490 Dr. R. Bornstein on the Influence of Light 



mental methods here employed and the results obtained, we 

 shall see that in the first method, where the more powerful 

 currents were employed, there was, coeteris paribus, a much 

 smaller sensitiveness to light ; and we may therefore conclude 

 that 



The diminution of conductivity produced by the electrical 

 current, which icas described above as an electrical after-effect, 

 is accompanied by a diminution in the sensitiveness to light. 



In all the previous experiments a coloured gas-flame had 

 been employed as the source of light. In order to test, in 

 this respect, the facts which had been discovered, a few of the 

 thin silver plates Were exposed to sunlight, and the change in 

 their conductivity was examined therewith. At first the 

 direct rays of the sun were allowed to pass through the 

 closed window ; and the effect produced was found to corre- 

 spond to that due to a rise of temperature ; so that either there 

 was no direct luminous action, or else it was overpowered by 

 the heat developed in the silver and glass by the sunlight. 

 The window was then opened, and by means of a glass con- 

 denser the sun's rays were brought to a focus at a short dis- 

 tance in front of the silver plate, since, when focused upon 

 it, they generated too much heat. Under these circumstances, 

 the surprising result was obtained that here again exposure 

 to light produced an increase of 3*7 per cent, in the electric 

 conductivity. 



These experiments therefore prove that 



" The metals platinum, gold, and silver, just as was already 

 known in the case of selenium and tellurium, experience, under 

 the action of luminous rays, an increase in their electric con- 

 ductivity, the magnitude of which, as far as the observations 

 go at present, may amount to from o to 5 per cent, of the total 

 conducting-power. 



Having thus stated the prominent facts, it appears neces- 

 sary to refute an objection which might possibly be made. It 

 appeared, as was described above, that thin layers of different 

 metals experience, under the action of luminous rays, a 

 change in their resistance which is contrary to that which 

 the resistance of thicker pieces of the same metals was proved 

 to undergo by heating ; and hence it was inferred that the 

 phenomenon which had been noticed could not be due to any 

 rise of temperature. It was therefore necessary to show that 

 the electrical resistance of platinum, gold, and silver, when 

 used in these thin layers, increases with a rise of temperature, 

 just as it does in the case of larger pieces. This is, a priori, 

 by no means certain, since we are almost entirely ignorant of 

 the molecular constitution of these thin metallic plates. It 



