[lubovich & pearen] INFRA-RED SPECTROSCOPY 



207 



When the thalofide cell was used it was mounted on the camera 

 of the grating with its active surface in the focal plane of the latter. 

 It was joined in series with a resistance of 240,000 ohms., a Tinsley 

 galvanometer and a battery, the potential of which could be varied 

 from zero to 80 volts. The slit of the spectrograph was about 1.8 

 mms. wide and a second slit about 8 mms. wide was placed directly 

 in front of the cell to limit the radiation entering it from the grating. 



The unilluminated thalofide cell with this circuit gave a steady 

 "dark current" of about 330 mms. deflection when the potential 

 difference applied was 40 volts. The deflections in excess of this 

 dark current deflection were read with the cell illuminated, readings 

 being taken at given intervals as the cell was moved along the focal 

 plane of the grating. As the cell in any given position did not reach 



JO fÀ 



Fig. 22 



Fig. 23 



.Au 



a steady state at once, the reading was taken one minute after the 

 commencement of each exposure. These readings have been plotted 

 as ordinates in Fig. 23 and the wave-lengths used as abscissae are 

 those obtained by calibrating the grating with the lines of the second 

 order spectrum falling within the range investigated. It was assumed 

 that each peak on the curve corresponded to a wave-length in the 

 spectrum and on this basis the constitution of the latter was deter- 

 mined. The wave-lengths in the spectrum of mercury as deduced 

 by the photographic method and by the thalofide cell method in the 

 present investigation are given together with the determinations of 

 the wave-lengths by McLennan and Shaver in Table I. 



