326 Mr Campbell, Discontinuities in Light Emission. 



to the present time, any results being obtained. It Avas found 

 that the balance of the two currents through the cells was upset, 

 if the total intensity of the light was changed. Whether the two 

 currents were excited by different lamps lit by the same current 

 or by the light from a single lamp split into two parts, a change 

 in the current supplying the lamp or lamps was attended by a 

 very serious displacement of the balance of the currents. 



A great deal of work has been done in the effort to discover 

 the cause of this defect and to remove it : but it has remained as 

 mysterious as ever. It was thought at first that the cause must 

 lie in some asjmimetry of the photo-electric cells*. 



That this explanation is not sufficient was shown by focussing 

 the two beams, not on photo-electric cells, but on thermo-elements. 

 Though these elements were much less sensitive than the cells, 

 there was no doubt that the balance of the thermo-elements was 

 upset in exactly the same way as the balance of the photo-electric 

 cells by a change in the total illumination. It appears then that 

 the effect is connected rather with the optical train employed than 

 with the detectors : and additional evidence for this view was 

 found in the fact that the sign of the change in the balance could 

 be changed by comparatively trifling alterations in the mirrors 

 and lenses. But in no case were attempts to reduce the change 

 to zero successful. 



The magnitude of the change in balance (measured relatively 

 to the whole current passing through either cell) increases very 

 rapidly with an increase in the total light. By placing diaphragms 

 before the lamp and causing a given increase in the illumination 

 by shunting a constant resistance in the lamp circuit, it was found 

 that the change in the balance varied approximately as the cube 

 of the total light. 



It Mas thought at one time that the change in the balance 

 might be due to the fact that there was a portion of the current 

 passing to the electrode which was independent of the light. It 

 is easy to see that, if there were such a portion, the balance 

 between the currents would hold only for one intensity of the 

 light. The experiments that have just been noted show that 



* It was thought at one time that the reason might lie in the fact that the 

 em-rent through the cells was not strictly proportional to the intensity of the light 

 acting upon them : and observations were made with the object of testing this 

 proportionality. The variations in the intensity of the light were produced by 

 passing it through a photographic lens in which diaphragms of known aperture 

 could be inserted. It was found that the current was not proportional to the light, 

 being relatively smaller for smaller intensities of the light. It was discovei-ed sub- 

 sequently that a similar result for ultra-violet light acting on zinc had previously 

 been noted by Griffith {Phil. Map. vi. 14, 1907, p. 297). But the matter is not 

 very important for our purpose, because all that is necessary, in order that a 

 change in the total light should not alter the balance, is that the current should 

 be the same function of the lia-ht in the two cells. 



