Vapour in the Visible and Ultra-violet Regions. 313 
plate-holder, perforated with a number of vertical slits, each 
about 2 mms. wide, the widths of the opaque bars and slits 
being equal. This slide was left in the plate-holder during 
the exposures, and screened off the spectrum at regular in- 
tervals. In order to obtain greater deviations a large tube 
of Jena glass 80 ems. long was used. It was heated in « 
combustion furnace, the burners of which were turned down 
low, the best results being obtained when the tops of the 
flames only partially enveloped the tube. 
As it was desired to extend the observations as far down 
into the ultra-violet as possible, the spark was substituted for 
the are, and to avoid loss of light by reflexion one of the 
mirrors was dispensed with. The spark was furnished by a 
large induction-coil operated with an alternating current of 
110 volts, a leyden-jar of large size having been joined in 
parallel with the terminals. Cadmium electrodes were used, 
the spark-gap being placed close to the horizontal slit, which 
was mounted at the centre of curvature of a concave reflector 
of magnalium of 1 metre radius. The convergent beam from 
the reflector passed through the sodium dispersion-tube and 
came to a focus on the slit of the spectrograph. The bottom 
of the tube having been raised to a full red heat in the com- 
bustion-furnace, the photographic plate was exposed with the 
slotted slide in position. The anomalously dispersed spectrum 
was thus recorded in small sections corresponding to the slits 
in the slide. The tube was then allowed to cool off, the slide 
drawn out a distance equal to the width of one of the slits, 
Fig. 6. 
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and a second exposure made which recorded the position of 
the undeviated spectrum. A drawing of the plate-holder, 
with its perforated slide and the deviated and undeviated 
spectra obtained with it, are shown in figs. 6 & 7. 
