Page:Elementary Text-book of Physics (Anthony, 1897).djvu/473

§376] form of a flattened spheroid, of which the polar diameter, parallel to the optic axis, is equal to the diameter of the ordinary spherical wave, and the equatorial diameter is to its polar diameter as 1.658 is to 1.486. Prom these two wave surfaces the path of the light may easily be determined by construction by methods already explained in § 334, and exemplified in Fig. 141, in which $$ic$$ represents the direction of the incident light, and $$co$$ and $$ce$$ the ordinary and extraordinary rays respectively.

376. Polarization of the Doubly Refracted light.—If a second crystal be placed in front of the first in any of the experiments described in the last section, there will be seen in general four images instead of two; but if the second crystal be turned, the images change in brightness, and for four positions of the second crystal, when its principal plane is parallel or at right angles to the principal plane of the first, two of the images are invisible, and the other two are at a maximum brightness. If one of the beams of light produced by the first crystal be intercepted by a screen, and the other allowed to pass alone through the second crystal, the phenomena presented are easily followed. If the principal planes of the two crystals coincide, only one image is seen. If the second crystal be now rotated about the beam of light as an axis, a second image at once appears, at first very faint, but increasing in brightness. The original image at the same time diminishes in brightness, and the two are equally bright when the angle between the principal planes is 45°. If the angle be 90° the first image disappears, and the second is at its maximum brilliancy. As the rotation