upon the Velocity with which it is traversed by Light. 259 
going researches on air and water being no longer applicable, 
recourse was had to the following property of light established 
by the researches of Malus, Biot, and Brewster. When a ray of 
polarized light traverses a plate of glass, inclined towards its 
direction, the plane of polarization of the transmitted ray is in 
general inclined towards that of the incident ray. The magnitude 
of the rotation of the plane of polarization which is thus caused 
by the two refractions at the two surfaces of the plate of glass 
depends, first, upon the angle of incidence; secondly, upon the 
azimuth of the primitive plane of polarization with reference to 
the plane of incidence ; and thirdly, upon the index of refraction 
of the glass forming the plate. 
The angle of incidence and the azimuth of the primitive plane 
of polarization remaining the same, the rotation of this plane 
increases with the index of refraction of the glass plate. Now 
since this index is inversely proportional to the velocity with 
which waves of light are propagated through the glass, it follows 
that the magnitude of the rotation of the plane of polarization 
increases when the velocity with which light traverses the glass 
plate diminishes. The determination of any change in this 
velocity is, therefore, reduced to that of the corresponding change 
in the rotation of the plane of polarization. 
In the first place it was deemed necessary to determine the 
change in the rotation which any given increase or decrease of 
the index of refraction could produce. By direct and comparative 
measurements of these indices and rotations, in the cases of flint 
and ordinary glass, it was found that when the index was in- 
creased by a small fraction, the rotation increased by a fraction 
43 times greater than the first. 
The question next arises what change, according to the hypo- 
thesis of Fresnel, ought to be produced in the velocity of light 
when it traverses glass in a state of motion? -The answer is 
based upon the following data. 
' The greatest velocity at our command is unquestionably that 
of the earth in its orbit. At noon, during the period of the sol- 
stices, for instance, the direction of this motion is horizontal and 
from east to west ; from this it follows that when a plate of glass 
receives a ray of light coming from the west, it ought to be con- 
sidered as really moving to meet the ray with the immense velo- 
city of 31,000 metres per second. When, on the contrary, the 
incident ray comes from the east, the glass plate must be con- 
sidered as moving with this velocity in the same direction as that 
of the propagation of the waves of light, by which latter it is in 
reality overtaken. 
Now, according to the theory of Fresnel, the difference between 
the velocities of the light in these two extreme cases would be 
