428 HISTORY OF OPTICS. 



cannot discriminate them from one,) this compound 

 or duplicated pulse does produce on the retina the 

 sensation of a yellow." The reason for the produc- 

 tion of this particular colour, in the case of which 

 he here speaks, depends on his views concerning 

 the kind of pulses appropriate to each colour ; 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 anticipation of the expla- 

 nation ultimately given of these colours ; 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 jus- 

 tice, looked upon as the great author of the undula- 

 tory theory at the period now under notice, is Huy- 

 ghens, whose Traite de la Lumiere, containing a 

 developement of his theory, was written in 1678, 

 though not published till 1690. In this work he 

 maintained, as Hooke had done, that light consists 

 in undulations, and expands itself spherically, nearly 

 in the same manner as sound ; and he referred to 

 the observations of Romer on Jupiter's satellites, 

 both to prove that this difference takes place suc- 

 cessively, 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 he draws the 

 conclusion, so long looked upon as the turning-point 



