Resonance Spectra of Iodine. 263 



wave-length. Such a re-emission I have named u Resonance 

 radiation." The other lines, together with the 546 green 



Fig. L 



Exdt.Lifie 

 I 1 



1 1 



1 



i 



60 61 



1 1 



1 1 



/?£SO<V/}A'C£ SP£CT/?VM OP /o £>///£ 



line, form the " resonance spectrum.' 5 In it we find two lines 

 of shorter wave-length than that of the exciting line (one 

 of them being double, however), and two strong and two very 

 weak lines, between the green and double yellow lines of the 

 mercury spectrum, and eight or ten lines in the red and 

 orange. One line coincides almost exactly with the longer 

 of the two yellow Hg lines which are absent in the resonance 

 spectrum if all diffused and reflected light is cut off. 



It seems probable that we shall be able to do more with 

 iodine vapour than Las been done with sodium vapour, as the 

 work can be done at room temperature with glass bulbs. 

 I intend to continue the work, using other monochromatic 

 souices of light for the excitation of the vapour, and study 

 1 he behaviour of the resonance spectia in a magnetic field. 

 It will be extremely interesting as well to observe the probable 

 changes in the resonance spectra when the exciting source of 

 light is placed in a strong magnetic field. 



Berlin, Dec. 1st, 1910. 



Supplementary Rote added January 16tli. 



Since the preparation of the above note I have been able 

 to make further experiments through the courtesy of Prof. 

 Cotton, of the Ecole Normal, who very kindly placed his 

 laboratory at my disposal. The wave-lengths of the lines 

 have been re-determined, and a number of very faint ones, 

 not shown in fig. 1, added to the list. 



In the following table the small numerals denote intensities, 

 and the letters a and b indicate the lines which probably 

 belong to the same series. The approximate constancy of 

 wave-length differences is apparent at once, and we are 



