reached by the latter. The object of Lorentz in his papers was to explain the transmission of waves in moving media, beginning with the explanation of astronomical aberration. Singularly enough this was the one phenomenon which was better explained on the emission than on the undulatory theory, and which had proved a stumbling-block for the latter. If the ether is a substance, the question arises whether it is carried along by the earth in its motion, or whether it remains fixed. Lorentz assumed that it remains fixed, and thus satisfactorily explained aberration. But if the earth moved through the ether, the velocity of light between terrestrial points should be affected in the same way that the velocity of sound is affected by the wind. To test this a celebrated experiment was made by Michelson in 1881, repeated by Michelson and Morley in 1887, and several times later, which showed the failure of the earth's motion to influence the velocity of light from a terrestrial source. This classical experiment may prove to be the beginning of the end of the ether. It is evident that if light is propagated through the ether in waves which have a velocity peculiar to the ether, and not influenced by the velocity of the source, then light will take longer to reach a point a given distance from it when both are moving in the direction of the line joining them when the second point is ahead than when it is behind, in the ratio of the sum of the velocities of the source and the waves to their difference. The time for the light to go to the forward point and come back is greater than it would be if the system stood still by an amount inversely proportional to where is the ratio of the speed of the source to that of light. In the case of the earth this is about one part in one hundred millions, and it was shown by Michelson that no such effect existed. Michelson assumed that this showed that the ether was fixed to the earth. For the contrary explanation, Lorentz adopted an hypothesis already proposed by Fitzgerald, namely, that all bodies in motion are thereby shortened in the direction of their motion, in precisely this ratio. This hypothesis, though startling, has now obtained great weight. In connection with it, Lorentz introduced the idea of local time, which is different for different points of the same system moving with a uniform velocity of translation. The modification, by the motion, of both distance and time leads to a most fundamental principle for all our physical notions, called the principle of relativity, which, though brought about by Lorentz, was most clearly expounded by Einstein, who is probably the high priest of the ultra-modern school. The principle of relativity assumes as a postulate that all phenomena are the same if observed with reference to a body moving with constant velocity with respect to the ether as if with respect to a body at rest. If this is so, and no experiments have contradicted it, we have as much right to suppose the ether at rest with respect to one body as another. It seems then unnatural to characterize one body as moving relative to a fixed ether. Hence
