36 HISTORY OF OPTICS. 



by supposing that the rays in a denser medium move more easily, and 

 hence that the pulses become oblique ; a far less satisfactory and con- 

 sistent hypothesis than that of Huyghens, of which we shall next hare 

 to speak. But Hooke has the merit of having also combined with his 

 theory, though somewhat obscurely, the Principle of Interferences, in 

 the application which he makes of it to the colors 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 propagated a kind of fainter ray," which 

 comes behind the other reflected pulse ; " so that hereby (the surfaces 

 AB and EF being so near together that the eye cannot discriminate 

 them from one), this compound or duplicated pulse does produce on 

 the retina the sensation of a yellow." The reason for the production 

 of this particular color, in the case of which he here speaks, depends 

 on his views concerning the kind of pulses appropriate to each color ; 

 and, for the same reason, when the thickness is different, he finds that 

 the result will be a red or a green. This is a very remarkable antici- 

 pation of the explanation ultimately given of these colors ; and we may 

 observe that if Hooke could have measured the thickness of his thin 

 plates, he could hardly have avoided making considerable progress in 

 the doctrine of interferences. 



But the person who is generally, and with justice, looked upon as 

 the great author of the nudulatory theory, at the period now under 

 notice, is Huyghens, whose Traite de la Lumiere, containing a deve- 

 li'pement of his theory, was written in 1078, though not published till 

 1G90. In this work he maintained, as Hooke had clone, that light 

 consists in undulations, and expands itself spherically, nearly in the 

 same manner as sound does ; and he referred to the observations of 

 Homer on Jupiter's satellites, both to prove that this difference takes 

 place successively, and to show its exceeding swiftness. In order to 

 trace the effect of an undulation, Huyghens considers that every point 

 of a wave diffuses its motion in all directions ; and hence lie draws 

 the conclusion, so long looked upon as the turning-point of the com- 

 bat between the rival theories, that the light will not be diffused 

 beyond the rectilinear space, when it passes through an aperture ; 

 " for," says he, " although the partial waves, produced by the particles 

 comprised in the aperture, do diffuse themselves beyond the rectilinear 

 space, these waves do not concur anywhere except in front of the 



Micrographia, p. 66. e Tracts on Optics, p. 209 



