MODERN THEORIES OF HEAT AND LIGHT 



from the surface, the color vanished with the inclina- 

 tion, and was equal at equal inclinations on either side. 



"This experiment affords a very strong confirma- 

 tion of the theory. It is impossible to deduce any ex- 

 planation of it from any hypothesis hitherto advanced ; 

 and I believe it would be difficult to invent any other 

 that would account for it. There is a striking analogy 

 between this separation of colors and the production 

 of a musical note by successive echoes from equidistant 

 iron palisades, which I have found to correspond pretty 

 accurately with the known velocity of sound and the 

 distances of the surfaces. 



"It is not improbable that the colors of the integu- 

 ments of some insects, and of some other natural bod- 

 ies, exhibiting in different lights the most beautiful 

 versatility, may be found to be of this description, and 

 not to be derived from thin plates. In some cases a 

 single scratch or furrow may produce similar effects, 

 by the reflection of its opposite edges." s 



This doctrine of interference of undulations was the 

 absolutely novel part of Young's theory. The all- 

 compassing genius of Robert Hooke had, indeed, very 

 nearly apprehended it more than a century before, as 

 Young himself points out, but no one else had so much 

 as vaguely conceived it; and even with the sagacious 

 Hooke it was only a happy guess, never distinctly out- 

 lined in his own mind, and utterly ignored by all others. 

 Young did not know of Hooke's guess until he himself 

 had fully formulated the theory, but he hastened then 

 to give his predecessor all the credit that could possibly 

 be adjudged his due by the most disinterested observer. 



223 



