32 Mr. A. H. Pfund: A Study 
behind slit B and exposed to visible radiation; and the 
deflexion of galvanometer G,; was recorded. Then the 
selenium cell was brought into position and exposed ; the 
reading which its galvanometer Gg indicated was also recorded. 
Having rotated the prism slightly, so that light of greater 
wave-length passed through the slit, the thermopile was 
replaced, and upon exposnre was found to give rise to a 
larger galvanometer-deflexion than before. Now by means 
of the device at K the intensity of the light was cut down to 
such an extent that the galvanometer G,, indicated the same 
reading as at first. This meant that, though the wave-length 
had been changed, the energy was still the same. The 
selenium cell was again brought into position and exposed, 
and the deflexion of its galvanometer was observed. The 
width of the slit at A was changed at intervals to approach 
more nearly the condition of equal purity of spectrum, although 
it did not matter much in the final results whether this pre- 
caution was taken or not. 
The results are given in Table I. (p. 33) and are plotted 
in fig. 3. 
Fig. 3. 

“60 



40 


Gy 
oS 



ine] 
SS) 






Sensibility, 
S 


——> Wave Length 
Discussion of Results. 
When the first sensibility curve had been obtained, it was 
thought that the prominent maximum (near 0:7) might be 
a function of the metal contained in the selenide. In con- 
sequence, cells with electrodes of tin, nickel, copper, lead, 
