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 j8 
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 and respectively. If the acute bisectrix 
falls in the direction of the minimum refractive index a (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 2 V. 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 
from the observed angle by the equation sin V = 
When, however, the observations are made with the crystal 
immersed in a liquid, the observed angle must necessarily 
differ from 2E 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 
