in the Seventeenth Century. 27 



and M will determine the directions AN and A M of the two- 

 refracted rays* within the crystal. 



Huygens did not in the Thtoi-ie de la lumiere attempt a detailed 

 physical explanation of the spheroidal wave, but communicated 

 one later in a letter to Papin,f 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 propa- 

 gated 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 disco veryj 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 disturbnnce) 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. 



t Huygens' (Envres, ed. 1905, x., p. 177. 



+ T/ieorie de la lumiere, p. 89. 



