Conceive a beam of plane polarized light to move among a number of particles, all small compared with any of the wavelengths.
The crystals are feebly doubly refracting, and in polarized light exhibit a banded structure parallel to the cube faces.
In polarized light they show a weak grey colour with a black cross, the arms of which are parallel to the cobwebs in the eyepiece of the microscope and remain stationary when the section is rotated.
Porphyritic crystals often contract less than the surrounding glass, which accordingly becomes strained, and in polarized light may show a weak double refraction in a limited area surrounding the crystal.
Faraday had for a long time kept in view the possibility of using a ray of polarized light as a means of investigating the condition of transparent bodies when acted on by electric and magnetic forces.
