simultaneously. In this idea he found no difficulty; as he says: “It is certain that a space occupied more than one kind of matter may permit the propagation of several kinds of waves, different in velocity; for this actually happens in air mixed with aethereal matter, where sound-waves and light-waves are propagated together."
Accordingly he supposed that a light-disturbance generated at any spot within a crystal of Iceland spar spreads out in the form of a wave-surface, composed of a sphere and a spheroid having the origin of disturbance as centre. The spherical wave-front corresponds to the ordinary ray, and the spheroid to the extraordinary ray: and the direction in which the extraordinary ray is refracted may be determined by a geometrical construction, in which the spheroid takes the place which in the ordinary construction is taken by the sphere.
Thus, let the plane of the figure be at right angles to the intersection of the wave-front with the surface of the crystal; let AB represent the trace of the incident wave-front; and suppose that in unit time the disturbance from B reaches the interface at T. In this unit-interval of time the disturbance from A will have spread out within the crystal into a sphere and spheroid: so the wave-front corresponding to the
ordinary ray will be the tangent-plane to the sphere through the line whose trace is T, while the wave-front corresponding to the extraordinary ray will be the tangent-plane to the spheroid through the same line. The points of contact N
