HERMANN VON HELMHOLTZ 



band in the green, and it was found that the refrac- 

 tive index rises (as in normal bodies) for rays from 

 the red to the yellow. But all the rest of the 

 transmitted light, consisting of the more refrangible 

 rays, is less refracted than the red. From some 

 experiments of De Klecker, published in 1879, it 

 would appear that c The addition of fuchsine to alcohol 

 alters the speed of propagation of the (so-called) less 

 refrangible rays, but not perceptibly that of the more 

 refrangible.' * 



The phenomenon had been examined by many 

 physicists ; but the explanations were unsatisfactory. 

 Helmholtz's first solution was founded on the sup- 

 position that in transparent media certain ponderable 

 molecules participate in the vibrations of the ether 

 surrounding them. Mathematical difficulties arise if 

 we suppose that there is discontinuity between the 

 surfaces of these particles and that of the ether 

 everywhere in contact with them, so he further 

 assumes that there is continuity, that is, that 

 there is no abrupt transition. Now, imagine light 

 falling on such an arrangement. Part of the vibra- 

 tions transmitted by the ponderable molecules is 

 transformed into irregular vibrations, that is to say, 

 into heat. Thus part of the light is absorbed or 

 disappears. The ponderable medium opposes to the 

 movement of the vibrating molecules a resistance like 



1 Tait, Light, p. 157. See also Jamin et Bouty, Court de Physique, t. 

 iii., p. 542. 



230 



