Mr Campbell, Discontinuities in Light Emission. 513 



Discontinuities in Light Emission. II. By Norman Camp- 

 bell, M.A., Fellow of Trinity College. 



[Bead 6 June 1910.] 



§ 1. The following pages are a continuation of a former paper 

 presented to the Society under the same title*. In the former 

 publication it was stated that the experiments had been un- 

 successful in attaining their main object: they have continued 

 unsuccessful. 



The difficulty which had prevented the attainment of the 

 desired results was that of finding a source of light which should 

 have at the same time great intensity per unit area and great 

 constancy. The first characteristic was necessary in order that it 

 might be possible to split the light from the source into two 

 beams of sufficient intensity by means of an optical system of 

 lenses and mirrors. The second w^as necessary because it was 

 found that the relative intensity of the two beams into which the 

 light from the source was split changed with a change in the 

 total intensity of the light. Accordingly, unless the total in- 

 tensity was quite constant, fluctuations due to alterations in the 

 total intensity would be imposed upon and would mask the 

 fluctuations which it was desired to observe. 



Throughout the earlier experiments the source of light used 

 was a Nernst lampf. The town supply had been used as a source 

 of current, and it was thought that the fluctuations in the in- 

 tensity of the lamp might be due to the changes in the p.d. of 

 the supply. Arrangements were made to use accumulators as a 

 source of current, but even with this source the intensity of the 

 light, as measured by the resistance of the filament, was far too 

 variable for the purpose of the experiments. The average in- 

 tensity of the light over a long period was very constant, but 

 there were considerable fluctuations of short period, which were 

 probably due to the cooling of the filament by air currents, but 

 could not be prevented by enclosing the lamp in an almost air- 

 tight enclosure. 



It had been found, on the other hand, that the constancy of 

 the light from an Osram wire lamp carrying the current from 



* Proc. Camb. Phil. Soc. xv. 1909, p. 310. 



t An attempt had also been made to use the sun shining in a cloudless sky as 

 a source of light, the two beams being divided by means of a half-silvered mirror. 

 The requisite constancy and intensity appeared to be obtained, so far as could be 

 judged with the use of the inadequate hehostat which was employed in the prelimi- 

 nary experiments. But it would be impossible to carry through the research with 

 such a source of light in a climate where the necessary conditions are fulfilled so 

 seldom. 



