Page:TolmanEmission.djvu/2

Rh A very complete emission theory of electromagnetism has been presented by Ritz. According to this theory light retains throughout its whole path the component of velocity which it obtained from its original moving source, and after reflection light spreads out in spherical form around a center which moves with the same velocity as the original source. In this article an experiment will be suggested whose performance would permit a decision between the Ritz and Einstein theories of relativity.

According to the first of the above emission theories, if a source of light is approaching an observer with the velocity $$v$$, the emitted light would have the velocity $$c+v$$ and after reflection from a stationary mirror would have the velocity c. We shall now show that measurements of the Doppler effect (in canal rays) do not agree with this theory.

Consider measurements of the Doppler effect in light from a moving source made with a concave grating arranged as shown in Fig. 1. Light from the source (canal rays) enters the slit and falls on the grating which is so mounted that its center of curvature coincides with the position of the line of the spectrum to be photographed at D, Hence the paths BD and CD traversed after reflection by the two rays of light ABD and ACD are equal, and