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

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ature. (3) Photographic registration will be used so as to make continuous readings possible. Thus recorded, the observations would be preserved and could be discussed later on, independently of the observer. These are the three points which represent a considerable improvement in comparison with the earlier apparatus.

It may be of some interest to mention that originally another apparatus was planned; this, however, was abandoned and the present interferometer adopted. The arms were to be 100 m long. The apparatus could not be turned, but the moving earth would have brought it into different positions relative to the ether. We now intend to try this, and the experiment is in preparation at Chicago.

Lorentz: In regard to the theoretical details raised by Dr. Michelson, I offer the following remarks: If the ether moves freely through matter, no such difficulties arise, as far as entrainment is concerned. If, on the other hand, we should be obliged by the facts to introduce a substantial ether again, it would, of course, be a very difficult problem to find out what its properties are. What would happen, for instance, in case matter should turn out to be only partly permeable for the ether, nobody can tell. For this reason the question about the ratio 19/20 could not well be raised before the properties of the ether were better known. We can even leave open the possibility that the motion of the ether may be irrotational. In this case the ether drift would of course have a component normal to the surface of the earth, and it would be rather large. This might very well be the case, and the effect mentioned by Dr. Michelson would be null. The relative velocity of the ether drift might increase with increasing distance above the surface of the earth, and still have no rotation. This, for instance, is the case in Planck’s modification of Stokes’s theory. A further possibility would be a compressible ether. This would remove even the necessity of having an irrotational motion of the ether. But it is sufficient for the present moment to point out that a motion of the ether with rot w = o would be sufficient to give a quantitative explanation of aberration phenomena and of Michelson’s result. I tell you all this only to show how numerous the different possibilities for the theory are, if we are compelled by new experiments to go back to the notion of a substantial ether.

Question to Dr. Kennedy: Your apparatus is so sensitive that it would detect a change in the optical path equal to 5/ = 2*io"3X.



Now this is not the sensitivity which you calculated theoretically. I should like to ask how you found this sensitivity. I do not ask this for myself, because I know how you have done it, but for the sake of the audience, because I think the method you applied is so very beautiful. Then I suggest also that you tell us if you could detect the orbital effect on the assumption of a drag of 19/20.

Dr. R. J. Kennedy: Answering first the second question, I think that the effect due to the orbital motion of the earth should be observable with my apparatus.

As to the first question, I thought that the method of determining 31 was rather crude. A weight of 5-6 kg on the slab on which the apparatus was mounted produced a shift of one fringe. I determined the minimum weight (about 10 g.) which produced an effect just observable. The ratio of the two weights gives, then, hl/\.

I might explain also that I got rid of the surplus scattered light, using a different method from that suggested by Professor Michelson for his new device. I used polarized light, impinging under the proper angle on the glass plate (Brewster’s angle) so that no light at all was reflected. [See description in Kennedy’s report. Michelson exclaims: “Very nice indeed.”] The method I used is not my own invention. It has been suggested somewhere in Comptes rendus (1911), if I remember rightly.

The shift of azimuth (50° to the west) in Miller’s experiment seems to indicate that some spurious effect is present, dependent only on the position of the apparatus relative to the meridian, which shifts the azimuth of the whole effect to the west. The result must then be considered as a superposition of spurious effect and ether drift. This explanation would probably require a magnitude for the effect due to ether drift smaller than anything that could have been observed with the devices used. It might also explain, as I think, the difference between the results obtained by Mr. Miller and myself.

Piccard’s experiment does not seem to be of great value. As far as I can make out, he worked just at a time of the day when hardly any effect was to be expected.

Lorentz: I do not think that Kennedy’s last remark is quite right. Piccard really ascended at the time when the constellation of Hercules rose above the horizon.

Kennedy: Piccard ascended twice. Once, when the sidereal time