SECT. XX. LENGTH OF THE UNDULATIONS. 167 



curvature of the lens, and the actual breadth of the 

 rings in parts of an inch, it was easy to compute that 

 the thickness of air at the darkest part of the first ring 

 is the 80 oa part of an inch, whence all the others have 

 been deduced. As these intervals determine the length 

 of the waves on the undulatory hypothesis, it appears 

 that the length of a wave of the extreme red of the 

 solar spectrum is equal to the 00000266th part of an 

 inch ; that the length of a wave of the extreme violet is 

 equal to the 0*00001 67th part of an inch; and as the 

 time of a vibration of a particle of ether producing any 

 particular color is directly as the length of a wave of that 

 color, and inversely as the velocity of light, it follows 

 that the molecules of ether producing the extreme red 

 of the solar spectrum perform 458 millions of millions 

 of vibrations in a second ; and that those producing the 

 extreme violet accomplish 727 millions of millions of 

 vibrations in the same time. The lengths of the waves 

 of the intermediate colors, and the number of then* 

 vibrations, being intermediate between these two, white 

 light, which consists of all the colors, is consequently 

 a mixture of waves of all lengths between the limits of 

 the extreme red and violet. The determination of these 

 minute portions of time and space, both of which have 

 a real existence, being the actual results of measure- 

 ment, do as much honor to the genius of Newton as 

 that of the law of gravitation. 



The phenomenon of the colored rings takes place in 

 vacuo as well as in ah- ; which proves that it is the dis- 

 tance between the lenses alone, and not the air, which 

 produces the colors. However, if water or oil be put 

 between them, the rings contract, but no other change 

 ensues ; and Newton found that the thickness of differ- 

 ent media at which a given tint is seen, is in the inverse 

 ratio of their refractive indices, so that the thickness of 

 laminae which could not otherwise be measured, may be 

 known by their color ; and as the position of the colors 

 in the rings is invariable, they form a fixed standard of 

 comparison well known as Newton's scale of colors ; 

 each tint being estimated according to the ring to which 

 it belongs from the central spot inclusively. Not only 

 the periodical colors which have been described, but the 



