16 "Waterliouse — Electrical action of Light upon Silver. [No. 1, 



an emulsion of silver sulphide in potassic sulphate, the exposed plate 

 ■was positive, the direction of the currents being the same for all rays, 

 the strength of the current heing least for the rays passing through 

 the green glass. 



With plates coated with an emulsion of silver nitrate in gelatine 

 in a weak solution of barium nitrate, the exposed plate was positive. 

 The effect of the red rays was very small, and of the blue rays very 

 great. 



One of the most important points in Professor Minchin's observa- 

 tions is his discovery of the formation of an invisible developable 

 deposit on silver plates coated with an emulsion of silver bromide, by 

 the action of the electrical current from a single bichromate cell passing 

 through the plates when immersed in water containing a little potassium 

 bromide. He found (J) that the plate connected with the carbon pole, 

 the cathode, was without the employment of any developer visibly black- 

 ened in its immersed part, (2) that, no visible change took place on the 

 other plate attached to the zinc, but when the plate was developed 

 with an ordinary pyrogallic acid developer its immersed portion was 

 also blackened. These effects were entirely due to the passage of the 

 current and were strictly confined to those portions of the sensitive 

 plate through which the current passed. 



The special bearing of these observations upon the formation and 

 composition of the invisible or visible developable photographic image 

 formed by the action of light, does not appear to have been generally 

 recognised. I began last year a series of observations on this subject 

 which quite confirmed Professor Minchin's : unfortunately they were 

 interrupted before completion, but I hope to resume them in due course, 

 after the completion of the present series, and bring them before the 

 Society on a future occasion. 



Professor Minchin also found that by coating silver plates with 

 eosine and gelatine, comparatively strong currents were obtained and 

 the plates were very sensitive to variations in the light. The current 

 generated by daylight in one of these eosine cells was sufficiently 

 Rtrong to produce the photographic action on a silver bromide plate 

 without any preliminary exposure of the bromide plate to gaslight. 

 He also describes a curious case of inversion of the current occurring 

 in the eosine and other cells, which I have also noticed, the initial 

 current being such as to make the exposed plate positive to the other. 

 This current, however, was of very short duration and was succeeded 

 by a steady and much stronger normal current in the opposite direction, 

 the exposed plate being negative to the unexposed. On suddenly 

 shutting off the light from the plate the instantaneous effect was to 



