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, in which i is the angle formed by the incident rav 



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 i i sin i . 



tion becomes - — = 7-7, but - — is the expression for the index 

 sinr V sinr 



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. 



