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.
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-
CONFERENCE ON MICHELSON-MORLEY EXPERIMENT 395
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.