Mr Campbell, Discontinuities in Light Emission. 317 



ments, the value of a, ^, a, h is of little importance. If a = 6 and 

 ajp is changed from infinity to 1, the value of Ot^ is only decreased 

 in the ratio 5 to 3. 



This conclusion may be surprising till it is remembered 

 that f{t) represents bhe motion of the needle after a sudden 

 disturbance. The values of the time constants of the instru- 

 ment, supposed smaller than that of the high resistance, affect 

 only the initial part of the motion. 



Accordingly it is clear so far that the sensitiveness of the 

 instrument is the one thing that really matters. A Dolezalek 

 electrometer will be much preferable to any form of electroscope 

 with smaller capacity but smaller sensitiveness ; for the capacity 

 of the system apart from the electrometer will not be insignificant 

 compared with that of the electrometer. 



The actual experimental arrangements will now be described. 

 Although no results have been obtained as yet, I think that, in 

 all essentials, they have attained a final form. But since no results 

 are claimed as yet, a very detailed account will not be necessary. 



In all quantitative statements that follow, electrostatic units 

 are employed. 



§ 11, In the earlier experiments the source of light was a 

 Nernst lamp of 50 watts heated by the town alternating supply 

 which was the most constant source of current available. To 

 indicate changes in the intensity of the light a small thermo- 

 element in a vacuum was placed immediately under the lamp : 

 changes of 1 in 10,000 could be detected with certainty. Since 

 the constancy of the light proved insufficient an attempt was 

 made to use an Osram lamp of 8 volts and 16 watts, but, though 

 by the use of two such lamps some preliminary observations on 

 the fluctuations of two independent sources have been made, the 

 intensity of the light was insufficient for the main experiment. 

 It is hoped in the near future to be able to run a Nernst lamp of 

 50 or 100 watts off" accumulators of constant potential. 



When continuous current was used to excite the lamp, its 

 constancy was measured by making the filament one arm of a 

 Wheatstone bridge, for which the exciting battery served as the 

 source of current. A change of 1 in 10^ of the resistance could 

 be detected, or, since the illumination appears to vary approxi- 

 mately as the 10th power of the resistance, a change of 1 in 10* 

 of the illumination. 



The light from the lamp was rendered parallel by a simple 

 lens of 7'5 cm. focal length and 4'5 cm. aperture. The resulting 

 beam was split into two parts, either by interposing in its path a 

 half-silvered mirror, or two full mirrors, one behind the other, of 

 which the front mirror had a sharp edge ; the plane of the mirrors 



