J893.] "Waterhouse — Electrical action of Light upon Silver. 15 



light was admitted. It was also inversely proportional to the square 

 of the distance of the source of light from the apparatus. An oil lamp 

 was used. The instrument appeared to show an exact proportionality 

 between the intensity of the light and that of the current, and its 

 great sensitiveness and precision would enable it to be used as a very 

 delicate photometer. In these experiments he found that the electro- 

 motive force exerted by the November sun upon iodised silver plates 

 through an opening 30 mm. wide was -j-g- of a Daniell cell ; with a 

 petroleum lamp, at 8 inches distance, it was only 0"004 Daniell. 



Dr. J. Moser afterwards, in 1887, in working on Egoroff's plan 

 found that the photo-electric current might be greatly increased by 

 treating the chlorised, iodised or bromised silver plates with solutions 

 of erythrosin, benzo-purpurin and other dyes, and in sunlight he 

 observed currents of a strength equal to half a volt (Eder's Jahrbuch 

 der Pliotographie, 8fc, 1888, p. 297.) 



At the meeting of the British Association, in 1880, Professor Gr. M. 

 Minchin gave an account of his experiments on the generation of 

 electric currents by the action of light on silver plates which were 

 coated with emulsions of bromide, chloride, iodide and other salts of 

 silver in gelatine and collodion, as well as with eosine, fluorescine and 

 various aniline dyes, the object of these experiments being the solution 

 of the problem of producing a photographic image of an object at a 

 distance. A detailed account of these and other interesting experiments 

 on lio-ht-cells was read before the Physical Society, and published in 

 the Philosophical Magazine, for "March 1891. 



He found that when two pieces of clean silver foil attached to 

 glass plates were coated with an emulsion of chloride of silver in 

 collodion and immersed in distilled water containing a few grains of 

 common salt, the plates being connected with the terminals of a 

 Thomson's galvanometer and one of them screened from the light, 

 that on exposing the unscreened plate there was an electric current 

 produced, and the exposed plate was negative to the uuexposed. The 

 same effect was observed with plates coated with emulsions of silver 

 bromide in water containing a little potassium bromide. When the 

 plates were coated with iodide of silver in collodion by the wet silver- 

 bath method, the liquid being water containing a little potassium 

 iodide there was a reversal of the nature of the exposed plate, it being 

 positive to the unexposed. With coloured glasses in front of the 

 exposed plates it was found that the red rays produced comparatively 

 feeble currents, while those produced in the blue and violet rays were 

 very great, but the directions of the current were the same for all rays. 

 This agrees with Becquerel's observations. With plates coated with 



