milne, because when I was in Harvard, he came over and gave a series of lectures at the Ann Arbor Summer School.
Oh yes. Did you attend those lectures?
Yes, I attended the lectures and got a great deal from him.
Can you recall? That was 1930?
I should think, 1929. Dirac lectured at that summer school.
Well, the names may come, but I'm interested in what the exciting ideas were and what was the structure of these meetings? How did you enter into them?
Well I was a member of an audience, and Milne set out Schwarzschild's ideas in the form embodied in his (Milne's) Handbook der Astrophysik article. Chandrasekhar in his typescript "Recollections and Reflections on E. A. Milne" gives the best account of Milne and his work that I know of. I dissent only from his suggestion that Milne's later work was motivated from a wish to discredit Eddington.
How did people react to Milne in general?
As physicists they were interested but not as much as in Dirac.
Were these meetings held to bring more physics into astronomy?
Not primarily. They were just a summer school which was on physics as a whole, so that's why Dirac was there. Also Brillouin was there, so, there were three giants at this meeting: Oirac, Brillouin, and Milne. That gives sort of the general tone of the kind of summer school it was.
What other astronomers were there that you talked to?
T. Dunham and P.M. Petri of Victoria.
Did you know R. Atkinson?
Oh no. In 1929, Atkinson had probably already, with Houtennans, published his ideas on generation of energy in stars.
Right. So he hadn't come to Rutgers yet.
He was probably either in England or in Germany at the time. So the only astronomer that I remember was someone whom I think very very highly of, but whom you may not recall at the moment, Theodore Dunham.
Theodore Dunham, certainly. What were your contacts with him?
Quite a bit. You know that he started out as a doctor, at his father's wish, and went through a complete medical training, and only then was able to start doing astronomy. So that when I first met him, I suppose in the summer of 1928, he was working on a Coudé spectrum of… A case could be made out for including Dunham in a second eleven. He designed the superb Mt. Wilson and Canberra Coudé spectroscopes and in his own work set new standards for high-dispersion stellar spectroscopy. His investigation of interstellar absorption lines was the basis for Stomgren's later discussion.
Talking about Milne first, how would you class his work, in comparison with Eddington?
Eddington, yes. I think I have quite a clear idea about that. Eddington was a mathematical physicist; Milne an applied mathematician.
Did you have direct discussions with him?
Oh yes. I not only heard him lecture and spoke to him, in fact, we were in the same rooming house in Ann Arbor. He returned to England and I came over here a few years later, and until his death we were in almost daily contact. But he of course was off in realms where I couldn't follow him at all.
I'm interested in the contact you had with him — his personality, the way he looked at science, at astronomy — do you recall any specific discussions?
Well, I know that whenever I had a particular problem, I could talk to him directly, and he would provide an answer which may or may not have been acceptable. An answer which if it wasn't acceptable was due to my ignorance, not to any lack of insight on his part.
Would he be sensitive enough to realize that he wasn't putting enough physics into something, to allow you to understand something? Would he repeat or amplify his discussion?
Yes, up to a point. But his fundamental interest was in the mathematics, and not perhaps in the physics of the situation. But on the other hand, he could be exceedingly acute in seeing physical points. And there is a paper of his in the PHILOSOPHICAL MAGAZINE on unit mechanisms. Collisions of the first kind, second kind and of the third kind, not in the popular phraseology of the moment, and revealing real physical insight, which has not always been realized, I think, by subsequent physicists and astronomers. It's really a remarkable paper. A paper which, incidentally, lay on Newell’s desk at Cambridge for several months, before Milne could get it published. Kramer's paper anticipated some of Milne's results.
What were they?
Well, where equilibrium is maintained, so that a collision of the first kind results in the electron losing energy and the atom gaining energy, sufficient to lift it up to the next level. A collision of the second kind is where just exactly the reverse happens, both collisions bring radiation less. And if these two are in balance, then you have equilibrium. Collisions of the second kind were first discussed by Klein and Rosseland; Milne particularly saw the importance of unit mechanisms where you have photoionization and recombination.
I wasn't familiar with his paper. When was that paper?
That I can also tell you, if I step over the —
You're looking now at —
— at a particular paper.
This is a collection of Milne's papers that you have personally bound.
And which paper is this?
This is the one I'm talking about.
Ok, from the PHILOSOPHICAL MAGAZINE, 1924, Volume 47: "Statistical equilibrium in relation to photoelectric effect and its application to determination of absorption coefficients." It is something to do with opacities then.
