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 is not increased by a motion of the source of light towards the observer. The first postulate of relativity adds the idea that a motion of the source of light towards the observer is identical with a motion of the observer towards the source. The second postulate of relativity is seen to be merely the combination of these two principles, since it states that the velocity of light in free space appears the same to all observers regardless both of the motion of the source of light and of the observer. Since the first postulate of relativity has already been considered as sufficiently proved, we shall proceed at once to present certain evidence in favor of the assumption that the velocity of light is independent of the motion of the source. In the latter part of the paper, we shall also consider an entirely independent proof of the second postulate based on the Kaufmann-Bucherer experiment.

The principle that the velocity of light is independent of the velocity of its source has hitherto lacked experimental justification. It was obtained, however, as a direct consequence of the ether theory of light, which makes the velocity depend only upon the properties (i. e., elasticity or electrical nature) of a stationary transmitting medium, and therefore, as with sound or other wave motions, independent of the velocity of the source. Until within a few years the ether theory had been so extraordinarily successful in explaining even the most complicated phenomena of optics, that we should have accepted any of its experimentally unproved conclusions without hesitation. At the present time, however, since the experiments of Michelson and Morley and of Trouton and Noble stand in such direct contradiction to the predictions of the ether theory, we have no hesitation in considering any other assumption as to the velocity of light, which, although not in accord with the ether theory, would free us from the complications introduced by the theory of relativity.

Such an alternative assumption, as to the velocity of light, which