# Page:TolmanEmission.djvu/2

No. 2
137
THEORIES OF LIGHT.

A very complete emission theory of electromagnetism has been presented by Ritz.[1] 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.

The First Emission Theory.

According to the first of the above emission theories, if a source of light is approaching an observer with the velocity ${\displaystyle v}$, the emitted light would have the velocity ${\displaystyle c+v}$ 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.[2]

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

1. Ritz, Ann. de chim. et phys., 13, 145 (1908); Arch. de Génève, 26, 232 (1908); Scientia, 5 (1909). See also Gessamm. Werke. The Ritz electromagnetic theory does not seem to have received the critical attention which it deserves. It was the earliest systematic attempt to explain the Michelson Morley experiment on the basis of an emission theory and is the only emission theory which has been developed with any completeness.
2. In an earlier article (loc. cit.). the author showed that if an emission theory of light were true, there would be no change in the wave-length of light when the source is set in motion. This undisputed conclusion led the author to believe that with a suitable arrangement of grating no Doppler effect would be detected in light from moving sources if an emission theory should be true. It has been correctly pointed out by Stewart (loc. cit., p. 420), however, that the use of a grating to determine wave-lengths is based on a theory which assumes a stationary medium. Hence grating measurements of the Doppler effect do not afford a general method of testing all emission theories, but such measurements must be subjected to a more complete analysis. As shown in the sequel, however, such an analysis of existing measurements of the Doppler effect is apparently sufficient to disprove the Stewart emission theory. Such measurements are not suitable for deciding between the theories of Ritz and Einstein, however, since in general these two theories would only lead to the expectation of second order differences.