chief
axis. In some crystals the index of refraction for the extraordinary
ray E is greater than for the ordinary ray O ; and in others it is
smaller. The former are said to have positive (or attractive), the
latter negative (or repulsive), double refraction. Quartz is an example
of the former, the index of refraction, according to Malus, being for 0— 1.5484,
for E = 1.5582; and calc-spar of the latter, the index of O
being=l.6543, of E=l-4833. The index of E is in both cases taken at its
maximum.
According to Dufrenoy, the following table shows the index of refraction of a great number of minerals :
