deposited another will follow, and so the picture is built up. 

 Should the restrainer be inefficient, or extraneous light have 

 reached the plate, the whole surface will be darkened by 

 reduced silver, and " fogged " as it is technically termed. 



It was found that the exposure and development of the 

 sensitive plate might be delayed for a space of time, varying 

 from a few hours to many years, by washing away thesuper- 

 fluous silver nitrate, and coating the plate with an infusion 

 or solution of tannin, gallic acid, gum, tea, coffee, morphia, 

 albumen, and other substances, and then drying it ; thus 

 enabling the plates to be prepared in bulk at home, carried 

 abroad for exposure, and brought back for development. 

 This great gain in the matter of portability was, however, 

 counter-balanced by a very great loss in sensitiveness, 

 a long exposure being required. 



In order to obviate the necessity for employing the 

 objectionable silver nitrate bath, the plan was adopted of 

 combining silver bromide in the collodion itself, an emulsion 

 being thus found which could at once be poured upon the 

 plate, washed and dried. Experiments made, in order to ob- 

 tain a more convenient vehicle than collodion for this purpose, 

 resulted in a discovery which has within the last three or 

 four years revolutionised the practice of photography. Suffice 

 it to say then that gelatine was found to be that vehicle. 

 The original experimenter would, no doubt, have been 

 satisfied with substituting the aqueous solution of gelatine 

 for the expensive and dangerous gun cotton, ether, and 

 alcohol, even had he gained a degree of sensitiveness only 

 equaling that of bromized collodion. But it was soon found 

 that the gelatinous bromide emulsion when cooked for a week 

 at a moderate temperature, or boiled for half an hour, became 

 most exquisitely sensitive, plates thus prepared exceeding wet 

 collodion in rapidity, bringing down the exposure to fractions 

 of seconds. Nor is this all the gain. In place of having 

 to drag about the cumbrous tent with its wet-plate ap- 

 paratus, the amateur can now wander about armed only 

 with a camera and a few dry plates ; while beautiful 