Ok, I see, now I understand reasonably what's going on here. This is a very interesting volume, "Astrophysical Papers 23-28, E. A. Milne."
Written before I knew him; most important paper, yes. R.H. Fowler and Milne's paper on maximum absorption lines was the first one.
Yes, that's important.
You know that one, the very first one.
This is from the MONTHLY NOTICES, Vol. 83, No. 7 — "Intensities of absorption lines in stellar spectra and the temperatures and pressures in the reversing layers of stars, by Fowler and Milne." Well, in your contact with Milne, how did he feel about this paper being delayed by Newell?
Well, he was upset, naturally, perturbed about it, as we all are in these circumstances. Newell was a charming person, but perhaps a little dilatory in doing the job occasionally.
Then it was just in general that he was slow. It wasn't anything directed toward Milne?
No, not a bit, absolutely none at all.
The question arises; of course, did Milne ever talk with you about his feelings about differences of opinion with Jeans and Eddington?
Well, yes. They reached their height of course before I came to England. Chandrasekhar of course thinks that Milne's bitterness about Eddington colored all his future work, that Milne was determined to do Eddington down, and all his work was directed to that end. I think that's nonsense on Chandrasekhar's part, and based on not what Chandrasekhar himself had seen, but a story told him by W.M. Smart about contacts of Milne and Eddington. No, Milne was concerned with the job he was doing. If incidentally he thought that he did Eddington in, well, that was a bonus. But it had nothing to do with the motivation of Milne. Milne was interested in the problems, not in this personal difference of opinion.
So Chandrasekhar's opinion was based on Smart's statement?
That was the evidence submitted for the strong feelings which he claimed Milne had. Well, I think perhaps we'd better go on. I get a little bit tired as a result of this.
Ok, well, your next name?
Yes, the next one is Ejnar Hertzsprung, a Dane. I never met him. I've heard him talk. But his great contribution was that he took observations which had been made primarily at Harvard by Pickering, and Pickering's associates, and reinterpreted them. For example, the question of classification of spectra had been done by Pickering, in a completely unphysical way, so that the original classification contained C stars and D stars and E stars, and the hottest stars for example appear last on the list, in O's. This was without any physical interpretation and the only associate of Pickering's who had any idea what was happening was someone of whom you probably won't have heard, Miss Antonia Maury.
Could you give me an impression of how well Miss Maury and her work was known when you were at the Dominion Observatory in Victoria?
Almost not at all. She was just another of the female assistants at Harvard College Observatory.
After 1910, up to 1910, her classifications were not known?
The standard Harvard classification was probably in use well before 1905. By 1910 I should guess that the Revised Harvard Photometry with Pickering's magnitudes and the standard Harvard classes was if not published at least in preparation and that the Henry Draper Catalogue based on Miss Cannon's work was being planned.
Well, after the 1910 Solar Union Conference, in which your father participated, astronomers got together to form a special classification committee and to try to decide what classifications, should be adopted. Did your father, after he came back from the original meeting in 1910, talk at all about the question of stellar classification and the importance of establishing some meaning for the classification systems? Did he talk about Miss Maury, about Pickering, H.N. Russell, anything like that?
No, not to any marked extent. Classification was just a means of statistical investigation at that time, and really, Hertzsprung at that time was scarcely mentioned at all.
He was scarcely mentioned?
He was not well known either?
No, except to a few people including of course Henry Norris Russell who was the outstanding one but no, scarcely at all. I remember F. Schlesinger writing me and expressing surprise, even as late as 1933, that I was talking about Hertzsprung, saying that it wasn't generally known what Hertzsprung had done, not just what he had done but what its significance was.
Schlesinger was surprised that he wasn't more known?
Well, not surprised but at least pleased that I'd talked about Hertzsprung.
So he knew himself.
Oh, he knew, himself, yes. Schlesinger is clearly a "first eleven" astronomer.
It seems as though after the 1910 meeting, people began believing in Miss Maury's subdivisions a little more.
Well, I think perhaps not so much, but it provided a better method of classification. I don't really know but I would guess that well before the 1910 meeting, these spurious classes of Pickering and his associates had been dropped, and the B A F G K M sequence was generally adopted.
This is the feeling that you had as you were being trained in astronomy?
Oh no, not my feeling, I don't think.
Who did you have in mind?
I don't know that I had any feelings about it at all. I was just accepting what was given.
