Page:Optics.djvu/199

 It will be seen farther on that the same analogy extends also to another species of action that crystallized substances exert on light, which will be explained in the following article.

The polarization of light is a property discovered by Malus, which consists in certain affections that the rays of light assume on being reflected by polished surfaces, or refracted by these same surfaces, or transmitted through substances possessing double refraction.

Though it would be impossible here to give a complete exposition of the details of these phænomena, we will at least describe some of the experiments by which they may be exhibited.

The first and principal of these consists in giving to light a modification, such that the rays composing a pencil will all escape reflexion when they fall on a reflecting surface under certain circumstances.

As an instance, suppose a beam of sun-light $$SI$$ (Fig. 220.) falls on the first surface $$LL$$ of a plate of glass, smooth but not silvered, making with the surface an angle of 35° 25′: it will be reflected in the direction $$II'$$, making the angle of reflexion equal to that of incidence. Let it then be received on another plate of glass, smooth but unsilvered, like the former; generally speaking it will be again reflected with a partial loss. But the reflexion will cease altogether if the second glass be placed like the first, at an angle of 35° 25′ to the line $$II'$$, provided also it be so turned that the second reflexion take place in a plane $$I'L$$ perpendicular to that of the first, $$SIL$$.

In order to make this disposition of the glasses more clearly intelligible, we may imagine that $$II'$$ is a vertical line, that $$IS$$ lies north and south, and $$I'L$$ east and west.

Before we enter upon the inferences to be drawn from this remarkable experiment, I will make a few observations on the manner of performing it conveniently and accurately.

Many kinds of apparatus may be devised to attain this end. That which M. Biot usually employs, is represented in Fig. 221.