The structure of molecules in relation to their optical anisotropy.—Part I

One of the most significant facts relating to the scattering of light in gases is the imperfection of polarisation of the light scattered in a direction perpendicular to the incident beam. The late Lord Rayleigh and Born explained this phenomenon as being due to the optical anisotropy of the molecule, that is, to the fact that the polarisation induced in a molecule depends on its orientation with respect to the electric vector in the incident light. Lord Rayleigh’s theory does not go into the question as to how the anistropy arises, but merely assumes that there are in each molecule three principal directions of vibration, along which the induced polarisations are different. If A, B, C are the moments induced in a molecule when its three principal directions are respectively along the direction of the electric vector in the incident light, then the ratio of the weak component to the strong in the transversely scattered light is given by r= 2 (A2 + B2 + C2) - 2 (AB + BC + CA) / 4 (A2 + B2 + C2) + AB + BC + CA. We now possess reliable measurements of the imperfection of polarisation in many gases and vapours, from the work of Lord Rayleigh and of Raman and Rao. Recently there has been carried out at Calcutta further measurement of the same quantity, in a series of organic vapours, by Mr. A. S. Ganesan. Some of these results are collected together in Table I.