732 Prof. R. W. Wood on Hydrogen Spectra 



oat to a greater extent than shown in fig. 1 so that the 



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central part and the ends can be examine 1 separately through 

 the prism. 



We will now take up the very curious phenomena which 

 occur with this type of tube, beginning with the relation of 

 the Balmer series to the secondary spectrum. 



Suppression of the Secondary Spectrum, 



When the tube is first started the central portion usually 

 shows the Balmer lines, the secondary spectrum, and a trace 

 of the nitrogen bands. It should be operated with a heavy 

 current (say 0*2 amp.), which will heat it to a temperature 

 of perhaps 300°. Fresh moist hydrogen is flowing in at one 

 end and out at the other, and the Crookes dark-space is two 

 or three millimetres wide. 



With continued operation it will be found that the second- 

 ary spectrum gradually disappears from the central portion 

 of the tube, though it remains bright at the ends. It may 

 not be possible to banish it entirely if sealing-wax is em- 

 ployed for the capillary seals, as the presence of carbon 

 monoxide appears to be favourable to the appearance of the 

 secondary spectrum in the entire tube. 



If the tube is new, it may be several days before the 

 central portion reaches what I have called the black stage. 

 In this condition the colour is a fiery purple, and only the 

 first four Balmer lines are seen with the direct-vis'on prism, 

 the region between being quite black. If the tube is viewed 

 through a green colour filter which cuts off the Balmer 

 lines completely', the discharge is almost invisible. That it 

 can be seen at all results from the circumstance that there is 

 still a trace of the secondary spectrum. Photometric measure- 

 ments showed that the intensity of the secondary spectrum 

 along the central portion was about ^ of the value at the 

 ends. This was determined by reducing the intensity at 

 the ends with a calibrated neutral-tint wedge until it 

 matched the intensity at the centre, a green filter being 

 used of course. 



At one stage of the proceedings the mercury lines usually 

 appear, but they eventually vanish. In leaving the tube 

 over-night, the stop-cock between the tube and the pump is 

 closed, and the tube gradually fills with hydrogen, the 

 generator eventually going out of action of its own accord. 



When the tube has finally reached the black stage, which 

 in my opinion means that all hydrocarbon vapours have 

 been carried off and the inner walls well cleaned, we stop the 



