DR. T: R, MERTON AND PROF. J. W. NICHOLSON ON 



It does not appear to be necessary to add any further details of the experimental 

 arrangements in these cases, which have been dealt with in an earlier section in 

 general terms. 



The magnification of all th6 enlargements in these cases was the same, the interval 

 between AA6678, 3888, being 147 mm. as against 44'95 mm. on the original plate. 

 The magnification is therefore 



m= 147/44-95 = 3'270. 



The following notation will be adopted for brevity : 



(a,) = Spectrum of pure Helium at very low pressure, the resistance of the tube 



corresponding to about 2 cm. alternative spark gap. 

 (/>) = " Ordinary " spectrum of pure Helium from the capillary with 1 mm. dark 



space. 

 (c 1 ) = Spectrum of Helium containing a trace of Hydrogen, just sufficient to show 



the Hydrogen lines, Taken from the capillary (l mm. dark space). 

 (d) Spectrum as in (c), but with a larger admixture of Hydrogen. 



Table XII. gives the observed heights of the various lines: 



TAKLK XII. Observations of Helium under Various Conditions. 



The next table (Table XIII.) gives the corresponding photographic intensities of 

 the lines, as the results of calculation by the usual formula. 



