LAWS OF THE COLOURS OF THIN PLATES. 415 



the plate was about 1-1 78000th of an inch, or 

 three times, five times, seven times that magnitude, 

 there was a bright colour produced; but black- 

 ness, when the thickness was exactly intermediate 

 between those magnitudes. He found, also, that 

 the thicknesses which gave red and violet 2 were as 

 fourteen to nine; and the intermediate colours of 

 course corresponded to intermediate thicknesses, 

 and therefore, in his apparatus, consisting of two 

 lenses pressed together, appeared as rings of inter- 

 mediate sizes. His mode of confirming the rule, 

 by throwing upon this apparatus differently coloured 

 homogeneous light, is striking and elegant. "It 

 was very pleasant," he says, " to see the rings gra- 

 dually swell and contract as the colour of the light 

 was changed." 



It is not necessary to enter farther into the 

 detail of these phenomena, or to notice the rings 

 seen by transmission, and other circumstances. The 

 important step made by Newton in this matter 

 was, the showing that the rays of light, in these 

 experiments, go periodically through certain cycles 

 of modification, each period occupying nearly the 

 small fraction of an inch mentioned above ; and 

 this interval being different for different colours. 

 Although Newton did not correctly disentangle the 

 conditions under which this periodical character is 

 manifestly disclosed, the discovery that, under some 

 circumstances, it does exist, was likely to influence, 

 Opticks, p. 184. 



