Resonance Spectra of Iodine Vapour 



471 



The present investigation was made in the laboratory of 

 the Royal Institution, which was placed at my disposal 

 through the courtesy of Sir James Dewar, the investigation 

 of many points being made possible by the giant Nicol prisms, 

 condensing-lenses., and other apparatus not usually found in 

 physical laboratories. I am also under great obligation 

 to Mr. Twyman, of the firm Adam Hilger & Son, who 

 placed echelons, spectrographs, and other optical apparatus 

 at my disposal, and to Mr. Kenneth Mees, who very kindly 

 lent me his small prism spectrograph, with which most of the 

 photographs illustrating this paper were made. 



Thanks to improvements made in the method of illuminating 

 the vapour, it was possible to obtain the resonance spectrum 

 with sufficient brilliancy to permit of its examination with 

 the echelon, which is of great importance in connexion with 

 the proposed examination for a possible Zeeman effect. 

 I searched for the effect in the laboratory of M. Cotton in 

 Paris with a large Weiss electromagnet, but at that time it 

 was impossible to obtain the spectrum with a brilliancy 

 sufficient to permit of its examinntion with any spectroscope 

 of high resolving power, and nothing was accomplished. 



Fie. 1. 



The arrangement of the apparatus built up at the Royal 

 Institution is shown in fig. I. Two large quartz mercury 

 arc-lamps made by the Westinghouse Cooper-Hewitt Co., 

 of London, which I have found to be much more powerful 

 than any lamps which I have used previously, were used for 



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