Bakerian lecture: A study of the line spectrum of sodium as excited by fluorescence

It was recognised in the early days of spectrum analysis by Stokes and Kirchhoff that the absorption of light by a sodium flame or by sodium vapour was conditioned by resonance of the vibrating sodium molecules to that period in the exciting light which was removed. In more recent days our knowledge of these phenomena has been much extended by E. Wiedmann, and later, R. W. Wood, who showed that the absorption just mentioned was accompanied to some extent by lateral re-emission. L. Dunoyer has also made important observations. This is the phenomenon known as resonance radiation. [Since this paper was written I have to lament the death of my beloved father, Lord Rayleigh. I found the following rough memorandum, dated September, 1897, among his papers, not put away carefully, but in a pile of letters and pamphlets of no value. It was written long previously to my own interest in the subject, and I think he had quite forgotten having considered the matter; at all events, he never referred to it when I showed him some of my own experiments. The memorandum runs thus:—“A moderately fed soda flame stops light of D quality incident upon it. This may be seen to some extent with sunlight, but better with electric arc, or perhaps with another and brighter soda flame. What is the nature of this stopping? It is generally spoken of as absorption. But this seems unlikely. It must be a molecular operation. If the molecule in the Bunsen shines on excitation, how can it fail to do so as the result of energy of the right sort already falling upon it, and certainly operative upon it? It seems more likely that the energy is re-radiated without absorption,i. e ., that the light isscattered . If so, it would probably be scattered as from small particles of the ordinary sort with polarisation effect.”—July 21,1919.]