double refraction
birefringence

By this time, he had also researched double refraction (birefringence) in crystals.

Birefringent is relared or characterized as a double refraction.

In this picture we can see calcite crystal laid upon a paper with some letters showing the double refraction.

It is used for the prisms of the polariscope, because of its strong double refraction.

The power of double refraction is possessed by all crystals except those of the isometric system.

This is due to double refraction as the light bends twice, when entering and leaving the prism.

With this double refraction, the two subsidiary images appear as one above and one below the main image.

Non-crystalline materials have no double refraction and thus, no optic axis.

Some solid materials under specific conditions can demonstrate double refractions and optic axes.

An object viewed through Iceland spar appears as two because of double refraction.