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In footnotes or endnotes please cite AIP interviews like this:
Interview of Milton Humason by Bert Shapiro in circa 1965, Niels Bohr Library & Archives, American Institute of Physics, College Park, MD USA, www.aip.org/history-programs/niels-bohr-library/oral-histories/4686
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Focusses on Humason's interests in observational astronomy. Origins and extension of velocity-distance relations in the 1920s; his work with Edwin Hubble, Walter Baade, Alan Sandage and Nicholas Mayall. Own spectroscopic work. Comparison of observing conditions at Mt. Wilson Observatory and Mt. Palomar Observatory; the most distant object observed at Mt. Wilson; living conditions and relations between astronomers at Mt. Wilson. Description of life since retirement.
The velocity-distance relationship started after one of the IAU meetings, I think it was held in Holland. And Dr. Hubble came home rather excited about the fact that two or three scientists over there, astronomers, had suggested that the fainter the nebulae were the more distant they were and the larger the red shifts would be. And he talked to me and asked me if I would try and check that out. Well, our trouble was that our spectrographs were extremely slow — that was back in about 1927 or ‘28. We had prisms in the spectrographs then and they were made of, a lot of them, of yellow glass which didn’t let the ultra-violet light through and the exposures were extremely long. I agreed to try one exposure, and as I remember it lasted over a period of about two nights. I exposed the plate for two nights and got one of the brighter nebula whose velocity wasn’t then known. Of course Slipher had taken the brighter ones — you’re probably aware of that in Flagstaff and he had velocities of some — I think — 25 nebulae that he’d gotten in Flagstaff or red-shifts. Some of them were negative, some of them positive, and the largest one was about 1800 kilometers per second.
So I took this one and I got a very poor spectrum but enough to know that the velocity turned out to be over 3,000 kilometers. Right here, here I am I don’t know what that was. I could get it for you but, well, it turned out to be over 3,000 kilometers which was quite exciting at that time. And they wanted me to continue with that work. It wasn’t very happy kind of work because it took long exposures — I mean exposure ran into hours and hours and the objects were hard to see on the slit of the spectrograph and the spectrographs were slow but Dr. Hale was still alive at that time and he got quite excited about it and called me down to the Hale Laboratory in Pasadena there, and talked to me, and told me if I ever was willing to go ahead with it that he’d do everything he could to see that we got a faster spectrograph. Well, Dr. Anderson who was at Mount Wilson at that time and had not yet, I think this is correct, had not yet taken charge of the 200-inch down there but was at Mount Wilson, and he got interested in the problem of a faster spectrograph. He worked on the design of a lens in his office there and came up with a modified design something on the order of a microscope objective.
Our spectrograph and camera was built along the lines that he designed and it turned out to be very fast — at least we thought it was fast compared to what I had worked with before. So it cut the exposures down to 4 and 6 hours — these same things that were taking me two to three nights to get before. Well, we went ahead from there and for a long time I worked with that camera and that spectrograph and got some probably 80 red-shifts and as predicted they did increase. The red shifts increased or the velocities got greater as the nebulae became fainter. In other words the indication was they were more distant. And then Don Hendricks was in the optical shop at that time and the first designs of the Schmidt telescope had been made and used as a telescope but Hendricks decided it could be adapted to a spectrograph — to a camera lens for a spectrograph. So he went along after a lot of argument about whether he should do it or not but Dr. Hale felt as though he ought to go ahead on it. He designed a camera lens which we called the Schmidt lens or Schmidt spectrograph and again we made a big step forward in the speed — I mean then we were able to go to extremely faint nebulae and the largest velocity I got on Mount Wilson with that camera turned out — just as the two hundred inch started — on the order of 46,000 kilometers per second. Then, of course, Dr. Hubble did the direct work on these objects, measured their brightness, and if that were plotted against the red-shifts, well, then you got a linear relationship between the red-shifts and the brightness or magnitude of the nebulae. And that’s the way it started. Well, of course, the first one was.
Could you describe that?
Well, I don’t know how, I don’t remember the NGC number of that anymore and I don’t remember the exact velocity, I can get it.
Well, just describe your feelings about it, if you could.
