10 



it. To his great surprise the next morning he found there- 

 upon a full}' developed picture. After several experiments 

 he discovered that the unexpected result was owing to the 

 action of fumes from an open bottle of mercury which lay in 

 the neighbourhood of the plate. This was the first instance 

 of what is called the development of the latent image. 



At the same time that Daguerre in France was availing 

 himself of the action of iodine on a silver plate, Mi*. Fox 

 Talbot, on this side of the Channel, was combining the 

 silver with the iodine upon paper. His process which, 

 slightly varied in detail, is still used, is an excellent one, 

 though now considered very slow, A piece of silver is 

 washed with silver nitrate solution, dried, and then floated 

 on one of potassium-iodide ; so as to leave silver iodide on 

 and in the paper. On this surface, as in the case of 

 Daguerre's experiments, an image was obtained after very 

 long exposure in the camera. Here again, by a somewhat 

 similar accident it was found that a latent image was formed 

 in a much shorter period, which might be developed by a 

 wash of gallic acid in the presence of free silver nitrate. 

 The Talbotype labours under the great disadvantage that the 

 prepared paper must be used very soon after sensitizing. 

 We saw also that it is very slow. The keeping property of 

 the paper but not its rapidity was improved by the intro- 

 duction of the waxed paper process, which was practised with 

 great success by a distinguished member of this Society. 



But it was the introduction of the collodion process 

 wiiich gave the first real impetus to the practice of pho- 

 tography, and brought it within the range of e very-day life. 

 Several different vehicles had been suggested for the sensitive 

 iodide of silver — albumen, gelatine, gum arabic, — and it oc- 

 curred to Mr. Archer to utilize collodion for the purpose. 

 Collodion is, as you know, simply gun cotton, or pyroxyline, 

 dissolved in a mixture of ether and alcohol. In this col- 

 lodion is further dissolved potassium iodide. A small quantity 

 being poured upon a glass plate, the solvent quickly evapo- 

 rates, and a very thin, delicate, and transparent film 



