394 PROCEEDINGS OF THE AMERICAN ACADEMY 



of the telescope, which was 35.3 inches long, was filled 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 Preface to the " Laws of the Aberration 

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

 effect of motion on the phenomena of sound and light. From his own 

 analysis he concludes: 1. that a motion of the prism and telescope 

 perpendicular to the direction of a star produces no effect on the re- 

 fraction ; 2. that when the motion is in the direction of the star, the 

 velocity of the light is changed according to Fresnel's fraction of that 

 motion ; and 3. that for any intermediate direction it is changed 

 to the extent of tliat fractional part of the motion multiplied by 

 the cosine of the angle between the direction of the motion and the 

 direction of the star. 



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

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

 the motion of the refracting medium. When a ray of polarized light 

 passes through an inclined plate of glass, the plane of polarization is 

 changed, according to certain laws investigated by Mahis, Biot, and 

 Brewster. The degree of change depends upon the inclination of the 

 ray, the azimuth of the plane of primitive polarization, and the index 

 of refraction of the glass. The incidence and azimuth being constant, 

 this rotation of the plane of polarization increases with the index of 

 refraction. This index being inversely as the velocity of light, the ro- 

 tation is smaller the greater this velocity. Fizeau used two bundles 

 of glass, four plates in each, and slightly prismatic, inclined to one 

 another. One bundle was made of common glass ; the other of flint 

 glass. The angle of incidence for the ray was 58° 49'. When the 

 azimuth of the primitive plane of polarization was 20°, the rotation of 

 the plane of polarization was 18° 40' and 24° 58' for the two bundles. 

 By Fresnel's hypothesis the change in the velocity of light from the 



motion of the medium is ± (- — ^ — j v. The greatest available velocity 



for the medium is that of the earth in its orbit, viz. 101,708 feet per 

 second (31,000 meters). At the time of the solstices this motion is 

 horizontal, and from east to west at noon. If the incident light comes 

 from the west, the velocity of light is diminished by Fresnel's fraction 

 of the velocity of the earth. If the light comes from the east, its 

 velocity is increased by the same amount. The change in the index of 



