$2 Dr. JR. T. Beatty on 



distribution in different wave-lengths is a constant fraction 

 of that emitted by a black body at the same temperature. 

 The author has made use of this result by allowing the light 

 from a definite narrow strip of the spectrum to fall on the 

 cell, and observing the photo-currents obtained when the 

 energy consumption of the lamp was varied. Since, 

 according to hummer *, the total energy emitted by a 

 carbon lamp follows Stefan's law the electrical energy 

 supplied per second to the filament varies as the fourth 

 power of the absolute temperature. Hence 



.E rTi 4 

 E x - LtJ ' 



The energy distribution of a grey body is given by 



c{K~ 5 e— r-j| (Wien's law), hence the logarithm of the ratio 



of the photo-currents in the two cases is 



_c 2 rl _ r-i 



xLt tj- 



T 



Hence on plotting ,~- against this logarithm, T being the 



1 c 



variable, we get a line whose slope is T-kr* 



A, -1-1 



This was done for a number of wave-lengths between 

 400 and 660 /ll/jl. In every case a straight line was obtained 

 and the values of Tj thus calculated lay between 1880° and 

 1920° abs., the deviations from the average being most 

 marked at the wave-lengths to which the cell is least 

 sensitive. 



The temperature of the filament at its normal burning 

 voltage was accordingly taken as 1900 c abs. It may be 



noted that the method gives J? an d not T 1? so that the 



Ii 

 results are independent of any uncertainty as to the value 

 of c 2 . The value for the temperature given above was 

 obtained by putting c 2 equal to 1*445. 



The energy per unit breadth of spectrum falling on the 

 slit of the monochromator may now be calculated, an 

 arbitrary multiplication factor being involved. 



The numbers so obtained must be multiplied by ^, the 



dispersion of the prism, in order to get the corresponding 

 energies which would leave the emergence slit if no losses 



* Electrotechnische Zeitschrift, xxxiv. Heft 50, 1913, pp. 1428-1449 . 



