SECT, xx.] NEWTON'S KINGS. 193 



by black intervals. The size of the rings varies with the 

 colour of the light. They are largest in red, and decrease 

 in magnitude with the succeeding prismatic colours, being 

 smallest in violet light. 



Since one of the glasses is plane and the other spherical, it 

 is evident, that from the point of contact, the space between 

 them gradually increases in thickness all round, so that a cer- 

 tain thickness of air corresponds to each colour, which in the 

 undulatory system measures the length of the wave producing 

 it (N. 195). By actual measurement, Sir Isaac Newton found 

 that the squares of the diameters of the brightest part of each 

 ring are as the odd numbers, 1, 3, 5, 7, &c. ; and that the 

 squares of the diameters of the darkest parts are as the even 

 numbers, 0, 2, 4, 6, <fec. Consequently the intervals between 

 the glasses at these points are in the same proportion. If, 

 then, the thickness of the air corresponding to any one colour 

 couldbe found,its thickness for all the others wouldbe known. 

 Now, as Sir Isaac Newton knew the radius of curvature of the 

 ^ens, 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 gg^o P art 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 0'0000266th part of an inch ; that 

 the length of a wave of the extreme violet is equal to the 

 00000167th part of an inch ; and, as the time of a vibration 

 of a particle of ether producing any particular colour is directly 

 as the length of a wave of that colour, and inversely as the 

 velocity of light, it follows that the molecules of ether pro- 

 ducing 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 colours, and the number of their 

 vibrations, being intermediate between these two, white light, 



