PERIODICAL COLOURS. 309 



that, according to very simple laws, the discovery of 

 which in any age would suffice to immortalize a physi- 

 cist. 



The differences of route which produce these conflicts 

 between the rays, accompanied hy their entire mutual 

 destruction, have not the same numerical value for the 

 differently coloured primary rays. When two white rays 

 cross, it is then possible that one of their chief constituent 

 parts, tlie red, for example, may alone be in the condition 

 fit for mutual destruction. But white, deprived of its 

 red, becomes green ! Thus intei'ference of light mani- 

 fests itself in the phenomena of coloration. Thus the 

 different elementary colours are placed in evidence with- 

 out any prism to sei)arate them. We should, however, 

 remark that there does not exist a single point in space 

 where a thousand rays of the same origin do not proceed 

 to cross one another after reflexions more or less oblique, 

 and we shall perceive at a glance the whole extent of 

 the unexplored region which interferences open to the 

 investigations of experimenters. 



When Young published this theory, many phenomena 

 of periodical colours had been already ofi"ered to the no- 

 tice of observers ; and we should add, had resisted all 

 attempts at explanation. Among the number we mio-ht 

 instance the coloured rings which are formed by reflexion, 

 not on thin films, but on mirrors of thick glass slightly 

 concave ; the irridescent bands of different breadths with 

 which the shadows of bodies are bordered on the outside, 

 and in some instances covered within, which Grimaldi 

 first noticed, and which afterwards uselessly exercised 

 the genius of Newton, and of which the completion of 

 the theory was reserved for Fresnel ; the bows coloured 

 red and green, which are perceived in greater or less 



