The extraordinary ray, produced by the double refraction of the ordinary ray, o.
In this direction a ray suffers no double refraction, in all others it does.
The difference, 0.1723, between these two indices gives a measure of the bi-refringence or double refraction.
This direction of no double refraction is called the optic axis of the crystal.
At the same time the polarization of light by double refraction was discovered.
Soon after, reflexion, refraction, and double refraction were accounted for on that principle.
The double refraction of Iceland spar was first treated in a work published by Erasmus Bartholinus, in 1669.
The number and distance apart of the rings depend on the strength of the double refraction and on the thickness of the crystal.
The difference between the two indices is a measure of the strength of the double refraction or birefringence.
The difference γ − α represents the strength of the double refraction.