Energy Distribution in Spectra. 63 



due to absorption and reflexion took place. Thus the actual 

 relation between photo-current and wave-length, when the 

 carbon lamp is used, may be employed to work out the curve 

 connecting the wave-length with the photo-current per unit 

 energy incident on slit. 



This relation is shown in fig. 10 (PI. II.), and we are 

 now in a position to express all the previous curves in terms 

 of the energies emitted by the different hydrogen lines. 



Figs. 7 E and 8 E. (PL II.) were thus drawn by changing 

 the photo-current ordinates o£ figs. 7 and 8 into energy ordi- 

 nates by the help of fig. 10. 



It will be seen that the greater part of the energy is 

 carried by H a , and a smooth curve joins the ordinates for 

 H a , Hp, H y , H$. As the shcipe of this curve changes with 

 the pressure no attempt has been made to find a relation 

 between energy and wave-length, though at very low 

 pressures where the atoms can radiate their energy com- 

 pletely, such a relation would probably be capable of a 

 simple interpretation. 



Jolly * records similar energy curves obtained with a 

 thermopile, but his tubes were excited by an induction-coil 

 with a capacity in parallel with the tube, and so his curves 

 are not comparable with those obtained in the present 

 investigation — they show, however, a general resemblance. 



The curves 7 and 8 giving photo-currents as ordinates 

 show the details better than the energy curves 7 E and 8 E ; 

 in the latter the large amount of energy in H a necessitates 

 a small scale of ordinates for the rest of the spectrum. 



Indeed, a great advantage of the photo-electric cell is that 

 it can be made very sensitive to the weak lines in the blue 

 and ultraviolet. 



The author intends to continue the work by investigating 

 the discharge at low pressures, and also the spectra of 

 hydrogen compounds, such as HC1 and H 2 S, in order to 

 elucidate further the variations in the energy distribution. 



In conclusion, he wishes to express his thanks to 

 Mr. John Wylie, B.A., for valuable help and advice in 

 designing and constructing much of the apparatus used ; 

 to Miss Muriel Campbell, B.Sc, for a series of observations 

 to test the properties of the photo-electric cell, and to 

 Mr. E. W. McClelland for assistance in taking the final 

 readings. 



* Phil. Mag. (6) xxvi. November 1913, p. 801. 



