A. A. MICHELSON
The addresses by Professors Michelson and Lorentz were followed by a detailed account of the results obtained by Professor D. C. Miller, who, fortunately, was also able to be present.
I. PROFESSOR A. A. MICHELSON (UNIVERSITY OF CHICAGO)
In 1880 I conceived for the first time the idea that it should be possible to measure optically the velocity w of the earth through the solar system. There had been earlier attempts to discover first-order effects, based on the idea of a system moving through a stationary ether. (First-order effects are proportional to w/c, where c = light-velocity.) Talking in terms of the beloved old ether (which is now abandoned, though I personally still cling a little to it), one might have expected that the aberration of light would be different for a telescope filled with air and with water, respectively. The experiments, however, showed, contrary to the then-established theory of light, that no such difference was present.
FresnePs theory was the first to account for this result. Fresnel assumed that matter was able to drag along the ether partially (entrainment of ether), giving it a velocity wr, so that
w' — pw.
He was able to determine p (FresneFs coefficient) in terms of the refractive index
This coefficient is easily obtained from the negative result of the following experiment. Two light-beams travel along the path (Fig. 1; 0,1, 2, 3, 4, 5) in opposite directions and give rise to a set of interference fringes. 7 is a tube filled with water. Now if the whole system moves with the velocity w through the ether, a shift of fringes would be expected on moving the tube from position I to II.
l I 2.
CONFERENCE ON MICHELSON-MORLEY EXPERIMENT 343
No displacement is observed. By assuming a partial entrainment of the ether, FresnePs coefficient p may readily be determined from this experiment. It may also be found in a very simple and direct fashion with help of the Lorentz transformation.
FresnePs result was accepted universally by investigators of his time, including Maxwell, who pointed out that, while there could be no first-order effects, there might, nevertheless, be second-order effects (proportional to u?/c2). Now with o km/sec. for the motion of the earth, w/c = io-4, and w2/c2 = io~8, a quantity too small to be measured, according to Maxwell.
It seemed to me, however, that by making use of light-waves, one might devise an adequate arrangement for measuring such second-
Fig. 2 Fig. 3
order effects. Consider an apparatus, including mirrors, moving with the velocity w through the ether. Suppose two light-beams to travel back and forth in the apparatus, one parallel to w, the other at right angles to w. According to the classical theory, the change in light-path resulting from w should be different for the two beams and produce an appreciable shift of the interference fringes. The first device tried for the measurement of second-order effects is indicated in Figure 2. This arrangement, however, involved very great difficulties and was soon abandoned; and fortunately, because it led to the construction of the interferometer, which has proved of great value in many subsequent experiments.
The interferometer (Fig. 3) is known to all of you. A set of fringes is obtained by superposition of the two beams traveling from the source to a glass plate and then to mirrors 1 and 2, respectively,