OF NATURAL PHILOSOPHY. 207 



by M. Fresnel, as decisive, in his mind, of the ques- 

 tion between the two great opinions on the nature 

 of light, which, since the time of Newton and 

 Huyghens, have divided philosophers. (See 207.) 

 When two very clean glasses are laid one on the 

 other, if they be not perfectly flat, but one or both 

 in an almost imperceptible degree convex or promi- 

 nent, beautiful and vivid colours will be seen between 

 them ; and if these be viewed through a red glass, 

 their appearance will be that of alternate dark and 

 bright stripes. These stripes are formed between, the 

 two surfaces in apparent contact, as any one may 

 satisfy himself by using, instead of a flat plate of 

 glass for the upper one, a triangular-shaped piece, 

 called a prism, like a three-cornered stick, and 

 looking through the inclined side of it next the 

 eye, by which arrangement the reflection of light 

 from the upper surface is prevented from inter- 

 mixing with that from the surfaces in contact- 

 Now, the coloured stripes thus produced are ex- 

 plicable on both theories, and are appealed to by 

 both as strong confirmatory facts ; but there is a 

 difference in one circumstance according as one or 

 the other theory is employed to explain them. In 

 the case of the Huyghenian doctrine, the intervals 

 between the bright stripes ought to appear absolutely 

 black ; in the other, half bright, when so viewed 

 through a prism. This curious case of difference 

 was tried as soon as the opposing consequences of 

 the two theories were noted by M. Fresnel, and the 

 result is stated by him to be decisive in favour of 

 that theory which makes light to consist in the 

 vibrations of an elastic medium. 



