PERIODICAL COLOURS. 309 
that, according to very simple laws, the discovery of 
which in any age would suflice to immortalize a physi- 
cist. 
The differences of route which produce these conflicts 
between the rays, accompanied by 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, the red, for example, may alone be in the condition 
fit for mutual destruction. But white, deprived of its 
red, becomes green! Thus interference of light mani- 
fests itself in the phenomena of coloration. Thus the 
different elementary colours are placed in evidence with- 
out any prism to separate 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 offered to the no- 
tice of observers ; aud we should add, had resisted all 
attempts at explanation. Among the number we might 
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 