Oh! Well, something like Byron, I mean I’d been working in spectroscopy doing spectroscopic work for years and you develop a plate and you grab a magnifier and look at it and there it is. All I can say is that I was a little surprised and quite happy to see that it did have a large red-shift and it was shifted to the red and not to the violet and so on, and then as we went on, all of them were shifted to the red. And it turned out then that only the very near nebulae have a negative shift that is to the violet like the Andromeda Nebula — the very close ones where it doesn’t act on those that are too close to us. But after you leave those why everything — there has never been a single instance when they didn’t go to the red and always kept on this linear relationship — stayed right on there. I called Dr. Hubble up the next day and he was very happy about it and anxious to see the plate. When I came down he took a look at it and then started talking about doing more and that’s when I was undecided whether I wanted to go ahead with it or not because of these tremendously long exposures. But I finally did, and I enjoyed very much working with Dr. Hubble — for one thing — and as I said, the largest velocity we got at Mount Wilson was about 46,000 kilometers per second.
Then we went down to Palomar — it jumped to, I just have to get my stuff out of there, I guess. (break) Ordinarily with the old spectrographs one used rather small plates but still large enough so that they could be handled easily and developed and all. As you went to the higher speed cameras and spectrographs, especially the Schmidt, the plates had to become smaller, and they became very small until I think the plate I used on Mount Wilson — the size of the plate that was used on Mount Wilson there with the Schmidt spectrograph was of the order of about 8 millimeters by 8 millimeters square. Or 10 by 10 maybe. They became very difficult to handle. I mean, we had to develop ways to take care of those things in the fixing bath and the developer. If you let loose one of those little things [with] your finger [it] is liable to flop right over and land on the film side and stick to the bottom of the tray. So we made wire holders for them and slipped them in the wire holders first and then they were never taken out of there till they went through the fixing bath and through the wash and dry. Then we pasted them on glass slides so that they could again be handled all right. Those things were then put on a measuring machine in Pasadena. The measurements were made down in Pasadena.
How did you measure one? Could you describe the method of me measuring?
Well, I mean, you measured it [laughter]. You look in the microscope and of course on each side of the nebular spectrum you have a comparison spectrum. And the spectrum we used was helium because the dispersion was so short that any other would crowd up so you couldn’t separate the comparison lines. So on each side of the nebular spectrum was a spectrum of helium put on there. Then we measured the lines in the nebulae with respect to those helium lines whose wavelengths were known and the distances were separated from those would give you the velocity. Probably no difficulties with the cameras except the first ones were slow and we gradually got faster and faster cameras and spectrographs. The problem at Mount Wilson was the Cassegrain platform was very uncomfortable and your feet were on cold iron all night long there and the position would get extremely awkward at times. They will get your eye up to the eye piece of the spectrograph. It became much easier when we went to Palomar and went up in the cage there where you had the spectrograph right in front of you all the time. All you had to do is bend over and look in the eye piece. [Gap] Oh, he at the very beginning, he took a few spectra with the old cameras the slower ones. He was very anxious to get more or get them fast, if you like, to see how far out we could go and whether that thing stayed linear. The relationship stayed linear and he took quite a number of spectra in there at the beginning and there we used…
But did the velocity distance relationship have any meaning to you or special significance. I mean how did you feel about the expansion theory?
Well, I of course, I’m an observational astronomer, I mean not a theoretical one, but I was greatly interested in knowing the theories that were put out about the expanding Universes or whether the Big Bang thing, and all that business. But I knew nothing about that end of it myself and I have always been rather happy that my end of — my part in the work — was, you might say fundamental, it can never be changed — no matter what the decision is as to what it means. Those lines are always where I measured them and the velocities if you want to call them that or red shifts or whatever they are going to be called eventually, will always remain the same. Pat McDoward who had a lot to do with the… You’re not taking this down?
Yes, I am.
Oh no, no, not this (laughter)
You don’t want to record it.
Whatever we are talking about.
We are talking about the lines, about the …
Oh yes I know, well, the position of these lines would never change. Now that wasn’t true of the other type work — the direct work which Hubble did and other people did in measuring the magnitudes of these objects because all of the distances were based on the Cepheids in the Andromeda Nebula in the large and small Magellanic Clouds. And it wasn’t long before Dr. Baade came up with the so-called Population II type stars which changed everything. I mean it made the magnitudes of distant things a little different but their distances became somewhat different and that thing is still changing today. I mean as they refine the work which they are doing down there now [which is] mostly electronically, I mean measuring now instead of on photographic plates which had a rather large error. Your measurements had a rather large error. Why they’re doing it electronically and they get a reading and it is pretty accurate, it goes into the hundredths of a magnitude instead of around a tenth of a magnitude.
