MAGNU8S0N — ANOMALOUS DISPERSION OF CYANIN. 261 



be neglected. Ketteler 1 in 1870 determined the value of the 

 constants, in Briot's formula, for several substances and found 

 that the calculated values for the index of refraction agreed 

 closely with those observed. 



It was at this time that Christiansen and Kundt brought the 

 phenomenon of anomalous dispersion into prominence. These 

 remarkable observations could not be explained by either 

 Cauchy's or Briot's equations, but rendered necessary the con- 

 struction of a new theory. 



Sellmeier 2 made the first attempt and laid the basis for fur- 

 ther work. His assumptions are as follows : 



1st. The existence of an ether of constant elasticity and density. 



2nd. The retardation of the light is entirely due to the inter- 

 action between the ether and the material particles. 



3rd. The intensity of the interaction depends upon the ratio be- 

 tween the natural period of vibration of the material parti- 

 cles and of the wave length, and that an absorption line re- 

 sults when this ratio is unity. 



4th. That each material particle is entirely free to vibrate with- 

 out affecting the other particles. 

 Several minor assumptions are made in the development of 



the first equation to simplify the calculations or even to make 



them possible. 



From these assumptions Sellmeier deduces the following 



formula for the refractive index : 



m t a ag 



. ! ? *> -d' (13). 



m' a" 



Where t equals period of vibration of *■. 



Where d equals natural period of- vibration of material particle. 

 Where a equals amplitude of center of gravity of material par- 

 ticle. 

 Where a equals amplitude of ether vibration. 

 Where m' equals mass of ether in unit volume. 

 Where m equals mass of matter in unit volume. 



i Ketteler, Pogg. Ann. CXL., p. 1. 



a Sellmeier, Pogg. Ann. CXLV, pp. 399, 520 ; CXL VII., pp. 387, 535, (1873). 



