Mr CciAnphell, Discontinuities in Light Emission. 327 



such a cause cannot account for the whole of the effect : but it was 

 found that by introducing a current independent of the light the 

 magnitude of the effect could be decreased very considerably. 

 Such a current was introduced by raising the end of the high 

 resistance R distant from the electrode to a suitable potential, 

 instead of connecting it to earth and the earthed quadrants of 

 the electrometer. The magnitude of this potential had to be 

 found by trial. But though the effect could be reduced greatly 

 by this device, it could not be entirely abolished. The best state 

 of affairs that could be produced was when, on changing the total 

 light, the spot on the scale first moved rapidly a short distance in 

 one direction and then gradually took up a final position on the 

 other side of the zero corresponding to the new balance. 



§ 20. The change of balance was of especial importance in the 

 earlier experiments, because the most constant source of potential 

 which was available for a Nernst lamp was subject to variations of 

 as much as 5 °/^. But at the outset it was not even thought 

 worth while to make special efforts to obtain a more constant 

 source, until all possible investigations into the matter had been 

 made. If there were any uncertainty whether the observed 

 fluctuations were due to changes in the total intensity of the 

 light or to changes in the relative intensity of the two beams, 

 the experiments could be of no possible value. It was only when 

 all efforts to get rid of the difficulty had failed that it seemed 

 desirable to proceed further. 



It appeared, then, that errors due to this source could be 

 detected, if not eliminated, by comparing the fluctuations due to 

 one of the cells with those due to both of the cells illuminated by 

 totally independent sources. Changes in the total intensity of 

 the light, even if they affect the latter experiment, must be much 

 more serious in the former. And, if it were found that these 

 changes were so small that the fluctuations in the former case 

 were, as theory indicates, half those in tlie latter, then it would 

 be certain that the fluctuations were due to changes in the 

 relative intensity of the two sources and not to changes in the 

 total intensity of the light. 



In order to measure the fluctuations due to one source 

 separately, the current through the photo-electric cell was 

 balanced by Meyer and Regener's method of sending an equal 

 and opposite current through the high resistance R by applying 

 to its ends a P.D. by means of the potentiometer. It is not in- 

 tended in this paper to give any actual results, but it has been 

 found that the total light from two Osram lamps run from the 

 same battery in parallel is so constant that the theoretical relation 

 is fulfilled within the limits of error: the fluctuations of one 



