III. On the polarisation of light by reflection from the surface of a crystal of Iceland spar

In the year 1819 Sir David Brewster communicated to the Royal Society (“Phil. Trans.,” 1819, p. 145) an account of some experiments he had made on the polarisation of light by reflection from the surface of double refracting substances, and showed that Malus’ statement with regard to Iceland spar was incorrect. Malus said that Iceland spar behaves towards the light it reflects like a common transparent body, and that its polarising angle is about 56° 30', and that whatever be the angle comprehended between the plane of incidence and the principal section of the crystal, the ray reflected by the first surface is always polarised in the same manner (“Théorie de la Double Refraction,” pp. 240, 241).