Now those hundredths become very important when it has to do with this velocity red-shift problem and Sandage has been working on that a lot. And Dr. Baade did before he died. Well, that’s about the story. I mean what I started to say was that I was always glad that my work could not be changed although one doesn’t know what they’ll eventually come up with to explain the red-shifts. Well, one of the problems up there was a small vibration in the hundred-inch telescope for instance. That was because of the fact that it floats in mercury and the clearance between the mercury and the outer tank in the float were extremely small because the mercury became very costly and to put a large bunch in there meant quite a bit of money. In those days we didn’t have the money they seem to have to throw around now and so the clearance was small and there was a sort of, you call it a fulminate [foam] that forms on the top of the mercury, that whitish stuff that forms on there, and it would build up and roll up, and it would cause this vibration in there. Sometimes it would be very exasperating and almost cause us to stop work at times. Well, we eventually took part of the lining out of there and it increased the space in there. It helped to do away with that.
Then, the mercury was extremely hard to hold, to contain. It always leaked and I expect that underneath the hundred-inch today there’s almost a mercury mine under there because it always goes down, mercury does. It gets out, it just starts working clear down, it goes through the dark rooms, the lower floors and into the dirt. And we were always sweeping it up to try to save it. Then we had, of course, slower plates then and Dr. Mees of the Eastman Kodak Company was a great help and almost an inspiration in his interest in astronomy. He did everything he possibly could to speed up the plates for us, to make the kind of plates that we wanted, and that’s when other people first started getting the red sensitive, or the color sensitive plates. Up to that time they had to be dyed up there on the mountain. We used to dye it. Eastman started making those and they continued and always worked closely with the astronomers. Sometimes the results have been good, other times they haven’t.
What was Dr. Hubble’s response to the shifts, to the redshifts?
Well, he was greatly excited about it and very happy and went ahead of course. (pause) I don’t know what to say now…
Did his expansion theory rest on the early observations or did it build slowly as you gave him more data?
Well, I don’t want to record this now. I mean can I say a word or two? Don’t record it. [Break] Dr. Hubble of course was greatly excited and interested in the whole thing and he kept that way up until the time of his death and there never was a time when I was on the mountain and came down, and the first thing I could expect was a knock on my door, and he’d come in and want to know what I’d gotten up there and want to see them. He’d always be pleased with what I had and could hardly wait to get in and plot these along with the others to see if they still continue to stretch out there. But that continued up till the time of his death. He wasn’t well during the last few years there. In fact [he] couldn’t do very much at the two-hundred inch in the end because of his health.
Was there any difficulty in identifying the lines on the plates?
No, no difficulty as far as I went, that’s because I used two well-known lines, the H and K lines of calcium. They are rather strong lines. They are close together but separated, and it’s very easy to identify them, first by their separation, their own separation, and second, by other lines. If they were H and K then you’d expect to find another line like H Gamma or H Delta in a certain place. I don’t think ever, in other words, we never made a wrong identification, or in other words, got a velocity that wasn’t right. If we’d wrongly identified the lines then our velocity would have been off and that never happened.
Do you remember the relationship between the measuring machine and kilometers per second?
No, oh that’s a kind of complicated thing to know, I mean, no, I couldn’t tell you that now. I wouldn’t have the material here to look it up. Now, Dr. Bowen can tell you that, maybe. He’s down there with his log tables and everything, he could grab those and…
I just wondered if you had a rule of thumb or a simplified [method]. Just way of checking immediately.
Oh, no, no, you measured them and all you had down was the readings from your dial on the measuring machine. You gave that measure to one of the girls there — a computer — and she went up and there there was a formula that she’d use to convert these measurements into red shifts or into velocity per second. Then she would bring it back and they’d all be converted and every line you measured ought to agree or else it wasn’t right. I mean it ought to approximately be in agreement.
Could you describe the taking of the most distant one? What was the most distant one you took? You mentioned 46,000 kilometers.
That’s at Mt. Wilson.
That’s at Mt. Wilson, yes, that’s what I was particularly interested in. Can you remember the taking of that particular line?
Well, yes, I do and that one, the red shift on that one, was never published because I was not sure of it and would not agree to it. Dr. Hubble wanted to publish it because it was a large one, it put it out to the end of the relationship at that time — the distant end. But I wasn’t sure of it at all and the very first thing when the spectrograph was finished at Palomar, the first thing I did was take that nebula down there, and without any trouble at all, I got a nice spectrum of it down there and that was correct, the one at Mt. Wilson. But we never published the value that I got from the Mount Wilson spectrograph.
