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 relief which would follow the adoption of this idea, the evidence which we are about to present is all in favor of the older idea of the velocity of light which has led to the second postulate and the complicated theory of relativity.

The Doppler Effect.
The Doppler effect, that is the influence which a motion of the source of light has upon the frequency of the emitted ray, has been the subject of much refined experimental work. It is natural to consider what relation this effect bears to our own problem, concerning the influence which a motion of the source of light has upon the velocity of the emitted ray.

If we have a body which is emitting the periodic disturbance which we call light, and then start the body in motion towards an observer, it is evident that the frequency with which the disturbances will reach the observer will be increased, since, in order to get to him, each successive disturbance has to travel a less distance than the one preceding it. As we shall see, this change, in the frequency of the light, will be produced in nearly the same amount, whether or not the velocity with which the light travels towards the observer is affected by the motion of the source. We shall find, however, that, with regard to the actual distance in space between successive disturbances (i. e., wave-length) the two hypotheses as to the velocity of light lead to quite different conclusions. This difference presents a method of deciding between the two rival hypotheses.

We shall proceed to an actual derivation of the effect which a motion of the emitting source has upon the wave-length of light, first, assuming that the velocity of the light is independent of the motion of the source, and then that the velocities of the light and source are additive.

It is evident that we must make no use of arguments based on preconceived notions as to the nature of light, but must restrict ourselves to purely kinematic considerations which would be equally