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INTERFERENCES. 2038 
transform at pleasure light into darkness, day into night? 
The process will excite more surprise than even the 
result. It consists in directing upon the paper, but by a 
route very slightly different, a second ray of light, which, 
taken by itself, would also have brilliantly illuminated 
it. The two rays in mixing together, it might be ex- 
pected, would produce a yet more brilliant illumination ; 
no doubt, it would seem, could exist 'on this point; but 
in point of fact, under certain conditions, they entirely 
destroy each other, and we find ourselves to have created 
darkness by adding one portion of light to another. 
A new fact requires a new term; this phenomenon, 
in which two rays in mixing together destroy each 
other, either wholly or partially, is termed “an inter- 
Serence.” | 
Grimaldi had long ago (before 1665) formed some 
notion of the action which one beam of light may exer- 
cise upon another; but in the experiment which he cites 
this action was but obscurely manifested; and, besides 
this, the conditions which were essential to its produc- 
tion had not been pointed out, and thus no other experi- 
menter followed up the inquiry. 
In searching after the cause of the iridescent colours 
"with which soap bubbles shine so brilliantly, Hooke 
believed that they were the result of interferences ; he 
even very ingeniously pointed out some of the circum- 
stances which cause their production ; but it was a theory 
destitute of actual proofs. And as Newton, who: knew 
of this theory, did not deign even once in his great work 
to discuss it critically, it remained more than a seasactd 
in oblivion.* 
* The silence of Newton as to Hooke’s attempt at explaining the 
colours of films by the wave theory may, we conceive, be fully ex- 
