Fringe Science

The Colmascope and the Polariscope are devices consisting of polarizers oriented with their polarization axes normal to each other andwith a gap between them that allows one to place objects in the gap in order to observe birefringence. It is useful for detecting stresses in glass and plastic, for identifying and orienting crystals, and for determining uniformity of transparent materials. The modern device uses two sheets of flat polarizing material. But the device predates George Eastman’s invention of Polaroid sheet. How did they use crossed polarizers before that time, and how did that differ from the current device? And what are Landolt fringes?