No, I don't think my father was particularly interested. My father's great contribution, his really outstanding contribution, was on the question of the effect of image formation and collimator illumination on the accuracy of line positions in spectroscopy. This was really quite outstanding. As a result of which, for example, with the 15-inch telescope at Ottawa, he was getting spectra for line displacements for stars of the 5th magnitude, which were scarcely obtainable with the 40-inch at Yerkes. And of course, the second point where my father was important was that he saw the importance of large telescopes and applied engineering principles to their design, Well, could I go on?
Hertzsprung, of course, saw the period luminosity law which had been lying in front of Pickering and his associates, completely unrealized. And of course, the effect of that on astronomy was revolutionary. And it was Hertzsprung who looked at spectra and saw, two things — first of all, what Miss Maury called C-stars, and what Miss Maury hadn't seen, was that there were line differences of ionized elements. These could be correlated with intrinsic brightness, so that spectroscopic parallaxes are Hertzspring's discovery.
As of 1905 he started working on them.
He had pointed out the possibility.
You mentioned that this work was not very well known.
No, I don't think it was.
When did you first hear of it yourself actually?
Oh, I suppose through H. N. Russell, in the first instance and because there was a meeting of the American Astronomical Society in Ottawa. Oh, I don't know when, 1910, 1911.
You heard him speak?
I heard him speak then. And I imagine he stayed in the house and I couldn't help but be overwhelmed by his eloquence.
We'll certainly get back to him when we talk about Russell and that meeting.
Right, so then, let's go on.
What do you think Hertzsprung's philosophy of doing astronomy was?
Hertzsprung? I have no idea at all. See, as I say, I never met him. He was, as far as I was concerned, a very very remote figure, and his two basic papers appeared in obscure publications, Feitschrift fus Wissenschafliehe, Photographis, which requires some digging to find, they're so obscure.
Was the ASTRONOMISCHE NACHRICHTEU available to you at that time?
Oh yes — well, at Ottawa and particularly in Victoria.
But not at Toronto?
No. Astronomy at Toronto at that time was still that of the 19th century.
I see. You did get your BA there?
I got it by the skin of my teeth, because I had volunteered for the Army then.
We'll talk about that eventually. So you learned first about Hertzsprung's work through Russell's talk?
Oh yes, definitely.
Did you follow Hertzsprung's work through the twenties and thirties?
Not at all. It was Russell who made Hertzsprung known to the astronomical world — of course, through the spectacular idea of giants and dwarfs, which is almost irrelevant.
Why is it irrelevant?
Well, because this is a selection phenomenon. There are darned few giants about. Most of the stars are on the Main Sequence. Well, then, what lay behind Hertzsprung's work of course was the next name on the list, Norman Lockyer, who was also a somewhat disregarded figure during his later life. True, he discovered helium. He had with Janssen observed prominences without a solar eclipse, and had realized that atomic spectra revealed differences between ions and neutral atoms, and he actually classified them — you find him talking about silicon I, silicon II, silicon III, and silicon IV, which are just spectra of the same atom only under different degrees of ionization.
When did he talk about this?
Norman Lockyer? Well, almost his whole work was this. He was primarily a laboratory spectroscopist.
But he used the terms "enhanced" and "proto elements"?
When did he actually use any numerical designations?
In his publications.
When did you first read Lockyer?
Oh well, I never read him very much, actually, but knew about him of course, because I was in A. Fowler's lab for a few months after the end of the First World War.
I didn't know that, I hope that we'll talk about your experience with Fowler.
Very good, he's the next one on your list.
Yes, he's the next one on my list, and of course, he carried Lockyer's work on. He was a pupil of Lockyer's, actually his assistant, in a small lab in the Imperial College.
That was the lab that Fowler had built himself?
Well, yes, but it was Lockyer who started the thing, and where Lockyer did his preliminary work.
You mean the lab at South Kensington became the Imperial College?
Well, it was part of the Imperial College. Lockyer's work until his retirement was carried out under the aegis of the Solar Physics Committee (possibly financed by the Indian Government) at the Royal College of Science, South Kensington. Here there was astronomical equipment as well as a spectroscopic laboratory. On Lockyer's retirement in 1901 the astronomical equipment was transferred to Cambridge where the Solar Physics Observatory under Professor H.F. Newall was established. With the equipment went, I believe, two of Lockyer's assistants; F.E. Baxendall and Butler (?). At Cambridge Newall, a physicist had already brought a 26-inch refractor. This transfer of Lockyer's astronomical work to Cambridge was probably warmly contested and even as late as 1919 Fowler was still regretting the loss of the telescopes.
No, scarcely at all. But it was clear that this was where Fowler's work had all started. And then of course, Fowler went on dramatically to reproduce helium 4686 in the lab, an outstanding event because it fitted in so beautifully with Bohr's atom and stationary states and so on.
