Richard Chace Tolman

“ We are now ready to deduce our principle as to the velocity of light. Consider a source of light and an observer B who measures the velocity of the light coming from this source. If the observer B is at rest and marks off a length of one centimeter in the path of light, he finds, of course, that the light takes the time 1/c to pass from one mark to the other. We wish to prove, however, that the relation which we have just derived, requires that the observer will also obtain the time 1/c even if he is in motion towards or away from the source. ”

“ For, if light or any electromagnetic disturbance which is being emitted from a source, partakes in the motion of that source in such a way that the velocity of the source is added to the velocity of emission, then a system consisting of the source and its surrounding disturbances acts as a whole and suffers no change in configuration when the velocity of the source is changed. ”

“ 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. ”