INTERFERENCES. 203 



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- 

 ference" 



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 century 

 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- 



