THE WAVE THEOEY. 271 



between the glasses corresponding to this distance that 

 Newton required to know, and this he found by calcu- 

 lation to be 3-7Wot a °f an inch. This, be it remem- 

 bered, is the distance corresponding to the fifth ring. 

 The interval corresponding to the first ring would 

 be only a fifth of this, or, in other words, about 

 180 1 000 th of an inch. Such are the magnitudes with 

 which we have to deal before the question ' What is 

 Light? ' can be scientifically answered. 



Newton's explanation of the rings which he was the 

 first to discover, though artificial in the highest degree, 

 is marked by his profound sagacity. He was hampered 

 by the notion of the 'corporeity ' of light. He could 

 not get over the objection raised by himself as to the 

 impossibility of shadows in a fluid medium. He held 

 therefore that light was due to the darting forth of 

 minute particles in straight lines ; and he threw out the 

 idea that colour might be due to the difference of ' big- 

 ness ' in the particles. He endowed these particles with 

 what he called fits of easy transmission and reflection. 

 The dark rings, in his immortal experiment, were pro- 

 duced where the light-particles were in their trans- 

 missive ' fit.' They went through both surfaces of the 

 film of air, and were not thrown back to the eye. The 

 bright rings occurred where the light-particles were in 

 their reflective fit, and where, on reaching the second 

 surface of the film, they were thrown back to the eye. 

 The cardinal point here is, that Newton regarded the 

 recurrence of light and darkness as due to an action 

 confined to the second surface of the film. And here 

 it was that Young came into irreconcilable collision 

 with him, proving to demonstration that the dark rings 

 occurred where the portions of light reflected from both 

 sides of the film extinguished each other by inter- 

 ference, while the bright rings occurred where the light 



