Page:TolmanPostulate.djvu/15

The first method would be to measure the wave-length of light from some moving source, since it is shown that the first hypothesis would lead us to expect a change in the wave-length of light from moving sources and the alternative hypothesis would not. The existing measurements of the wave-length of light from moving astronomical sources, made with reflection gratings, are not of such a nature that we can definitely decide that the velocity of light is independent of the motion of the source. We can, however, state that, if the alternative hypothesis is true, then a reflecting surface acts as a new source of light and light coming from such a surface has the normal velocity as from any other stationary source. It is further shown that measurements with a transmission grating would more nearly allow a definite decision of the question.

The principle that the velocity of light from a stationary mirror is the same as for light from any stationary source leads to a second method of deciding between the two hypotheses as to the velocity of light. An apparatus is described in which interference fringes were produced between light which had come direct from the sun and light which had first suffered reflection. Since the velocity of the reflected ray would in any case be unaffected by the motion of the original source, we should expect a shift in the position of the fringes to accompany a change in the velocity of the direct ray. No shift in the fringes was observed in examining light first from the approaching and then from the receding limb of the sun. We conclude that the velocity of light from the two limbs of the sun is the same, which confirms the principle that has led to the second postulate of relativity.

An entirely different method of proving the second postulate of relativity is afforded by the results of the Kaufmann-Bucherer experiment. In fact, by a combination of the first postulate of relativity with the principle that the mass of a moving electron is greater than that of a stationary one in the ratio $1:\ {\sqrt {1-\beta ^{2}}}$, it was found possible to deduce all the conclusions of the theory of relativity as to the units of length and time in a moving system, and finally to deduce the second postulate of relativity itself.

Massachusetts Institute Of Technology,

Boston, Mass.,

January 6, 1910