It has now been known for several years that the molecules of a gas consisting of positive and negative charges set into forced vibrations by the electric field of an incident light-wave are responsible, not only for the refractive properties of the medium, but also for lateral scattering and extinction as exemplified on a large scale by the blue of the sky and the colour of the setting sun. In fact, until comparatively recently, observations on the extinction of solar radiation of various wave-lengths by the earth’s atmosphere have provided the only dataf by means of which the theory of molecular scattering and extinction could be tested, this by satisfactory evaluations of the number of molecules per cubic centimetre of air under standard conditions of temperature and pressure, making use for the purpose of Rayleigh’s well-known extinction formula based on the idea of the
symmetrical molecule
,
i.e.
, one in which the dispersion electrons move in the direction of the electric vector in the lightwavewave. As far as observations were then available, theory gave a tolerably good account of measurements of sky-intensity, both as regards quality and polarization. The difficulty in this case is to take into account the illumination of the atmosphere by itself, a problem capable of reasonably simple solution in terms of integral equations only if the curvature of the earth is disregarded. From the experimental point of view, satisfactory observations are made difficult by the omnipresent and ever-varying dust content of the atmosphere at ordinary levels, to say nothing of the “ haziness ” and extinction due to the presence of water-vapour.
On the complex anisotropic molecule in relation to the dispersion and scattering of light
