Conference on the Michelson-Morley experiment held at the Mount Wilson observatory Pasadena, California February 4 and 5, 1927

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Michelson: What is the probable error for this | km/sec.?

Miller: This \ km/sec. is itself the probable error in the measurement of the magnitude of the effect, as determined from the calculations. Since no effect has yet been detected which can positively be attributed to the orbital motion one can only say that such an effect, if present, must be less than \ km/sec.

Michelson: Excuse me if I insist on this point. This estimation of the probable error is based on an interpretation of the experiments which does not intend to find the effect of the motion of the earth at all. Can you not find the probable error in discussing the observations from the point of view of finding the orbital motion?

Miller: I have not calculated the error from such a point of view.

Michelson: It would, however, be possible to do so. I should really like to see such calculations carried out.

Had I known earlier of the beautiful and ingenious apparatus of Mr. Kennedy, I probably should not have undertaken my experiments now going on in the same form. In any case, the problem in question must be investigated further. Even a more precise repetition of experiments with older devices already used will be of great value for the reliability of the results. We have now to find out definitely what actually is the truth, without going at it with any prejudice.

I am happy in respect to Mr. Kennedy’s experiment that I had the idea of this device, too. I also had intended to use the photometric comparison of the field produced by light which is reflected from a divided mirror, the two half-surfaces being at a distance of a fraction of a wave-length. But it did not occur to me that the separation could be made so nicely by sputtering. I intended to take the layer off by acids. The apparatus is indeed so beautiful that I should like to work with a similar device, in case Mr. Kennedy has no objections.

As far as Mr. Piccard’s remarks are concerned (see Lorentz) I must say that every beginner thinks himself lucky if he is able to observe a shift of 1/20 of a fringe. It should be mentioned, however, that with some practice shifts of 1/100 can be measured, and that in very favorable cases even a shift of 1/1000 of a fringe may be ob-



served. For this purpose the fringes must be extremely black. We are sufficiently advanced with our new apparatus to show such fringes [the apparatus was on view in the laboratory]. The main thing, of course, is to eliminate all stray light, which comes especially from the silver-coated plate. The ordinary plate gives rise to reflections at both surfaces. I now get rid of scattered light by this simple device illustrated in Figure 21, consisting of two prisms, with a halfsilvered surface where they are in contact, oriented so that the incident light is not quite perpendicular to the face of the first prism. Very black fringes may be obtained by this combination of prisms. There are still some difficulties as to the separating surface which I hope to overcome, however, very shortly. Probably the precision will reach 1/1000 of a fringe.


Fig. 22

I should like to make some remarks on the interpretation of Mr. Miller’s experiments. It seems to me to be very hard to explain them. Indeed, why should the ether be dragged along by the earth to the extent of 19/20 and not some other fraction? If this really occurs, then we must suppose that there will be a great difference between the drag on the surface of the earth and a thousand miles above it. There the drag would probably be zero. Assuming then, for illustration, some kind of an exponential decrease of drag with altitude we should expect a large difference between the shift at sea-level and on Mount Wilson. In this case another arrangement of apparatus could be used to observe the effect. Two rays of light could be sent around a vertically mounted rectangle (Fig. 22). A shift of several hundred fringes might then be expected. No shift, however, seems to exist, according to experiments made in the Ryerson Laboratory.

To conclude, I might still mention some advantages of the new apparatus: (1) The fringes are very black. (2) The frame will be built of invar so as to make it very insensitive to changes of temper-