How far did you go with the 200 inch telescope on this velocity distance relationship?
Well, I went out to, again I’m guessing, to 72,000 kilometers per second, I’d have to look it up. I don’t see how you’re going to use this, I mean that might be wrong. Are you going to check these values? The Lick velocities are in there also, of course. Mayall was at Lick at that time when we wrote this paper. We decided it would be handy to have them all on one list so that they could be used by other people, rather than having to look them up on several lists. So we published together and Alan Sandage did the theoretical work at the end there and the photometry, that is, the magnitude work. Except that he took some of the results that Hubble had already obtained, used some of those. But they were refined because that was after things changed a little bit. We thought those were pretty good but they turned out not to be good enough because, as I say, just a few hundredths of a magnitude make quite a difference in that relationship out at the end there. Now, this velocity that I mentioned, say 72,000, isn’t anything to what they are getting now. All that occurred since I left there and I don’t know the first thing about it except I’m amazed at it. I wouldn’t know what to think of it all and I think a lot of other people don’t either. I mean these star-like objects that they are getting that huge red shift from. Way beyond whatever I’d ever gotten.
Quasars? Quasi-stellar radio objects?
That’s right, but there again you have three to four different theories about it by different people, the Burbidges, people in England, people over here, Caltech, so I don’t know what they are. It’s hard to believe that they are at that distance. There again, that might upset the whole velocity distance relationship if those turned out to be distant nebulae or at the distance that those red shifts indicate. Well, it’s hard to see how they could get them and I have the feeling they are closer in, but there again if they are, then where’s your relationship going? But I don’t know enough to talk about it. That’s all happened since I’ve been up here.
Could you describe the observations on Mt. Palomar as opposed to the ones at Mt. Wilson, the ones you made at Mt. Palomar?
Well, I practically have, right here.
Yes, could you think of any major differences between using the 200-inch — the prime focus cage?
Well, of course, the major difference for an observer is the ease of observing at Palomar compared to what it was on Mt. Wilson there. I mean you had to stretch out into nothing up there and by the time you spend a night up there, your neck would be stiff and your eyes would hurt. You couldn’t sleep well or anything. But up here, it was a very nice arrangement and that seat rotated and everything went smoothly. (laughter) And the dickens of it all was that you know it all the time and knows this. They were eating better in both places than they were in their own homes, I’ll tell you that. They couldn’t buy those steaks and things. (laughter)
Olin Wilson was shocked when I told him it isn’t the people who have anything at home who are crabbing or grousing. Why it’s those who have nothing at home. (intermittent laughter…)
That’s right, (laughter) that’s right.
He was somewhat shocked or almost annoyed. In fact I was so cocky with astronomers, Milt was about the only one who would talk to me for years. Anything they wanted to say to me they had to say through Milt and vice versa. (more laughter…which includes female voice…)
Oh boy, we had our troubles down there, up on Mt. Wilson too. I think it’s true of every observatory.
What were some of the troubles on Mt. Wilson?
Oh, no (laughter) I mean you don’t want these down.
Little personal things. Some of the astronomers, I say things too plainly — good and decent people kind of and others — their intentions are good but their morals are loose. (laughter) They don’t get along with a night assistant, at least they make it tough for a night assistant. That’s a better way to put it and if they’d want to do that, they can really do it, too, I’ll tell you. Then they would come down and tell you, it’s all the trouble with the night assistant. Things like that, that’s all. (laughter)
You were describing some of the things at the 200 inch that were different at the 100. Can you think of any other things that were strikingly different?
Well, of course, the big difference and the exciting thing at first was the fact that you were riding with a telescope. I mean you were up there moving along. My exposure used to run over a whole night or two or three nights sometimes. And they were carried on from one night to the next. You start in the east and in the morning when you got out of that cage, you’ll be way over in the west over there. All that was completely different.
When you first started, were you impressed with the collecting power of the mirror? The fact that this was (inaudible)
Oh, yes indeed, it’s like Byron said, the time we went up there and had our first look through there, those things, to me they looked like diamonds and it happened to be a very black night that night. The field was black like black velvet and these things were just standing there, right down here they looked as though you could reach down and touch them. Yes indeed. Now up on Mt. Wilson we had trouble up there with reflections also. In other words, we got a lot of background light in the field. The field never looked gray and partly because of the lights from the city. The Pasadena and Los Angeles lights lit up the sky so the sky was brighter up on Mt. Wilson and it looked grayish up there, and you didn’t look at it the same way. You could look at it, but I mean it wasn’t quite the same as having this thing right in front of you.
