As has been emphasised by Prof. Pringsheim in his recent admirable report on the Raman effect (Die Naturwissenschaften, Aug. 3, 1928), there is a far-reaching and fundamental analogy between the behaviour of electrons and of light quanta during their collisions with material particles. The new lines appearing in the spectrum of the scattered light are the result of inelastic collisions or of super-elastic collisions of the light quanta with the molecules of the medium, according as the shift of frequency is towards longer or shorter wave-lengths. As has already been pointed out in an earlier communication (NATURE, July 7, 1928, p. 12), the lines with enhanced frequencies are usually of much smaller intensity than those degraded in frequency to an equal extent. The natural explanation of this is that comparatively few molecules are normally present in an excited state and therefore in a position to communicate energy to the light quantum. As the temperature rises, we should expect the number of such molecules to increase, and the proportion of super-elastic to inelastic collisions to rise pari passu.