TRANSACTIONS OF SECTION A. 469 



«ven a pm-ely logical consideration we miglit conclude that his statement cannot 

 be accepted. 



The photographic effect of a current which is passed through a sensitised 

 plate is a point ot fundamental importance. By placing two plates, each coated 

 with Liverpool Emulsion, in a cell containing distilled water and a few grains of 

 bromide of potassium, and putting this cell into the circuit of a bichromate cell 

 for a few seconds, it will be found that — 



(a) the plate connected with the carbon pole is, without the employment of a 

 developer, visibly blackened in its immersed portion ; 



(6) no visible change comes on the other plate ; but when this plate is developed 

 by pyrogallic acid, its immersed portion also becomes dark. 



This fundamental result was also obtained (though in a less marked degree) by 

 the action of a photo-electric cell, instead of the bichromate cell. To produce the 

 effect with greater ease in this case, expose the two bromide plates to gas-light for 

 about ten seconds before immersion. The localisation of the effect on the plate 

 through which the current passed was further shown by placing several silver 

 strips on the same plate of glass, coating all of them with a layer of Liverpool 

 Emulsion, and throwing some of them out of the circuit of the current. Only 

 those in circuit exhibit the photographic effect. Assuming that fluorescence ought 

 to operate a change of luminous energy into that of an electric current, the 

 author next replaced the silver salts by fluorescent siibstances. Eosine gave the 

 best results, but it is very easily soluble and it leaves the plate rapidly. A very 

 permanent eosine plate was obtained by making a mixture of eosine solution and 

 thin gelatine, pouring this over the plate, and then pouring a layer of collodion 

 over it. This was exceedingly sensitive to even dull sunlight, and when connected 

 with a galvanometer, indicated the faintest change in the light which it received. 

 As a perfect photometer it has a drawback. When the light is suddenly shut off, 

 the spot on the scale does not immediately return to zero. It was found that this 

 irregularity was due (partly at least) to the action of light on coUodion, and this 

 latter was specially examined. A less sensitive, but more regular, plate was made 

 by mixing eosine with thin gelatine and rendering the layer insoluble by immersion 

 in alum solution. 



Naphthalene red gives also very good results, and comes near satisfpngthe 

 requirements of a perfect photometer for continuous registration. Strong light 

 gives opposite results by the action of red and blue rays. Iodine-green — an 

 anihne dye — gives very strong currents, in the direction opposed to that of the 

 current given by an emulsion of iodide of silver — a result for which a theoretical 

 reason may be given. 



The E.M.F. of this cell for strong but oblique sunlight was in one experiment 

 found to rise so high as ~th of a Daniell. 



To prevent the solubility of several of the substances employed, mordants — 

 such as chloride of aluminium and borax — were employed ; but though the layers 

 on the plates were rendered insoluble, then- sensitiveness to light was almost 

 destroyed. 



A very ciuious case of inverse currents presented itself. Two clean silver 

 plates were immersed in a glass cell containing a solution of eosine. ^ When light 

 fell on one plate a current was suddenly set up in the direction opposite that given 

 by a plate coated with eosine and immersed in water. This was a small jerky 

 current, lasting for a second or so, and it was immediately succeeded by a large 

 current in the opposite direction which varied with the light intensity. When 

 the light was suddenly shut ofl" a further jerk in the latter direction took place, 

 and then the spot moved towards its zero position. The two plates having been 

 then left immersed in the cell for a f urtnighl,' ^ e re again used in the same manner 

 and it was foimd that the jerk had enormously increased ; but, although the light 

 was kept up, the spot stead'ily came back and moved in its normal direction beyond 

 its zero position — far beyond it if the light was strong, such as that of a candle at 

 & distance of three or four inches. These contrary currents appear to the author 

 to point to a mechanical action of light on the eosme in solution, as distinct from 

 the chemical action set up between the eosine layer and the silver plate in contact 