But at Mt. Palomar the background of the sky was dark?
Oh yes, most of the time, although in recent years it’s getting, I guess, a little brighter and brighter all the time. I don’t know.
Yes, there’s been some smog coming out all the way from the valley.
Yes that’s right, yes that’s right.
As a matter of fact I saw it when I was there the last trip.
Well, I think when we started to work there, you could say that it was clear 99% of the time. The smog didn’t reach down there for a long time. You could see it way up the valley there but it didn’t get down there. The sky was black — you could go outside and the Milky Way would be standing up there, a beautiful sight.
And how about your first observations, were you surprised by the amount of light that you got with the new mirror?
What was your first observation like with it?
Well, it was the observation of this faint nebula that I had observed on Mt. Wilson and was uncertain about it up there, and I had given it something like four nights on Mt. Wilson. I went down there and in five you: got a very nice spectrum.
And you reached the limits of that telescope as far as measuring spectra of distant galaxies, didn’t you?
With what I had, yes, and with what was known at that time. Now it wasn’t till I commenced to find these other things that they went beyond there. But we knew nothing about those then, I mean you didn’t even know they existed.
Did you expect a change in this distance velocity relationship?
Well, it would have been surprising because the Mt. Wilson observations indicated that it was going out there but what we did hope to find was instead of a linear relationship clear out that up at the end, the distant end would start to either curve up or curve down. Now they both would mean something, I can’t any more tell you what that is. (laugh) Sandage would know that. He’s got it plotted in that contribution I told you about. There’s a plot in there showing a linear thing as far as we went. And then a curve down and a curve up and telling what they would mean. One case meant one thing and one case another thing.
But none of your observations showed any departure on the linear relationship?
No, a possible slight departure up at the end there. I think, of course, the 200-inch was designed, built with the hope that we could get out to the limit or “the horizon” I’ve heard Hubble call it. Well, we never, there is apparently no horizon, at least as far as the 200-inch goes — it just continues — the number of stars continue to build up and the nebulae go on, they get fainter and fainter, and so we did not accomplish anything like that.
Was that a disappointment?
Well, no. Nothing in astronomy is a disappointment. I mean you make an observation to try and find out what it’s going to do. Now you think it’s going to do something, but all of the interesting problems come from the things that didn’t turn out the way you thought they might. I mean it’s those things you want to follow up and not the ordinary things. You want them to fit and if they don’t fit you discard it and don’t include it in your list or something. But it’s the things that don’t fit that really ought to be followed instead of the other things that do fit.
Did you want to continue and do more or what? (inaudible)
Oh, yes, I did. I wanted to continue but (laughter) I came to retirement age and then I even talked to Dr. Bowen. Maybe I could come down later during the sun spot minimum. Well, he is a very wise person, very kind, and he said, “Well, Milt, let’s see how you feel at that time.” That would have been three to four years later and by that time I knew he was right. (laughter) I couldn’t go down, although I did go down and do some work with the Schmidt telescope down there but a completely different kind of work. Not on nebulae at all. Yes, on nebulae but not on spectroscopic ones.
Have you stayed in touch with things since then?
No, sir, (laughter) I’m honest. I have to tell you. When I came up here my son wanted to buy me a telescope for a present. I said, “My God, Bill, I’ve looked in an eye piece all my life, I don’t want to look in any more eye pieces.” I said, “No, no, no.” I want to fish and that’s what I’ve been doing ever since I’m up here. (laughter) Steelhead and salmon, that’s my business now. (laughter) Talking with these old-timers around here. I go fishing with a Chinaman that was born and raised here. He’s 77. Believe it or not, they wrote an article about him the other day. His grandfather came over from China in a sampan and just landed, he wasn’t going anywhere. Just crossing the ocean and where he landed was Casper, out above here. That’s the reason they settled here. And the family name is Hee and there are a lot of them scattered around now. A big Chinese shop in Los Angeles, one of them. Some of them were extremely wealthy. Those Chinese they are awfully wealthy sometimes, they fool you. (laughter) But I enjoyed going up and down the river with him. If we don’t catch fish, then I hear about all the old days and… The lumber, he, of course, worked at the mills here, they all do. That’s all it is up here. You work for the Union Lumber Company, or you don’t work for anybody. (laughter)