Fowler was a very interesting person, I think. First of all much of his early work was published by Lockyer as Lockyer's own.
I do not know but have seen it suggested.
He never talked about that kind of situation?
Never at all, no, never at all.
Did he always seem to be very fair to his assistants?
Oh, absolutely, yes. I simply have no idea about Lockyer. Of course Lockyer was probably somewhat peculiar and got the backs up, I think, of many of his contemporaries. He was you know, the founder and first editor of NATURE. A great man in his own peculiar way.
What was Fowler's technique in the lab? How did he go about doing things?
Oh, very very quietly. There was nothing dramatic about Fowler at all; just honest, self-critical, persistent work — the only kind that I can really admire.
When were you working with him, just after the war?
Well, yes, after the Armistice. There was a so-called Canadian "Khaki" University set up, and to it thanks to my father, I was seconded from the Army. I chose to go with Fowler, and he allowed me to take some laboratory spectra. I didn't do anything; I'm a damn poor experimentalist, and I just absorbed the atmosphere.
Did he give any lectures?
Fowler at that time was probably working on his Physical Society Report. The only lectures I heard while I was there were lectures by the second Lord Rayleigh, the great Lord Rayleigh's son, which I could understand, more or less. Lectures by Comrie whom you won't probably know (an applied optician) for the rest, scarcely anything at all. Fowler didn't lecture. But what he did for me was to talk about Rydberg, and showed me how to apply Rydberg's method to find spectroscopic terms a thing which he later published, after I'd returned to Canada, in his PHYSICAL SOCIETY report on series in spectra with a table at the end that enables you to work out spectral terms and series via Rydberg.
This was about 1921 that he published this?
Yes, I think so, yes.
You left for Dominion in 1919.
Yes. I was back in Canada in July, 1919, yes.
Did you continue corresponding with Fowler at that time or with any others?
No. Not really. I asked him to send me a photograph and that was about the extent of it.
Did he do that?
He did, yes. The only time I've ever gotten a [???] fan photograph or indeed asked for one. Well, now the next name — Lockyer, Fowler — and then we have Saha. We have M.N. Saha. Did you have any contact with Saha?
Not at that time, no, didn't even know he existed. He must have come to England just as I was leaving, in fact, when I was back in the army again for a month or two. So I didn't really hear about Saha until I saw Russell after the war. Saha was only incidentally an astronomer, but a great thermodynamicist; it was in Fowler's lab and with his advice that Saha applied thermodynamics to astronomical spectra. Well, now then, this is where Russell comes in. He knew about and appreciated Hertzsprung. He realized what Lockyer had been doing and fully understood Saha.
Could you go back to the beginning, to your first contact with Russell?
When I returned to Canada, my father got me an appointment at Victoria. And so I went straight out there, and started measuring velocities of stars, in collaboration with my father, Harper and R.K. Young. I suppose Russell, was a peripatetic astronomer and in the course of his journeys must have come out to Victoria some time. I simply can't remember.
You say you heard him speak in Ottawa.
Oh yes. Oh yes. And of course, I knew all about Russell and what a stimulating person he was.
What were your first impressions of him, do you recall the speech, the lecture?
Well, of course, enormous enthusiasm. Russell had to contend, of course, with a very different view of astronomy which was held by the great doyen of astronomy at that time, W.W. Campbell at the Lick Observatory. Campbell had a very different view, a quite legitimate but mistaken view, about what was the physical development of stars.
What was Campbell's view?
Campbell's view was determined purely by his measures of stellar radial velocities, and so, he thought of the physical development as the development as a function of velocity. And some attempt to relate these velocities to the adopted spectral classification of the time.
Yes. J.C. Kapteyn also had that idea.
Oh yes. Well, now, you see, Kapteyn was another person who, following Herschel, saw the solution of astronomical problems in a statistical approach. And it was a wonderful idea, but it's difficult to get representative samples of stars to apply the statistical approach. The Victoria radial-velocity program was carried out in Kapteyt’s so called "selected areas.” Apart from the radial velocities themselves the program itself made little contribution to astronomical knowledge.
Did you realize this at the time?
I suppose, in a vague sort of way. Certainly there was no enthusiasm on my part.
Going back to the 1910 Ottawa meeting, what do you recall? Was W.W. Campbell there too?
No, I shouldn't think so.
Were you aware that Russell and Campbell had very different ideas?
Very vaguely, at that time. That came very much later. Russell in 1921 was given the Gold Medal of the Royal Astronomical Society, and he made a speech at the Royal Astronomical Society Club, in which he talked about the "Lick Observatory point of view."
