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LXVI. Resonance Spectra of Iodine by Multiplex Excitation. 

 By R. W. Wood, Professor of Experimental Physics, 

 Johns Hopkins University, and Adams Research Fellow of 

 Columbia University *. 



[Plates XVI.-XIX.] 



IN a previous paper (this journal, Oct. 1911) I have 

 described the general nature of the resonance spectra 

 of iodine excited by the light of the mercury arc, and the 

 remarkable transformation of the simple resonance spectrum 

 into the complicated band spectrum produced by collisions 

 of the iodine molecules with molecules of gases of the helium 

 group. In the course of a very extended study of resonance 

 spectra with which I have been engaged off and on for the 

 past seven years, I have come to the conclusion that no very 

 great advance could be made until some method was devised 

 for varying the wave-length of the exciting light by very 

 small amounts. 



In the previous work the spectra excited by a large number 

 of widely separated wave-lengths have been studied, but no 

 very general conclusion as to the nature of the molecular 

 mechanism could be drawn from the accumulated material. 



During the past winter I have advanced a step in the 

 right direction, and have made a very good beginning on 

 the work which I have always hoped would be possible. 



Some very astonishing results have been obtained which 

 throw a good deal of light on certain points that I never 

 felt able to explain in the course of the work on sodium 

 vapour. 



I have found, for example, that the resonance spectrum of 

 iodine is quite different w hen excited by the green line of the 

 Cooper-Hewitt mercury lamp (commercial glass lamp), from 

 the resonance spectrum excited by the same line from the 

 mercury arc in quartz (high temperature arc) : in other 

 words, the resonance spectrum suffers profound changes 

 when small changes occur in the structure of the exciting 

 line. In all of the previous work I have assumed that only 

 one of the hundreds of absorption-lines was operated upon 

 by the exciting line, though in one or two cases I ventured 

 the hypothesis that in some cases the exciting line might be 

 broad enough to act upon two absorption-lines simultaneously. 

 (See paper by R. W. Wood and F. E. Hackett on the 

 resonance spectra of sodium photographed with the concave 

 grating, Astro-physical Journal, vol. xxx. No. 5, 1909.) 



The complexity of the iodine absorption spectrum has been 



* Comijiunicated bv the Author. 





