Page:Elementary Text-book of Physics (Anthony, 1897).djvu/472

458 axis perpendicular to the paper, one of the images remains stationary, and the other revolves around it. The images lie in a plane perpendicular to the paper, and parallel to the line joining the two obtuse angles of the face by vrhich the light enters or emerges. The entering and emerging light is supposed in this case to be normal to the surfaces of the crystal. If the crystal be turned with its faces oblique to the light, the line joining the images will, in certain cases, not lie parallel to the line joining the obtuse angles of the faces. If the distances of the two images from the observer be carefully noticed it will be seen that the stationary one appears nearer than the other. If the obtuse angles $$A$$ and $$X$$ be cut away, and the new surfaces thus formed at right angles to the optic axis be polished, images seen perpendicularly through these faces do not appear double. By cutting the crystal into prisms in various ways its indices of refraction may be measured. It is found that, of the two beams into which light is, in general, divided in the crystal, one obeys the ordinary laws of refraction, and has a refractive index 1.658. It is called the ordinary ray. The other has no constant refractive index, does not in general lie in the normal plane containing the incident ray, and refraction may occur when the incidence is normal. It is the extraordinary ray. The ratio between the sines of the angles of incidence and refraction varies, for the Fraunhofer line D, from 1.658, the ordinary index, to 1.486. This minimum value is called the extraordinary index.

375. Explanation of Double Refraction.—In § 334 it was seen that the index of refraction of a substance is the reciprocal of the ratio of the velocity of light in the substance to its velocity in a vacuum. It is plain, then, that the velocity of light for the ordinary ray of the last section is the same for all directions, and that, if light emanate from a point within the crystal, the light, following the ordinary laws of refraction, must proceed in spherical waves about that point as a centre, as in any singly refracting medium. The phenomena presented by the extraordinary light in Iceland spar are fully explained by assuming that the velocities in different directions in the crystal are such as to give a wave front in the