OPTICAL PROPERTIES OF CRYSTALS 257 



For the relations between axes of elasticity and refractive 

 index, see Chapter X, page 234. 



In uniaxial crystals the optic axis is coincident with the princi- 

 pal (vertical) or c-axis of the crystals; hence uniaxial crystals 

 in sections normal to their vertical axes will behave like isotropic 

 crystals. In biaxial crystals the optic axes always lie in the 

 planes of maximum index of refraction 7 and of minimum index 

 of refraction a. The direction of medium refractive index /3 

 lies in a plane which is normal to that in which the two optic 

 axes lie. This direction of medium vibration is known as the 

 optic normal; with this direction the optic axes form angles 

 acute on one side, obtuse on the other. The lines bisecting 

 these angles are known as bisectrices. That bisecting the acute 

 angle is known as the acute bisectrix and that bisecting the obtuse 

 angle the obtuse bisectrix; these directions through a crystal 

 are designated B Xa and B Zfl respectively. If the acute bisectrix 

 falls in the direction of the minimum refractive index __ (i.e., 

 in the direction through the crystal of greatest ease of vibration) 

 the crystal is optical negative ( ), but if the acute bisectrix lies 

 in the direction of the maximum refractive index 7 (direction 

 of least ease of vibration) the crystal is optically positive (+). 



The angle formed by the optic axes is known as the axial 

 angle. The true axial angle is designated by 27. The observed 

 axial angle as measured in the microscope is greater than the 

 true angle and is designated by 2E. This discrepancy is due to 

 the displacement of image and is proportional to the refractive 

 index of the crystal measured. The true angle may be calculated 



* E* 



from the observed angle by the equation sin V = - . 



p 



When, however, the observations are made with the crystal 

 immersed in a liquid, the observed angle must, necessarily 

 differ from iE and is designated by 2H. The value assigned 

 being followed by the name of the medium in which the obser- 

 vations are made. 



The magnitude of the optic axial angle varies with the color 

 (wave-length) of the light rays and with the temperature of the 

 compound. In the examples cited above relative to the sodium 



