michelson's recent reseaeches on light. 461 



produced by the motion of the luraiuary or of the observer. This 

 prize, consisting of a gold medal or 3,000 francs, was awarded in 1874 

 to Mascart. He maintained that in Arago's exjjeriment the change in 

 refraction produced by the fraction of the earth's motion was compen- 

 sated by the displacement of the observing telescope. Mascart re- 

 peated Babiuet's experiment with gratings, where the effects of the 

 motion of the telescope and of the grating would be additive, and found 

 the sum small compared with JJabinet's calculation, lie thinks that 

 the change in the length of the wave caused by the motion is compen- 

 sated by the <lisi)lacement of the measuring apparatus, lie concludes 

 that retlection, diffraction, double refraction, and circular polarization 

 are powerless to show the motion of the earth, either with solar light 

 or that from a terrestrial source. 



In 1871, Airy used a vertical telescopt-, and measure*! the meridional 

 zenith distance of y Draconis, the star by which Bradley discovered 

 aberration. It is about 100" north of the zenith. The tube of the 

 telescope, which was 35.3 inches long, was tilled with water. The days 

 of observation included the seasons of the equinoxes, when the star is 

 most affected in opposite directions by aberration. The observations 

 were repeated in the spring and autumn of 1872. No increase was 

 produced in the aberration by the water in the telescope. 



In 1873, Ketteler, in the i)reface to the "Laws of the Aberration of 

 Light," enumerates thirty-nine persons who have investigated the effect 

 of niotiou on the phenomena of sound and light. From his own analy- 

 sis he concludes: (1) that a motion of the prism and telescope per[)en- 

 dicular to the dii'ectiou of a star produces no effect on the refraction ; (2) 

 that when the motion is in the direction of the star, tlie velocity of the 

 light is changed according to Fresuol's fraction of that motion; and (3) 

 that for any intermediate direction it is changed to the extent of that 

 fractional part of the motion niul(ii)lied by the cosine of the angle be- 

 tween the direction of the motion and the direction of the star. 



In 1859, Fizeau proposed an experiment foi- ascertaining if the azi- 

 muth of the plane of polarization of a refracted ray is influenced by tlie 

 motion of the refracting inediuui. When a ray of polarized light i)asscs 

 tlirough an inclined plate of glass, the plane of polarization is changed, 

 according to certain laws investigated by Mains, Biot, and Brewster. 

 The degree of change depends upon the inclination of the ray, the 

 aziinnth of the plane of primitive polarization, and the index of refrac- 

 tion of the glass. The incidence and azimuth being constant, this rota- 

 tion of the plane of polarization increases with the index of refraction. 

 This index being inversely as the velocity of light, the rotation is 

 smaller the greater this velocity. Fizeau nsed two bundles of glass, 

 fonr plates in each, and slightly prismatic, inclined to one another. 

 One bundle was made of (tommon glass; the other of tlint glass. The 

 angle of incidence for the ray was 58<^ 40'. \Vhen the azimuth of the 

 primitive plane of polarij'iatiou wus 20°, the rotf^tion of the plimie of 



