All these polarization colors in the case of anisotropic sections are due to the amount of relative retardation effected in the section – the difference between the velocities of the two rays traveling in the section, or, expressed differently, the difference between the two refractive indices, the retardation increasing, of course, with increase of thickness. Taking quartz as an example, it can be found that differently oriented sections produce different polarization colors. If these colors are compared with those of Newton’s scale, it will be seen that all three colors are found near the thin end of the wedge – low down in the scale. The yellow color is the highest of the three, and that section of the mineral for which the difference of refractive indices is greatest must produce this color. No other section of quartz will show polarization colors higher in the scale than the yellow produced by the prismatic section. That is, such a section contains the two vibration directions having the maximum velocity difference for the mineral. This difference is best expressed numerically as a difference between the two refractive indices.

 

            It may be demonstrated experimentally that when the light vibrates parallel to the c crystallographic axis of quartz, the refractive index is 1.553, and that when the vibrations take place at right angles to this direction, the refractive index is 1.544. The difference, 0.009, is clearly a measure of the strength of the double refraction for this particular mineral, and this value is the birefringence.

 

            Every doubly refracting mineral agrees with quartz in this respect, that it has one particular section containing the two vibration directions giving the maximum and minimum velocities for the mineral. Light vibrating in these directions has the maximum difference of refractive index for the mineral. The difference is the birefringence of that mineral, and, stated numerically, gives us some idea as to what polarization colors the section of ordinary thickness is capable of producing when viewed between crossed nicols of the polarizing light microscopes.