Prof. Minchin's Experiments in Photoelectricity. 209 



nitrate of barium, the exposed plate was positive, the effect of 

 the red rays being exceedingly small and that of the blue very 

 great. 



Photographic Effect of the Current. — A fact of very great 

 importance in this subject is that which relates to the action 

 of an electric current which passes through a photographic 

 layer on a plate. 



Two silver plates, each coated with a film of Liverpool 

 emulsion (bromide of silver), were immersed in a tumbler 

 containing distilled water and a few grains of bromide of 

 potassium. One of these plates was connected with the zinc 

 and the other with the carbon pole of a bichromate cell. The 

 current was allowed to pass through the plates for a few 

 seconds, with the following results : — 



(1) The plate connected with the carbon pole was, without 

 the employment of any developer, visibly blackened in its 

 immersed part. 



(2) No visible change took place on the other plate ; but 

 when the plate was developed, by pouring over it the usual 

 pyrogallic acid developer, its immersed portion was also 

 blackened. 



The photographic result was also obtained when the bichro- 

 mate cell, which originated the current, was replaced by a 

 photoelectric cell exposed to light. A vessel containing dis- 

 tilled water and a small quantity of bromide of potassium was 

 placed in a dark room; in this vessel were immersed (or 

 partially immersed) two silver plates S', V, the first coated 

 with a film of Liverpool emulsion, the second uncoated. These 

 plates were connected by insulated wires with the plates, S, V, 

 of a glass cell containing water and a small quantity of 

 common salt, the plate S being coated with a layer of a 

 chloride of silver emulsion, and the plate V being uncoated. 

 Magnesium light was then allowed to fall on the plate S of 

 this latter cell for a few minutes. Now, whether S' was 

 connected with S or with V, the plate S' when taken out of 

 the vessel and developed was very sensibly blacker on its 

 immersed portion than on the unimmersed. The action on 

 the plate S' was assisted by exposing it for about ten seconds 

 to gaslight before the photoelectric current of the exposed 

 cell was passed through it. The effect is undoubtedly due to 

 the action of the current, because when the plate S' was left 

 immersed in its cell for twenty hours and then developed, no 

 blackening effect was produced. 



Not only is this photographic effect of the current impor- 

 tant, but it is also important to prove that the effect on a 

 sensitized plate is strictly confined to those portions of it 



