ON LIGHT. 311 



centre, do actually follow the law of arithmetical pro- 

 gression (as on the above theory they should do), being 

 in the proportions of the numbers o, i, 2, 3, etc. 



(95.) By measuring then the diameter of (say) the 

 tenth dark ring (for the sake of greater precision), cal 

 culating the corresponding interval, or versed sine, and 

 taking one-tenth of the result, we shall get the interval 

 corresponding to the first dark ring for any particular 

 coloured light and this, by what has been above shown, 

 is the half of a wave-length for such light. Proceeding 

 thus, Newton found for what he considered the most 

 luminous yellow rays, one 89,oooth part of an inch for 

 the interval in question, which gives for the length of an 

 entire undulation of such rays, one 44,5 ooth of an inch. 

 This comes exceedingly near to the result which later 

 experimenters have obtained for that purely homogene- 

 ous yellow light emitted by a salted spirit-lamp, which is 

 one 43,197^ of an inch. For the extreme red and ex- 

 treme violet rays, (as well as their limits can be fixed,) the 

 corresponding wave-lengths are respectively one 33,866th, 

 and one 70,555^1 of an inch. 



(96.) These, it will be observed, are the lengths of 

 the undulations in air. In water, glass, or other media, 

 they are smaller, in the inverse proportion of the refrac- 

 tive index of the medium ; for in such media the velo- 

 city of light, as we have seen, is less in that propor- 

 tion ; and the number of undulations per second re- 

 maining the same, while the space occupied by them is 

 less, their individual extent must of course be less in 

 the same proportion. This, too, is in accordance with 



