398 



Mr. A. LL Hushes on the 



On comparing the results in Table I. for the light which 

 had passed through quartz with Hull's results, it was found 

 (1) that the zero leak occurred at — 1*43 volts, whereas in 

 Hull's experiments it occurred at — 233 volts ; and (2) the 

 ratio of fast electrons to the total number was much lesst han 

 in Hull's experiments. To make sure that this was not some- 

 thing due to the particular piece of quartz used, or to the 

 state e of the blackened surface, the experiments were repeated 

 using a discharge in hydrogen at 2 mm. pressure as the 

 source of light and transmitting it through the same piece of 

 quartz on to the same plate. The results obtained are given 

 in Table II. and plotted in fig. 2 (dotted line) ; the scale is 

 taken such that the saturation portion of the curves for the 

 quartz effects with the different sources coincide. 



Table II. 

 Source of Light — Discharge in H 2 at 2 mm. 



Potential of D. 



Leak. 



4-4 volts 



-1-113 (=3-04x{°" U 



4-3 



115 amp/ 



4-2 



111 



4-1 



111 







88 



- -34 



61 



- -66 



41 



-1-00 



. 25 



-1-34 



13-7 



-166 



7-0 



--2 00 



4-!0 



-2-34 



-1-8 



-2-66 



• -3-3 



-3-00 



-3-8 



-4-00 



-40 



The zero leak occurred at — 2'18 volts, which is less than 

 Hull's value, viz. 2 33 volts. The difference is probably due 

 to the greater proportion of reflected light in these experi- 

 ments. If we suppose that no reflexion takes place, then a 

 curve of the form A (fig. 3) wall be obtained, and the point 

 ,x denotes the potential required to stop the fastest electrons. 

 If, however, a certain fraction of the light is reflected to the 

 case D, the curve B for the leak from the case will be similar 

 to A but reversed and flatter. What is actually observed in 

 the experiments is the sum of the two curves indicated by 

 the dotted line cutting the abscissa in x v The difference 

 between x and x\ increases with the amount of reflected 



