370 Prof. Wood and Mr. Moore on the Fluorescence 



is ti resonance phenomenon ; moreover the clamping factor 

 must be very small since we have interference with large 

 path difference in the case of sodium light. The non-luminous 

 va])Our employed in these experiments m.ay of course be in a 

 different state from the vapour in a sodium flame, but even if 

 this be the case it does not seem justifiable to assume a large 

 amount of damping, for this should cause a displacement 

 of the absorption-lines with reference to the position which 

 they occupy in the case of absorption by a sodium flame. 



It is worthy of note that lines corresponding in position 

 to the position of the fluted bands are absent in the emission- 

 spectrum of sodium vapour, except perhaps in the case of the 

 temperature-emission studied by Evershed, which does not 

 appear to have been studied under high dispersion. 



There seems to be no way of explaining the emission of 

 the green light when the vapour is illuminated \vith blue 

 light. It cannot be the result of the damping of the ions 

 whose free period is that of the blue light, for it is in all 

 probability a discontinuous spectrum complementary to the 

 fluted absorption-spectrum. 



It will be interesting to see whether the absorption of tlie 

 vapour is directly affected by the circumstance that it is 

 fluorescing at the same time. This was found to be the 

 case in some experiments made by Burke upon uranium glass. 

 It is also important to determine in what way the absorption 

 and fluorescence are influenced by pressure. 



These matters will be investigated in the near future. 



The statement that when the wave-length of the exciting- 

 light is that of the D-lines no fluorescence is produced re- 

 quires modification. Strictly speaking this is not true, though 

 it is almost certain that the D-line absorption is in no way 

 responsible. This is due to the fact that the beam from the 

 monochromatic illuminator is not strictly monochromatic, 

 being in fact a band varying from ten to twenty Angstrom 

 units in width. As we shall show in the part of this paper 

 dealing with the fluted absorption of the vapour, the fine 

 lines can be traced up to the very edges of the broad band 

 produced by the widening of the D-lines when the vapour is 

 very dense. It is unquestionably the absorption at these lines 

 which gives rise to the very feeble reddish fluorescence which 

 can be seen when the light furnished by the monochromatic 

 illuminator is symmetrical about the D-lines. Strictly mono- 

 chromatic light of the wave-lengths of Di and Dg, no matter 

 how intense, we feel sure would produce no fluorescence, 

 unless the flutings actually cross this region, which is very 

 probably the case. The only light which produces no fluores- 

 cence is p-reen light in the vicinity of A, = 5530 and the violet 



