78 HISTORY OP OPTICS. 



let" were as fourteen to nine ; and the intermediate colors of course 

 corresponded to intermediate thicknesses, and therefore, in his appara- 

 tus, consisting of two lenses pressed together, appeared as rings of 

 intermediate sizes. His mode of confirming the rule, by throwing 

 upon this apparatus differently colored homogeneous light, is striking 

 and elegant. " It was very pleasant," he says, " to see the rings gra- 

 dually swell and contract as the color of the light was changed." 



It is not necessary to enter further into the detail of these pheno- 

 mena, or to notice the rings seen by transmission, and other circum- 

 stances. The important step made by Newton in this matter was, the 

 showing that the rays of light, in these experiments, as they pass 

 onwards 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 colors. Although 

 Xewton did not correctly disentangle the conditions under which this 

 periodical character is manifestly disclosed, the discovery that, under 

 some circumstances, such a periodical character does exist, was likely 

 to influence, and did influence, materially and beneficially, the subse- 

 quent progress of Optics towards a connected theory. 



We must now trace this progress ; but before we proceed to this 

 task, we will briefly notice a number of optical phenomena which had 

 been collected, and which waited for the touch of sound theory to 

 introduce amona; them that rule and order which mere observation 



O 



had sought for in vain. 



CHAPTER VIII. 



ATTEMPTS TO DISCOVER THE LAWS or OTHER PHENOMENA. 



THE phenomena which result from optical combinations, even of a 

 comparatively simple nature, are extremely complex. The theory 

 which is now known accounts for these results with the most curious 

 exactness, and points out the laws which pervade the apparent confu- 

 sion; but without this key to the appearances, it was scarcely possible 

 that any rule or order should be detected. The undertaking was of 



2 Opticks, p. 184. 



