682 Prof. R. W. Wood on Resonance Spectra of 



I have examined the absorption spectrum of sodium with 

 this very powerful apparatus, and find that it is equally 

 complex, the distance between the lines being about the same 

 as in the case o£ iodine. Sodium, however, exhibits only a 

 single faint line within the green mercury line, and as this 

 appears only when the vapour has a considerable density, 

 the mercury arc is incapable of stimulating this vapour to 

 appreciable fluorescence. 



The bromine absorption spectrum is reproduced in coin- 

 cidence with that of iodine on the same plate (spectrum V) . 

 Further reference will be made to this when I come to the 

 subject of the use of bromine vapour as a ray filter for 

 modifying the intensity distribution in the green mercury 

 line, and for the present I shall only draw attention to the 

 somewhat suggestive circumstance that a large number of 

 the lines in the two spectra coincide. From a rather 

 hasty study of the original plate with a low power micro- 

 scope (the photographs reproduced are threefold enlarge- 

 ments), I have come to the conclusion that there are more 

 coincidences than can result from accident, a question which 

 can doubtless be settled by the study of a wider range of the 

 spectrum and a count of the percentage of coincidences with 

 the spectra thrown out of wave-length coincidence by varying 

 amounts. It appears to me that there may very possibly 

 exist in the two elements identical systems of electrons, which 

 give rise to similar frequencies in the two molecules. This 

 is a point which I am studying at the present time. 



It may be well to point out that the iodine absorption 

 spectrum observed with sunlight is far superior to the solar 

 spectrum for testing large gratings. 



The Resonance Spectra. 



As I have indicated in previous communications, the 

 resonance spectrum of the iodine vapour, excited to lumiuosity 

 by the light of the mercury arc. consists of three superposed 

 series of nearly equidistant lines, one excited by the green 

 line, the other two by the yellow lines. I have succeeded 

 in separating the former from the two latter by the inter- 

 position of suitable absorbing screens between the mercury 

 lamp and the iodine tube, and in the discussion of the results 

 I shall take up the series separately. 



If we photograph the resonance spectra with a spectrograph 

 giving fairly high dispersion we find that the lines which 

 form the series excited by the green line are in reality close 

 doublets, accompanied hy fainter companions, which however 

 do not appear except on fully exposed plates. Neglecting 