Is that speech recorded anywhere?
I'm afraid not. Probably only in my memory and from Russell's remarks on a visit to Victoria. It may be. No, I doubt it very much. There is a record of the meetings of the Royal Astronomical Society Club, but I doubt very much whether anything is said there.
What did he say about the Lick point of view?
He sort of laughed at it. And this leaked back to the Lick Observatory, and Russell got a rather chilly welcome on a subsequent visit there.
Did you remember anything specifically that Russell was talking about?
Well, of course, following Russell's visit or during his visit to Lick subsequent to his speech at the RAS Club, I don't know whether you remember, they saw a very bright object in the setting sky, and this brought some reconciliation of Campbell's and Russell's points of view.
Really? I recall reading about that — a bright star like object that appeared just after —
Yes, just after sunset.
But they really couldn't figure out what it was? I was thinking they were seeing Vulcan for the first time. Now, how did this reconcile them?
Well, I think just because they were thinking about the same thing, about which they couldn't disagree, at least.
You mentioned, Russell stayed with your father or stayed in your house?
At the 1910 meeting certainly. Well, I won't guarantee that at all, but certainly he was there for meals.
What was the dinner conversation like?
All Russell, pure and simple. No, he was an incredible person to talk to. I can remember talking to him at Harvard. He would come and sit down on the other side of the desk, and talk about his own work, on and on, and —
This is Cambridge Mass?
Yes. I could understand little bits of it, here and there, and found it fascinating. He would go on without a pause, and then, there would come an opportunity in which I could raise a question myself and say something about what I was trying to do, and he would listen for a moment or two and then just drop off to sleep.
Really? How did you feel when he did that?
Did you wake him up?
No, no, not a bit; I never minded at all. No, he and Mrs. Russell stayed with us, for a week or so, on a visit to England. I can't remember when, somewhere around 1934, 1935, something like that. He would talk to himself while he was shaving. He would just go on talking and talking. Then, occasionally Mrs. Russell would say to him, "You stop talking; now I want to say something." She was the only one who could ever dominate the conversation for more than a minute or two.
You know the story, after he'd been with us, the Russell’s went and stayed with Eddington in Cambridge, and presumably the same pattern of behavior persisted. And the only thing that is known is that, after the Russell’s had departed, Eddington was found lying on the sofa, completely exhausted. Quite apart from his own work Russell on his frequent visits to observatories throughout the States did much to introduce American astronomers to recent ideas on line spectra and their interpretation.
When he visited you at Harvard, it must have been just at the time that he was working on solar atmosphere.
Yes. That of course was vital because from pretty primitive material, he had deduced the great abundance of hydrogen, which no one had suspected. His greatest work of course, was all he did on analysis of laboratory spectra and Russell-Saunders coupling.
Would you say that he was an inspiration to you, to get back into solar studies?
No. I don't think so. I was always more interested in solar than in stellar physics. Russell had a prodigious memory. He would be taken to symphony concerts, and he would remember enough of the frequencies of various stationary states to be able to find which lines arose from transitions between the two.
While he was at the concert?
While he was at the symphony concert.
Were you ever with him when he did this?
No. But he told me so. At least, music bored him and this is what he did.
Did he make notes on backs of programs?
No, I don't think so. He had an incredible memory.
That's very interesting.
And of course he really knew physics. At least, I don't think astronomers around 1910 were thinking about physics at all.
But he was.
Do you recall his various discussions? Did he get the other astronomers to see the importance of the use of physics or what?
I don't think he was ever trying to influence astronomers. He was interested in the problems. They could follow along or drop off, as they saw fit. No, it was the problem that interested Russell. Russell's contribution to astronomy is perhaps most clearly shown in a number of papers in the Proceedings of the National Academy of Sciences. These included a most illuminating discussion of interstellar absorption in our galactic system, the stellar origin of the luminosity of the gaseous nebulae and a balance review of the controversy between Shapley and H.D. Curtis on whether the external galaxies were or were not part of the Milky Way system.
In 1910 do you recall his using the terms "giants and dwarfs?”
Oh yes, I think so.
Did he qualify the use of the terms at all? Did he explain what he meant by them?
Oh yes, he explained what he meant by them. Well, we've covered Russell and Saha.
I have a question about Russell's theory of stellar evolution. Did he talk about this to a certain extent in Ottawa?
Oh yes. Yes.
How did you feel about that different kind of a theory?
