Page:Elementary Text-book of Physics (Anthony, 1897).djvu/479

§ 379] of being more transparent to the extraordinary than to the ordinary ray. By grinding plates of tourmaline to the proper thickness, the ordinary ray is completely absorbed, while the extraordinary ray is transmitted. The best method of obtaining a polarized beam is by the use of a crystal of Iceland spar in which, by an ingenious device, the ordinary ray is suppressed and the extraordinary transmitted. Fig. 143 shows how this is accomplished. $$AB$$ is a crystal of considerable length. It is divided along the plane $$AB,$$ making an angle of 22° with the edge $$AB$$ and perpendicular to a principal plane of the face $$AC.$$ The faces of the cut are polished and the two halves cemented together again by Canada balsam in the same position as at first. In Fig. 144, which is a section through $$ACBD$$ of Fig. 143, $$ab$$ represents the direction of the light which is incident upon the face $$AC.$$ It is separated into two rays, $$o$$ and $$e.$$ Since the refractive index of the balsam is intermediate between the ordinary and extraordinary indices of the spar, and since the angle $$DAB$$ is so chosen that the ray $$o$$ strikes the balsam at an angle of incidence greater than the critical angle, the ray $$o$$ is totally reflected. The ray $$e,$$ on the other hand, having a refractive index in the spar less than in the balsam, is not reflected, but continues through the crystal. A crystal of Iceland spar so treated is called a Nicol's prism, or often simply a Nicol.

A pair of Nicol's prisms, mounted with their axes coinciding, serves as a polariscope. The first Nicol transmits a single beam of polarized light the vibrations of which are in the principal plane. When the principal plane of the second Nicol coincides with that of the first this light is wholly transmitted through it. If the second Nicol or analyzer be turned about its axis, whenever its principal plane makes an angle with the direction of the vibrations, these are resolved into two components, one in and the other at