Iodine by Multiplex Excitation. 675 



In my earlier paper I expressed the belief that much more 

 could be learned from the study of the resonance spectra of 

 iodine than was the case with sodium, since the vapour is 

 much easier to handle. This belief has been amply justified, 

 and I have so improved the method of investigation that I 

 can now secure with an exposure of ten minutes a far better 

 photograph of the resonance spectrum than was obtained 

 with an exposure of 2-4 hours in the first experiments which 

 I made in collaboration with Dr. Franck in Berlin in 1910. 



I shall first take up the subject of the resonance spectra as a 

 whole, photographed with my large three-prism spectrograph, 

 and then pass to the more interesting subject of the structure 

 of the resonance lines themselves as affected by the structure 

 of the exciting line. This part of the work was done with 

 the concave grating. The study of the absorption of the 

 iodine for the frequencies falling within the spectral range 

 covered by the exciting lines, owing to their finite width, 

 was done first with a six-inch plane grating in the fourth 

 order spectrum, used in conjunction with a spectrometer of 

 considerably over two metres focus, and subsequently with a 

 40-foot spectrograph which I installed at my East Hampton 

 laboratory in August of the present year., 



Apparatus and Methods. 



Thus far I have employed only the mercury arc for the 

 excitation of the vapour, for I have found that sufficient 

 variety could be obtained with this source alone to make 

 possible a very complete study of the genesis of resonance 

 spectra. It has, moreover, an advantage over most other 

 sources of monochromatic light in that it is extremely 

 brilliant, and can be kept running continuously for an almost 

 indefinite length of time. I have employed two different 

 types of lamp : a commercial Cooper-Hewitt, burning in a 

 long glass tube at a comparatively low temperature, and a very 

 powerful Westinghouse Cooper-Hewitt lamp (quartz) such as 

 is used for tie sterilization of water on a large scale. This 

 lamp is of almost insupportable brilliancy, and many times 

 more powerful than the small Heraeus lamps with which I 

 have worked previously. I have already had it in operation 

 over 2000 hours, and so far as I can see it is in as good 

 order as when first made. It operates at 220 volts and takes 

 7 amperes when first started, the current dropping gradually 

 during the first ten minutes of operation, becoming stationary 

 at about 3*5 amperes, the lamp then being red-hot. I had 

 my lamp constructed so that it could be used end-on if 

 necessary, which has 'proved advantageous in the case of 