Well, I didn't know enough about the current theory. Now then, the last three names in the First Eleven start with Schlesinger. He is I think really one of my great heroes. I think his work on photographic determination of stellar parallaxes was a model of astronomical observation. The George Darwin lecture to the Royal Astronomical Society, at the time when he was given the Gold Medal of the Society, gives a very good idea indeed of just how thorough and far seeing that work really was. In an unobtrusive way he had a marked influence on American astronomy. A really great man and the sorts of things that he did unobtrusively were remarkable.
Oort's work on galactic rotation was originally unknown to my father until Schlesinger drew his attention to it.
— this was during a meeting they had?
I think, just a letter to my father, and as a result of that, the work that my father was doing on the radial velocities of O and B types of stars could be immediately applied to the subject of galactic rotation.
That's a very interesting thing to know. Schlesinger was very active in the astronomical community?
Oh yes, very much so. Not in the sense that Russell was. Russell was going around talking to people. Schlesinger was just quietly working away at New Haven, and influencing people through the astronomical club at New Haven.
The Astronomical Club?
"The Astronomical Neighbors" was I think the name. "The Astronomical Neighbors" was possibly regarded by its founders, Schlesinger from Yale, Russell from Princeton and Shapley from Harvard, as something of a counterbalance to the enormous influence exerted at that time by the Pacific Coast astronomers. As far as I know the meetings were always held at Yale with Schlesinger as host and consisted of a dinner given by him at some University Club followed by a quite informal chat on astronomical topics. Apart from its founders the regular members were E.W. Brown of Yale and B. Boss of Albany (son of L. Boss). The meetings were held probably once a year and occasionally attended by guests e.g. my father, and Rosseland and myself, brought by Shapley and then occasional visits, when my father happened to be in the East at the time.
But you had never gone to these meetings?
Yes, when I went to Harvard, Shapley took me two or three times. It was a very very quiet meeting.
Schlesinger was very quiet?
Oh yes. Great man, though: really great man. Well, then, of course, the next names on the list are J. H. Oort and Edwin Hubble. No question about that at all. Hubble always reminds me of H. A. Rowland. Do you remember the story about Rowland, who was called as an expert witness in some trial? In the course of cross-examination, the attorney said to Rowland, after having heard his testimony, "Well, Professor Rowland, who is the greatest physicist in the world?" Rowland said, "I am, I suppose." And later Rowland's friends rather questioned this statement, and Rowland's reply was, "But I was under oath." Like Rowland in physics, Hubble had every reason for taking pride in his contributions to astronomy. For these were indeed outstanding. His resolution of apparently gaseous nebulosity of the Andromeda Nebula into stars and the identification of some of these as long-period Cepheid’s was perhaps the first time that direct photographs with the 100-inch at Mt. Wilson had been effectively used for other than taking beautiful pictures (Ritchey, Duncan). Hubble's use of the period-luminosity law and his discovery of the effect of interstellar absorption in our galaxy on the apparent distribution of the external galactics are too well known to need further mention. What however is sometimes forgotten is Hubble's beautiful work both before and after he came to Mt. Wilson on gaseous nebulae in our galaxy. The relation between the extension of nebulosity and the magnitude of the skillfully identified associated star provided the basis of Zanstra's theory. Hubble was perhaps the most consummate interpreter of apparently quite intractable observational material.
Did he? Did Hubble? You called it the Hubble-Zanstra effect?
Yes. H. Zanstra did an equally incredible job in making a bold hypothesis that would relate this to photoionization.
Most people simply refer to it as the Zanstra mechanism.
Yes. But he worked it out quantitatively, and showed, of course, that the temperatures of stars which he had to assume in the exciting star coincided with the generally accepted views of stellar temperatures in general.
You had done some work showing that there was a discrepancy between the color temperatures that you derived from Andromeda?
Oh yes, yes, that was subsequently. Yes. And of course, I determined intensities of lines in nebular spectra, and showed that they were not consistent with a purely excitation mechanism but more consistent with the idea of recapture rather than excitation.
That was very important. This was just about at the time that Bowen was working on his fluorescence mechanism.
Yes. Well, now, Zanstra was first, because he set out the whole physical picture, but this only applied to diffuse nebulosity and it only worked because he neglected the relatively small amount of radiation that was coming from forbidden lines. Bowen's great contribution, of course, was that he picked out the mechanism from his laboratory spectra. He showed that forbidden lines arose from transitions between states which, on current theory, were forbidden.
I'd like to have your recollection of your feelings about reading this paper by Bowen, and saying, "Well, gee, this seems like a violation" — did you think it was a violation of physics or what?
