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IN connection with the recent work of Prof. Strutt and of Lord Rayleigh on the molecular scattering of light, Sir Joseph Larmor has put forward the interesting suggestion (Phil. Mag., January, 1919, p. 162) that the additive property of the energy elements scattered by the individual molecules is secured by the irregular alterations in the wave-length of the scattered radiation produced (in accordance with Doppler's principle) by the thermal movements of the molecules. There is one interesting feature of the Doppler effect in the scattered radiation to which Sir Joseph Larmor does not specifically direct attention in his paper, and which it seems important to emphasise, namely, that the magnitude of the Doppler effect would depend on the angle between the primary and the scattered radiation, and would, in fact, practically vanish in directions nearly the same as that of the primary waves. This follows from the fact that the movement of an individual molecule would alter the effective frequency of the radiation received by it, and this has to be taken into account in calculating the effective frequency of the emitted radiation as received by the observer. In directions nearly the same as that of the primary radiation there would be practically a complete compensation, and the Doppler effect would vanish.

The Doppler Effect in the Molecular Scattering of Radiation