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 sun. A tube mounted on a small telescope stand was provided with a slit, an observing eye piece, and a mirror (plate glass) as shown in the diagram. The apparatus was adjusted to produce interference fringes between the light from the slit and from its mirror image, a method employed by Dr. Lloyd. The mirror was about 25 cm. in length and the distance from the end of the mirror to the eye piece was about 10 cm.



Let us suppose an interference fringe at the point f produced by the combination of a ray of light sf, coming direct from the slit and another smf which has suffered reflection from the mirror. If now the velocity of light were dependent on the velocity of its source, and we set the source in motion towards the slit, the time taken for a given wave-front to travel from the slit to the eye piece along the path sf would be increased by a greater amount than along the path smf, since for the distance mf, which in the apparatus was at least 10 cm., the ray is traveling with the normal velocity of light as from a stationary source, while for the whole path sf the light is traveling with an increased velocity. We should thus expect a shift in the fringes to accompany a change in the velocity of the source.

A lens of about 9 cm. aperture and 80 cm. focus was used to throw an image of the sun (about 8 mm. in diameter) on the apparatus, and light first from one and then from the other limb of the sun allowed to enter the slit. No shift in the fringes was observed. We may easily calculate the magnitude of the expected effect. Suppose we are receiving light from the approaching limb of the sun, the velocity of the ray sf would be greater than that of the 1 reflected ray by about 1.5 km. per second, so that a given wavefront in traveling the ten centimeters from m to f would fall behind the corresponding one which travels along sf by