Oh no, no, I didn't think that. I think the general view, at that time, was very well expressed by Eddington in a short paper in MONTHLY NOTICES, around 1932. It [Bowen's work] was a revelation at the time, and it was just then that I was completing some work on Andromeda, and it fitted in very nicely, in one sense. Then clearly [to follow my list] Unsold and Stromgren: Unsold following straight on after Schwarzschild and using the idea of dependence of the line absorption coefficient on frequency, which Schwarzschild had seen, and applying it to observation. His whole physical approach is summarized in the first edition of his PHYSIK DER STERNATMOSPHEREN. And Stromgren [is on the list] because he carried this a great many stages further, and restored Russell's abundance of hydrogen, both for its effect on theories of stellar interiors, but more particularly during the Second World War, for his model atmospheres, which was a revelation really at the time.
I see. Was there a period of time in the thirties when Russell's abundances weren't adhered to?
Oh yes. Unsold did not originally accept them.
I didn't realize that.
Oh no. In a perfectly legitimate way Unsold arrived at a very low abundance of hydrogen, in fact, from the jump at the Balmer discontinuity. And it was Stromgren, in a paper in a Festschrift for his father E. Stromgren, but then it was summarized in a paper which I have, which I perhaps had better give you the title of.
"Tables of Model Stellar Atmospheres," B. Stromgren Copenhaget; J,1944. From the Danske Widertskebrues. But the paper is in English, strangely enough.
Yes, fortunately for me.
So this was a review paper, and this again is in a personally bound volume.
It's bound up with some papers of mine.
Yes, very interesting, well, I hope to be able to get that.
Ok, well now that is Unsold and Stromgren.
All right. We've already talked about Zanstra and Bowen.
Have you had direct contact with Bowen?
When I was at Mt. Wilson for a month or two, I just saw him. I think this was before he'd actually published his paper so I had no idea, or if any idea, the vaguest idea, of what he was doing. As I say, we now come to Shapley. And Shapley, as I say, is a most interesting case. He produced an incredible hypothesis, with almost no data at all.
Of the next three names it is at least debatable whether Shapley should not be included in the first rather than the second eleven. For on the basis of Bailey's discovery of Cepheid variables in seven (?) globular clusters, Shapley found their distances and assuming that all the known globular clusters. Some 100 in all were similar in size and structure, found their spatial distribution. On the startlingly bold hypothesis that the spatial distribution of these relatively rare clusters delineated the structure of our galaxy Shapley found its size, shape and the eccentric position of the solar system — a hypothesis fully and surprisingly confirmed by Oort's investigation of galactic rotation. The heliocentric galaxy of Herschel and statistical astronomy was thus ended and a new era in astronomy begun. In spite however of Shapley's brilliance and originality I do not think either at Mt. Wilson where this work was done or elsewhere that his later work, apart from Shapley-Ames Catalogue, was ever fully accepted. This was probably the consequence of Shapley's somewhat uncritical use of observations both by himself and others and of his failure, in spite of Russell's clear warning, to realize that absorption within our galaxy is not necessarily wholly selective.
Under the direction of E. C. Pickering Harvard College Observatory with its outstations was probably in the early years of this century the first observatory in America if not in the world. Pickering's stellar photometry, objective-prism spectroscopy and the initiation of the sky patrol were responsible for making it so. In the early 1920's when Shapley succeeded Pickering as director the staff consisted of S.I. Bailey, E.s. King and a number of women assistants including notably Miss A. C. Maury and Miss A. J. Cannon. To them Shapley added W.J. Luyten with his proper-motion program based on Harvard plates and Cecilia H. Payne, a graduate student of Eddington, who applied Fowler and Milne's line maxima to Harvard O. G. spectra to write a notable Monograph on Stellar Atmospheres. Shapley also invited for period of a year or more a number of visiting astronomers among who were Rosseland, Gerasimovic, Opik, ten Bruggencate, Bok and others. Harvard, that under Pickering, had been something of an absolute monarchy, or so I suspect, became under Shapley's guidance a band of enthusiastic workers.
Of Trumpler and Baade, Trumpler earns his place in the second eleven for his work on open clusters, the establishment of absorption within our galaxy that with Hubble's earlier investigation enabled Bok to show why statistical investigations had failed to discover the eccentric position of the Sun in the galactic system. Baade's discovery of supernovae, resolution of elliptical nebulae into stars and the recognition of two stellar populations puts him like Hubble in the class of consummate observers.
