72 



Scientific Proceedings, Royal Dublin Society. 



started. At the end of a certain period, generally three minutes, another 

 reading was taken, and simultaneously the shutter was raised by means of a- 

 thread, thus allowing the beam of light to fall on the substance on the 

 testing plate. After another three minutes a third reading was taken and 

 the shutter replaced, and so on, thus finding the motion of the spot of light 

 during alternate periods, in which the substance under test was alternately 

 illuminated and in darkness. The motion during the " dark " intervals is 

 chiefly due to the natural ionization of the air in the testing vessel. During 

 the " light " intervals this is increased by the photo-electric current, if any. 

 The mean difference between the movements during the " light " and the 

 " dark " intervals is a measure of the photo-electric current. 



The following is a typical set of readings, obtained when the plate was 

 covered as uniformly as possible with a layer of freshly prepared leaf-powder 

 about 1mm. thick. The air-pressure in the testing vessel was kept very 

 close to 2 mm. of mercury throughout. The sensitivity of the electrometer 

 was such that 0'5 volt produced a deflexion of 179 scale divisions, and the 

 pressure of the grid was 231 volts. The interval between readings was 

 three minutes : — 



It will be observed that, as the deflexion increases, the rate of motion 

 decreases. This always occurs, and is chiefly due to the leakage of the 



