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between the two components which could exist in this kind of glass.

If we estimate the contraction from the change in density by means of the excess of the index above unity, as was done in the case of water and as assumed by Rayleigh, the above fraction would become

${\frac {1.77}{.77}}\times 4.5\times 10^{-11}$ or 10^-10,

approximately. This is 50 times smaller than $0.5\times \left(10^{-4}\right)^{2}$, the change to be expected on the "contraction" hypothesis, and is 30 times less than the sensibility obtained with water.

If, on the other hand, we take Wertheim's^[1] results for glass, we have approximately for Faraday's flint, 5 x 10¹¹n as Young's Modulus and $2.4\times 10^{7}$ dynes on a millimetre cube to give a relative retardation of one λ. From above we have

${\frac {6.5\times 10^{-5}\lambda }{445}}=1.46\times 10^{-7}\lambda$

as the relative retardation for 1 mm.

Thus, the force to produce the least observable effect is

$2.4\times 10^{7}\times 1.46\times 10^{7}=3.4$ dynes per 1 mm.

Young's Modulus for 1 mm. square becomes $5\times 10^{9}$ and the contraction becomes

${\frac {3.4}{5\times 10^{9}}}=6.8\times 10^{-10}$,

which is seven times smaller than that expected on the "contraction" hypothesis. If we correct by ¼ for Poisson's ratio, as we should if the interference problem were done on a glass support, the calculated contraction becomes

$0.5\times \left(10^{-4}\right)^{2}\times {\frac {4}{5}}=4\times 10^{-9}$

or six times larger than our margin for glass.

Hence, if the test is a valid one, the "contraction" hypothesis cannot explain the negative results of the interference experiments; and, with the same reasoning, we also conclude either that the aether moves with the embedded matter, or that the effect of the relative motion on the intermolecular forces and the possible consequent relative change in dimensions are very small.

Physical Laboratory,

University of Nebraska, Lincoln.

1. Ann. de Chim. et de Phys. (8) t. xl. p. 202.