EPOCH OF YOUNG AND FRESNEL. 427 



reasonings from them. In a short time, however, 

 the rival optical theory of undulations made its 

 appearance. Hooke in hisMicrographia (1664) pro- 

 pounds it, upon occasion of his observations, already 

 noticed, on the colours of thin plates. He there 

 asserts 3 light to consist in a " quick, short, vibrating 

 motion," and that it is propagated in a homoge- 

 neous medium, in such a way that " every pulse or 

 vibration of the luminous body will generate a 

 sphere, which will continually increase and grow 

 bigger, just after the same manner (though inde- 

 finitely swifter) as the waves or rings on the surface 

 of water do swell into bigger and bigger circles 

 about a point in it 4 ." He applies this to the expla- 

 nation of refraction, by supposing that the rays in a 

 denser medium move more easily, and hence that 

 the pulses become oblique, a far less satisfactory 

 and consistent hypothesis than that of Huyghens, of 

 which we shall next have to speak. But Hooke has 

 the merit of having also combined with his theory, 

 though somewhat obscurely, the Principle of Inter- 

 ferences, in the application which he makes of it to 

 the colours of thin plates. Thus 5 he supposes the 

 light to be reflected at the first surface of such 

 plates ; and he adds, " after two refractions and one 

 reflection (from the second surface) there is propa- 

 gated a kind of fainter ray," which comes behind 

 the other reflected pulse ; " so that hereby (the sur- 

 faces AB and EF being so near together that the eye 

 3 Micrographia, p. 56. 4 Ib. p. 57. * Ib. p. 66. 



