14 The Theory of the Aether 



an Homogeneous transparent medium LL, and DA, EB, FC, to be 

 small portions of the orbicular impulses which must therefore 

 cut the Rays at right angles : these Rays meeting with the plain 

 surface NO of a medium that yields an easier transitus to the 

 propagation of light, and falling obliquely on it, they will in the 

 medium MM be refracted towards the perpendicular of the 

 surface. And because this medium is more easily trajected than 

 the former by a third, therefore the point of the orbicular 

 pulse FG will be moved to If four spaces in the same time that 

 F, the other end of it, is moved to three spaces, therefore the 

 whole refracted pulse to H shall be oblique to the refracted Rays 

 GHK and /." 



Although this is not in all respects successful, it represents 

 a decided advance on the treatment of the same problem by 

 Descartes, which rested on a mere analogy. Hooke tries to 

 determine what happens to the wave-front when it meets 

 the interface between two media ; and for this end he intro- 

 duces the correct principle that the side of the wave-front 

 which first meets the interface will go forward in the second 

 medium with the velocity proper to that medium, while the 

 other side of the wave-front which is still in the first medium 

 is still moving with the old velocity : so that the wave-front 

 will be deflected in the transition from one medium to the 

 other. 



This deflection of the wave-front was supposed by Hooke to 

 be the origin of the prismatic colours. He regarded natural or 

 white light as the simplest type of disturbance, being consti- 

 tuted by a simple and uniform pulse at right angles to the 

 direction of propagation, and inferred that colour is generated 

 by the distortion to which this disturbance is subjected in the 

 process of refraction. "The Ray,"* he says, " is dispersed, split, and 

 opened by its Refraction at the Superficies of a second medium, 

 and from a line is opened into a diverging Superficies, and 

 so obliquated, whereby the appearances of Colours are produced." 



* Hooke, Posthnmo/is Works, p. 82. 



