French polariscope (class demonstration model), Babinet type

A beam of light consists of transversal waves. This means the light is vibrating perpendicular to the propagation direction. In ordinary light the planes of vibration are divided over all 360 degrees of an arc. In 1808 it was proved that light could be polarized. This means, for example, that a light beam is reflected against a mirror, or passes through a crystal, and only the horizontal vibration direction is left. Some substances are able to change the polarization plane. You can see this through a polariscope. These are four crystal polariscopes, designed by the physicist Félix Savart (no 413 on the photo), the opticist J.L. Hofmann (no 414 with the two-coloured brass tube), the mineralogist Henri Hureau de Sénarmont (no 415), and the mathematician Jacques Babinet respectively.