Page:Optics.djvu/205

 invariable in each molecule, and we will consider it only as relative to its actual direction, in order to leave each molecule at liberty to turn round its axis of polarization. According to these definitions all the results that we have hitherto obtained may be enounced very simply and clearly in the following manner:

When a ray of light is reflected by a polished surface, under the angle which produces complete polarization, the axis of polarization of every reflected molecule is situated in the plane of reflexion, and perpendicular to the actual axis of translation of that molecule.

If the incident molecules are turned so that this condition cannot possibly be fulfilled, they will not be reflected, at least under the angle of complete polarization. That happens when the axis of polarization of an incident molecule is perpendicular to the plane of incidence, the angle of incidence being properly determined à priori.

Generally speaking, when a polished surface receives a polarized ray under the angle at which it would itself produce polarization, if it be made to turn round the ray without changing that angle, the quantity of light reflected in different positions varies as the square of the cosine of the angle between the plane of incidence, and the axis of polarization.

When a ray of light has undergone polarization in a certain direction, by the process above described, it carries that property with it, and preserves it without sensible alteration, when made to pass perpendicularly through even considerable thicknesses of air, water, and, in general, any substance that exerts only single refraction, but double refracting media alter, in general, the polarization of a ray, and in a manner, to all appearance, sudden, communicating to it a new polarization of the same nature in a different direction. It is only when crystals are held in certain directions, that the ray can escape this disturbing influence. Let us endeavour to compare more closely these two kinds of action.

That of single-axed crystals has been studied by Malus, who has comprised its effects in the following law. When a pencil of light naturally emanating from a luminous body, passes through a