Page:Optics.djvu/186

 propagated, alternations of light and darkness, which, in point of disposition and intensity, agree most minutely with those observed in diffracted light.

This introduction of this principle has enabled M. Fresnel to embrace all the cases of diffraction with extraordinary precision; but an exposition of his results, though very interesting, would lead us farther than the plan of this Work would allow.

The rays of light, in passing through most crystallized substances, are generally divided into two parcels, one of which, containing what are called the ordinary rays, follows the usual mode of refraction; but the other, consisting of what are termed the extraordinary rays, obeys entirely different laws.

This phænomenon takes place in all transparent crystals, except those which split in planes parallel to the sides of a cube, or a regular octohedron. The separation of the rays is more or less strong, according to the nature of the crystal, and the direction which the light takes in passing through it. Of all known substances, the most powerfully double refracting, is the clear carbonate of lime, commonly called Iceland Spar. As this is a comparatively common substance, and may easily be made the subject of experiment, we take it as a first instance.

The crystals of this variety of carbonate of lime are of a rhomboidal form, as represented in Fig. 211. This rhomboid has six acute angles, and two obtuse; these last are formed by three equal plane angles: in the acute dihedral angles, the inclination of the faces is 74° 55′, and consequently, in the others it is 105° 5′. Malus and Dr. Wollaston have both found these values by the reflexion of light.

If a rhomboid of this description be placed on a printed book, or a paper marked with black lines, every thing seen through it will appear to be double, so that each point under the crystal, must send two images to the eye, and consequently, two pencils of rays. This indicates that each simple pencil must be separated into