and M will determine the directions AN and AN of the two refracted rays[1] within the crystal.
Huygens did not in the Théorie de la lumière attempt a detailed physical explanation of the spheroidal wave, but communicated one later in a letter to Papin,[2] written in December, 1690, “As to the kinds of matter contained in Iceland crystal," he says, "I suppose one composed of small spheroids, and another which occupies the interspaces around these spheroids, and which serves to bind them together. Besides these, there is the matter of aether permeating all the crystal, both between and within the parcels of the two kinds of matter just mentioned; for I suppose both the little spheroids, and the matter which occupies the intervals around them, to be composed of small fixed particles, amongst which are diffused in perpetual motion the still finer particles of the aether. There is now no reason why the ordinary ray in the crystal should not be due to waves propagated in this aethereal matter. To account for the extraordinary refraction, I conceive another kind of waves, which have for vehicle both the aethereal matter and the two other kinds of matter constituting the crystal. Of these latter, I suppose that the matter of the small spheroids transmits the waves a little more quickly than the aethereal matter, while that around the spheroids transmits these waves a little more slowly than the same aethereal matter, ... These same waves, when they travel in the direction of the breadth of the spheroids, meet with more of the matter of the spheroids, or at least pass with less obstruction, and so are propagated a little more quickly in this sense than in the other ; thus the light-disturbance is propagated as a spheroidal sheet."
Huygens made another discovery[3] of capital importance when
- ↑ The word ray in the wave-theory is always applied to the line which goes from the centre of a wave (i.e. the origin of the disturbance) to a point on its surface, whatever may be the inclination of this line to the surface-element on which it abuts; for this line has the optical properties of the "rays" of the emission theory.
- ↑ Huygens' Œuvres, ed. 1905, x., p. 177.
- ↑ Théorie de la lumière, p. 89.
