Prof. Minchin's Experiments in Photoelectricity. 211 



light was observed in a cell containing eosine in solution ; 

 and as the same effect was occasionally observed in cells of 

 other kinds employed in experiments made long afterwards, 

 I shall draw attention to the phenomenon here. 



Two clean silver plates were immersed in a glass cell con- 

 taining distilled water and a very small quantity of eosine. 

 One of these plates was screened and the other exposed to 

 light, both being connected with the galvanometer. Imme- 

 diately on exposure of the plate to light, there was generated 

 a current in which the exposed plate was positive to the other. 

 This current, however, lasted for only a second, and it was 

 then (the exposure continuing) succeeded by a steady and much 

 stronger current in the opposite direction, this latter being 

 the current which would exist if the plate had been coated 

 with an eosine-gelatine film in the usual way, any variation 

 in the intensity of the light being answered by a corresponding 

 variation in the strength of the photoelectric current. Now 

 when the light was suddenly shut off from the plate, the 

 instantaneous effect was to increase the existing current — the 

 effect being merely impulsive — after which (the plate being 

 screened) the current gradually disappeared. This result 

 was again and again reproduced, and exactly the same result 

 was found if the water in the cell contained a small quantity 

 of fluorescine instead of eosine, except that the initial and 

 final impulsive currents were much smaller with fluorescine 

 than with eosine. 



The plates of this cell having been left in the liquid and 

 kept in the dark for a fortnight, the action of light was again 

 tried ; and it was then found that, while the inverse currents 

 were produced as before, the initial current on exposure was 

 enormously increased both in magnitude and in duration. It 

 now disappeared gradually, and was succeeded by a current 

 in the reverse direction. 



When one of these plates was removed from the cell and 

 immersed in water in presence of a clean silver plate, it was 

 at once, on exposure to light, negative, like a silver plate 

 coated in the ordinary way with an emulsion of eosine. 



M. Becquerel, in the course of his experiments on the 

 electric action of light on plates coated with salts of silver, 

 made the observation that in the case of silver plates coated 

 with bromide, chloride, and iodide of silver, the nature of the 

 exposed plate (whether positive or negative) depends on the 

 thickness of the layer deposited on the surface of the plate. 

 Thus, he says (La Lumiere, vol. ii. p. 129) : — " By depositing 

 on one of these plates a thin layer of iodide, obtained by the 

 action of the vapour of iodine at the ordinary temperature, 



