For every reflected substance there is a particular angle of incidence, which gives a maximum of polarisation in reflected light. It is called the _polarising angle_ for the substance, and its tangent is always equal to the index of refraction of the substance; or, what amounts to the same thing, it is that particular angle of incidence which is the complement of the angle of refraction, so that the refracted rays are at right angles. This important law was discovered experimentally by Sir David Brewster. Tourmaline, like Iceland spar, is a negative uniaxial crystal; and its use as a polariser depends on the property which it possesses of absorbing the ordinary much more rapidly than the extraordinary ray, so that a thickness which is tolerably transparent to the latter is almost completely opaque to the former. Its pale cobalt blue colour enhances the beauty of certain crystal and mineral substances, but like Iceland spar, the paler and more perfect crystals are becoming scarce. _Selenite_ is another mineral of value in polarisation experiments. It is a native crystalline hydrated sulphate of lime. A beautiful fibrous variety called _satin-gypsum_ is found in Derbyshire. The form of the crystal most frequently met with is that of an oblique rectangular prism, with ten rhomboidal faces, two of which are much larger than the rest. It is usually split up into thin laminæ parallel to their lateral faces; each film should have a thickness of from one-twentieth to one-sixtieth of an inch. In the two rectangular directions these films allow perpendicular rays of polarised light to traverse them unchanged, termed their _neutral axes_. In two other directions, however, which form respectively angles of 45° with the neutral axes, these films have the property of double refraction, a direction known as the _depolarising axis_. [Illustration: Fig. 182.--Darker’s Selenite Films and Stage.]