THE UNDULATORY THEORY. 495 



molecules of the ether bears a sensible ratio to 

 the length of an undulation 19 . Professor Powell ob- 

 tained also, from the general expressions, a formula 

 expressing the relation between the refractive index 

 of a ray, and the length of a wave, or the colour 

 of light" . It then became his task to ascertain 

 whether this relation obtained experimentally ; and 

 he found a very close agreement between the num- 

 bers which resulted from the formula and those 

 observed by Fraunhofer, for ten different kinds of 

 media, namely, certain glasses and fluids 21 . To these 

 he afterwards added ten other cases of crystals 

 observed by M. Rudberg 22 . Mr. Kelland, of Cam- 

 bridge, also calculated, in a manner somewhat dif- 

 ferent, the results of the same hypothesis of finite 

 intervals 23 , and obtaining formulae not exactly the 

 same as Professor Powell, found also an agreement 

 between these and Fraunhofer's observations. 



It may be observed, that the refractive indices 

 observed and employed in these comparisons, were 

 not those determined by the colour of the ray, which 

 is not capable of exact identification, but those more 

 accurate measures which Fraunhofer was enabled 

 to make, in consequence of having detected in the 

 spectrum the black lines which he called B, C, D, E, 

 F, G, H. The agreement between the theoretical 

 formula and the observed numbers is remarkable, 



19 Phil. Mag. vol. vi. p. 266. so Ib. vol. vii. 1835, p. 266. 

 21 Phil. Trans. 1835, p. 249. 22 Ib. 1836, p. 17. 

 33 Camb. Trans, vol. vi. p. 153. 



