78 PHYSICAL PROPERTIES AND COMPOSITION 



by means of the characteristic equations, and such more 

 or less rationally based empirical relations as are associated 

 with these. The phenomena of the refraction of light 

 belong to this class, because, on the one hand additive 

 relations are found on comparing different substances, 

 closely resembling those discovered in regard to volume, 

 and on the other, this parallelism, together with the dielec- 

 tric constant, affords a certain theoretical basis. 



A. Velocity of Light and Refractive Index. 



The velocity of light has its highest value in vacuo, so 

 that the decrease in other media may be associated witli 

 the greater or smaller space occupied by matter, the 

 measure of which we have so far taken to be the volume 

 at the absolute zero, so that to measure the velocity of 

 light at that temperature would be a matter of much 

 importance. 



The ratio of the velocities in two media is known to be 

 given by the refractive indices, since, according to Snellius : 



v-, sin L 



= = n. 



v 2 sin r 



where L is the angle of incidence (angle with the normal 

 to the surface in the medium to which the index i refers), 

 r the angle of refraction, n the refractive index calculable 

 from the two numbers, and which accordingly as v^ refers 

 to vacuum, is always greater than unity. The refractive 

 index is, however, often given relatively to air, thus caus- 

 ing a reduction in the numerical value of constant relative 

 amount, but small, since the refractive index of air is only 

 slightly greater than unity. 



B. Influence of Wave-length. 



Whilst to start with we compare optical relations between 

 different substances to the corresponding volume relations, 

 we must now remark on a fundamental fact which makes 

 the problem much more complex. Whilst in the volume 



