﻿and Absorption by Resonating Gas Molecules. 693 



photographs, and have saved the many hours that would have 

 been spent in fussing with plate-holders and miss-tit plates. 

 The hinged back of the box can be swung aside and the 

 plate inserted in the supporting clamps in a few seconds, 

 the room being darkened of course. 



Fiar. 1 a. 



The Westinghouse quartz arc was enclosed in a box and 

 the collimator of the quartz spectrograph introduced through 

 a small hole. In this way diffused light, which would have 

 spoiled everything, was shut out. The lamp was started by 

 pulling a string which passed through a small hole in the 

 top of the box, and extinguished at the right moment by 

 opening a switch. 



The monochromatic light from the slit of the quartz 

 spectrograph was either made parallel, or brought to a focus 

 by means of a pair of quartz lenses of 30 cm. focus each, 

 the condition of the pencil of rays being tested with the 

 uranium glass plute. 



The photograph taken of the long tube closed with flat 

 quartz plates showed an image of the cone of rays traversing 

 the high vacuum precisely as if the tube were filled with 

 dense smoke. The tube was at room temperature, and the 

 density of the mercury vapour was about O'OOl mm., never- 

 theless an exposure of fifteen or twenty seconds was all that 

 was necessary. 



Owing to my absence from Baltimore I was unable to 

 take up the work again until this winter. In the meantime 

 I had secured a Westinghouse Cooper-Hewitt quartz mercury 

 arc which was very much more powerful than my old 

 Heraeus lamp. These quartz arcs emit the strong line of 

 wave-length 253(5, and it is this light alone which excites 

 the resonance radiation. 



To my surprise, on repeating the experiment of two years 

 ago with the new arc lamp, I found no trace whatever of the 

 luminous cone of light, even with an exposure of several 

 minutes. On looking up my notes, however, 1 found that in 



