156 Proceedings of the Royal Society of Edinburgh. 
[Sess. 
XIV. — The Absorption of Light by Inorganic Salts. No. XI. : 
Conclusion. By R. A. Houstoun, M.A., Ph.D., D.Sc. 
(MS. received February 10, 1913. Read March 3, 1913.) 
In this paper a short account will be given of the present state of the 
theory of the absorption of light, with special reference to the results 
gained in this series of investigations. 
Theories of the dispersion of light may be divided into two classes : 
(1) those in which the body is regarded as consisting of particles which 
vibrate under the influence of the light wave ; and (2) those in which the 
body is regarded as consisting of obstacles which diffract the light wave. 
According to (2), light is scattered, not absorbed ; a wave going through 
the body diminishes in intensity, but the energy lost is radiated out 
laterally without change of wave-length. Theories of the second type can 
account for a genuine absorption only by supposing light to be absorbed 
inside the obstacles according to theories of the first type. 
If we consider the theories of the first type we find that the particles 
may do either of two things. They may move about against a frictional 
resistance. This is the case of a metal. Or they may vibrate about their 
positions of equilibrium. In this case we get an absorption spectrum with 
well-defined bands, and this case alone will be considered in the present 
paper. 
Since the time of Sellmeier there have been various theories of dis- 
persion founded on the principle of sympathetic vibrations.* The best- 
known modern theories are those of H. A. Lorentz, Drude, and Planck. 
They assume that in the body various classes of electrons are in vibration, 
and from the equations of motion of these electrons they obtain the 
dielectric constant of the medium, and hence its index of refraction and 
coefficient of absorption. Though these theories agree in the main, they 
differ in some minor points. According to Drude, it is the electric intensity 
of the incident wave that acts on the electrons ; according to Lorentz and 
Planck, it is the latter modified by the presence of the surrounding 
electrons. Drude makes no attempt to explain the friction term in the 
equation of motion of the electrons. Lorentz explains it by the transfer- 
* For a clear account of them all, cf. Professor Pfliiger’s monograph in the fourth 
volume of Kayser’s Spectroscopie. 