The last four in the second eleven earn their place because of the improved observational facilities for which they were responsible and for their application to basic astronomical problems. Among them easily the first is George Ellery Hale. Financed by his father Hale designed and built the Kenwood Observatory in Boston and there took the first steps towards the realization of the spectro-heliograph. Next appointed Associate Professor of Astrophysics in the University of Chicago he was primarily responsible for the founding of the Yerkes Observatory with its 40-inch refractor, the perfected spectro-heliograph and the early states in the figuring of a blank (purchased by Hale's father) by Ritchey for a 60-inch reflecting telescope. At Yerkes Hale built the horizontal Snow Telescope with its fixed high-v dispersion spectroscope that was later transferred to the Mt. Wilson Observatory of the Carnegie Institution. With this equipment Hale showed that the widened lines found by Young sunspot spectra were due to Zeeman splitting, the result of magnetic fields in these spots. Two solar-tower telescopes were built at Mt. Wilson. The 60-inch reflector completed and finance obtained by Hale for the 100-inch Hooker reflector. A nervous breakdown in 1910 compelled resignation of his directorship of the Mt. Wilson Observatory but did not wholly stop his solar investigations that were continued with the assistance of Dunham in a privately-built observatory in Pasadena.
With the 36-inch refractor at the Lick Observatory W.W. Campbell made the first accurate determinations of stellar radial velocities. A comprehensive program of such velocities for both the northern and southern hemispheres was completed by him. For the southern station of Lick the first flexure-free spectroscope was designed and built by W.H. Wright whose indispensable assistance was responsible for much of the success of the Lick program. Wright also designed and built a quartz spectroscope for the then antique Crossley reflector at Lick and with it made a detailed study of the spectra of planetary nebulae including monochromatic photographs of these objects that 60 years later are still scarcely surpassed.
V. M. Slipher assisted Percival Lowell in his observatory at Flagstaff, Arizona. Lowell's primary interest was Mars and to Slipher was entrusted the task of spectroscopic detection of oxygen in its atmosphere. This was also a problem that was tackled at the Lick Observatory; neither investigation was conclusive and Slipher was left free to design and construct spectroscopes of high luminous efficiency. With them he obtained the first spectra of the faint external galaxies and discovered their high velocities, the procurers of the subsequent velocities of Humason and of Hubble's interpretation.
Unlike others in the two elevens my father, J.S. Plaskett, was a self-trained astronomer. With the death of his father Plaskett and his brothers ran the family farm until at about the age of 16 he was able to follow his natural bent as a mechanic; apprenticed first in Woodstock, Ontario and subsequently with the Edison Electric Company in Schenectady and the Canadian Edison Company in Sherbrooke, Quebec. Subsequently he was employed as the mechanic in the Physics Department of the University of Toronto. While so employed and studying at home he matriculated and with permission of professors in the Department was able to attend some university lectures. At the age of 34 after some years of private study on top of his full-time work as mechanic in the Department he passed the necessary examinations in mathematics and physics and was awarded the degree of B.A. Always deeply interested in optics and photography he worked in his spare time on the correct rendition of colors using so called orthochromatic emulsions. In 1903 he applied for and was appointed to the staff of the Dominion Observatory under the directorship of W.F. King. His first astronomical work was planning equipment for a joint Canadian-British expedition to observe in Labrador the solar eclipse of 1905. Because of clouds this expedition was unsuccessful but Plaskett made use of the coelostat to build a horizontal, strictly achromatic snow-type telescope for solar investigations. This instrument was used by him for a long series of determinations of solar rotation that had been inaugurated at the 1910 meeting of the International Solar Union.
Three achievements perhaps merit the inclusion of J.S. Plaskett in a second eleven. Firstly his investigation of the optics of the Brashear spectroscope attached to the 15-inch refractor of the Dominion Observatory and the effect on its performance of a new corrector lens designed by him. Supplementing this equipment with a Wright-type flexure-free single prism spectroscope he determined the orbits of a number of spectroscopic binaries extending this work down to stars of the 5th magnitude, a luminous efficiency scarcely attainable at that time with the Yerkes 40-inch refractor. Secondly his successful effort to persuade the Canadian Government to finance the building of a 72-inch reflector ultimately erected at Victoria, B.C., at the end of World War I. This instrument was designed and built by the Warner & Swasey Company and incorporated for the first time at my father's suggestion ball bearings to take the thrust of this massive telescope. Finally should be mentioned his successful use of this instrument notably in the determination of the radial velocities of 0 and B-type stars and the resulting application to galactic rotation.
 A list of astronomers prepared by Plaskett
 A. Fowler. Report on Series in Line Spectra (Physical Society of London, 1922)