180 Prof. R. W. Wood and Mr. S. Okano 



on 



greater part of the bulb, and showing the subordinate series 

 as well as the D lines. 



In the case of vapours for which the single line emitted 

 lies in the ultra-violet, and evidence of its presence is 

 obtained by the spectroscope, it is obviously necessary to 

 make sure that the potential difference between the terminals 

 of the hot cathode is not responsible for the appearance of 

 the line. 



When the single line lies in the visible region there is less 

 trouble, since, with diminishing voltage, the glow contracts 

 to a thin skin covering the positive electrode, which may be 

 either the auxiliary anode " a " or the positive terminal " b " 

 of the hot cathode, according to whether the applied potential, 

 or the potential due to the drop along the cathode filament, 

 is responsible for the emission. 



By using a very short filament we succeeded in reducing 

 the potential difference between its terminals to about 

 one volt; but even in this case we detected the D lines when 

 the spectroscope was directed towards the terminal "b." 

 They were so faint, however, that there was not much 

 chance of their being seen by reflexion from the walls of 

 the bulb. 



The anode wire in this case was perfectly straight, and by 

 viewing it " end on " the visibility of the faint luminous 

 glow surrounding it was enormously enhanced. To still 

 further increase the sensibility of the method, we formed an 

 image of the end of the wire on the slit of the spectroscope 

 by means of a lens (see fig. 1). This was accomplished 

 without difficulty by throwing an image of the sun on the 

 wire. On darkening the room we found that we could 

 observe the D lines at the wire " a " until the applied 

 potential was reduced to 0'5 volt, or perhaps a little less than 

 this. The exact point at which the line disappears depends 

 of course upon the condition of the eye. 



To remove entirely the possibility that the potential drop 

 along the tungsten wire was contributory, we employed two 

 methods. In the first or stroboscopic method, we employed 

 a brass disk with wide teeth and small holes as indicated in 

 fig. 2. This wheel interrupted the heating current, and by 

 viewing the tube through the apertures we observed the 

 condition at the auxiliary anode only at the moments when 

 there was no potential difference between the ends of the 

 tungsten filament. (The circuits were of course simpler than 

 indicated in fig. 2.) This method gave good results when 

 used with the steel tube (which will be described presently), 



