Maxwell. 295 



is connected by springs to another massive shell inside it, and 

 so on. The corresponding extension of the equation for the 

 refractive index is 



where p^ p 2 , . . . denote the frequencies of the natural periods 

 of vibration of the atom. 



The validity of the Maxwell- Sellmeier formula of disper- 

 sion was strikingly confirmed by experimental researches in 

 the closing years of the nineteenth century. In 1897 Rubens* 

 showed that the formula represents closely the refractive 

 indices of sylvin (potassium chloride) and rock-salt, with 

 respect to light and radiant heat of wave-lengths between 

 4,240 A.U. and 223,000 A.U. The constants in the formula 

 being known from this comparison, it was possible to predict 

 the dispersion for radiations of still lower frequency ; and it 

 was found that the square of the refractive index should have 

 a negative value (indicating complete reflexion) for wave- 

 lengths 370,000 A.U. to 550,000 A.U. in the case of rock-salt, 

 and for wave-lengths 450,000 ^to 670,000 A.U. in the case of 

 sylvin. This inference was verified experimentally in the 

 following year.f 



It may seem strange that Maxwell, having successfully 

 employed his electromagnetic theory to explain the propagation 

 of light in isotropic media, in crystals, and in metals, should 

 have omitted to apply it to the problem of reflexion and refrac- 

 tion. This is all the more surprising, as the study of the optics 

 of crystals had already revealed a close analogy between the 

 electromagnetic theory and MacCullagh's elastic-solid theory; 

 and in order to explain reflexion and refraction electro- 

 magnetically, nothing more was necessary than to transcribe 

 MacCullagh's investigation of the same problem, interpreting e 

 (the time-flux of the displacement of MacCullagh's aether) as 

 the magnetic force, and curl e as the electric displacement. As 



* Ann. d. Phys. Ix (1897), p. 454. 



t Rubens and Aschkinass, Ann. d. Phys. Ixiv (1898). 



