INTERFERENCES. 203 



transform at pleasure light into darkness, day into 

 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 &quot;an inter 

 ference.&quot; 



Grimaldi had long ago (before 1C 65) 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 &quot;deign even once in his great work 

 to discuss it critically, it remained more than a century 

 in oblivion.* 



* The silence of Newton us to Hooke s attempt at explaining the 

 colours of films by the wave theory may, we conceive, be fully ex- 



