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 p1, p2, … denote the frequencies of the natural periods of vibration of the atom.
The validity of the Maxwell-Sellmeier formula of dispersion was strikingly confirmed by experimental researches in the closing years of the nineteenth century. In 1897 Rubens[1] 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 tho 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 wavelengths 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.V. in the case of sylvin. This inference was verified experimentally in the following year.[2]
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 refraction. 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 eloctromagnetically, nothing more was necessary than to transcribe MacCullagh's investigation of the same problem, interpreting ė (the time-flux of the displacement of MacCullagh's aether) as the magnetic force, and curl e as the electric displacement. As
