THE DETERMINATION OF REFRACTIVE INDEX 
235 
stresses or strains. In the determination of the refractive indices 
of isotropic crystals it is obvious that the same value will be 
obtained in all directions through the crystals. In the case of 
anisotropic crystals, however, the rate of transmission of light is 
different in different directions through the crystals. In order 
to better appreciate the influence of these properties upon the 
refractive index, it is necessary to briefly consider a few funda¬ 
mental facts. 
A ray of light, when passing obliquely from one medium into 
another whose rate of transmission for light rays is different, 
will be deflected from its original path according to the equation 
sin i V . ^ , , ... 
^— = -777, m which t is the angle formed by the incident ray 
smr V j j 
and the normal, r the angle formed by the refracted ray and the 
normal, and V and V' the velocities of the transmission of the 
light in the two media. When the rays pass from a medium 
having a higher rate of transmission into one of lesser rate the 
deflection is toward the normal, but when passing from a medium 
with a lesser rate into one of higher rate the bending is away from 
the normal. In microscopic work the light rays are usually 
passing from air into a denser medium. If in the above equation 
we assign to the velocity of light in air the value of i, the equa- 
Sin 't I sm 7 / 
tion becomes — = 777, but — is the expression for the index 
sin r V sm r 
of refraction, from which it appears that the refractive index is 
inversely proportional to the velocity of the transmission of light 
in the medium. Since in anisotropic crystals, the rate of trans¬ 
mission of light rays differs according to the direction through 
the crystal in which the rays are sent, it is obvious that the 
refractive index of an anisotropic crystal cannot be expressed by 
a single value and further, that of the several values given by a 
double refracting crystal, the greatest index will be found in the 
direction through the crystal of the lowest rate of light trans¬ 
mission and the smallest index in the direction of the highest 
rate of light transmission. In other words, different values for 
the index of refraction will be obtained according to the position 
in which the crystals lie upon the stage of the microscope. 
