Iodine oy Multiplex Excitation. 



687 



a portion o£ the photograph obtained in this way will be 

 found reproduced on Plate XVII. spectrum F. This same 

 region excited by the quartz lamp will be found immediately 

 below (spectrum G). It is quite apparent that the component 

 lines which accompany the doublets are quite different in 

 the two cases. Moreover, in the latter photograph it will be 

 found that the doublets are displaced toward the region of 

 shorter wave-length, with respect to those in the spectrum 

 excited by the Cooper-Hewitt lamp. The difference between 

 the two spectra is more clearly brought out by the drawing- 

 reproduced in fig. 1, which was very carefully made from 



Fig. 1. 





IS 



totffir'Hftf'ftt 



LAMP 



, £*itt»tton by 

 j Qu*rt% M" 



photographs obtained with a very fine slit, which are not 

 suitable for reproduction. The component of the doublet 

 which has the longer wave-length in the case of the quartz 

 arc excitation is in coincidence with the component of 

 shorter wave-length in the photograph obtained with the 

 Cooper-Hewitt lamp, in which we find, however, a faint line 

 exactly in coincidence with the other component of the doublet. 

 What is more remarkable, however, is the absence of coin- 

 cidence in the case of some of the fainter lines. The line at 

 wave-length 5460 is accompanied by fainter companions 

 spaced in much the same way as are the companions accom- 

 panying the doublet, but the main line is not itself double. 



If we compare the groups at 5460, 5526, and 5658 in 

 spectrum G, we cannot but help being struck by the marked 

 similarity between them, and by the similarity between the 

 arrangement of the lines in the groups and the arrangement 

 of the iodine absorption-lines which occupy the region covered 

 by the green mercury line (spectrum 0). The resonance 

 groups are, however, on a vastly larger scale than the iodine 

 absorption-lines, the amplification being about thirty-fold. 

 This is most strikingly brought out in spectra K and L, 

 which represent enlargements of the groups at 5526 and 5658 

 respectively. The width of the 5461 mercury line, within which 

 all of the absorption-lines which respond to the excitation 

 fall, is shown on the same scale at M. Speaking figuratively. 



