11 



substances, such as white of egg (albumen), honey, beer, dextrine, 

 collodion, &c. One of the most useful of the processes for obtaining 

 negatives, was that of a thin film of collodion on glass. Cotton- 

 wool is insoluble in a mixtm-e of ether and alcohol ; but if the 

 wool be dipped into a mixtui'e of sulphm-ic and nitric acids, and 

 then carefully washed and dried, its properties are altered, and it is 

 converted into "gun-cotton," a solution of which in ether and alcohol i:- 



• collodion." The collodion, when impregnated with certain iodides 

 and bromides, is then poured upon a plate of glass, and a thin 

 film is formed on the glass. The plate is then dipped into a 

 solution of nitrate of silver, and must be placed in the camera while 

 wet. After exposm'e, the plate is washed with a solution of pro- 

 tosulphate of iron, in order to •' develop " the picture and render it 

 risible. This is the old " wet-plate " process ; but it is being 

 superseded by the " dry-plate " process, in which gelatine is 

 employed in the place of collodion. 



The Lectm'er then photographed a bust of Sir Walter Scott by 

 means of the dry plate process. The light used was a brilhant one 

 produced by burning magnesium wire. The negative picture was 

 developed and fixed in a small portable dark chamber ; the whole 

 process occupying about three minutes. The positive was ''printed" 

 from the negative by exposure for about ten seconds to gas-light. 

 The silver printing process is nearly the same to-day as it was when 

 first introduced — a little albumen is, however, added to give a better 

 and harder surface to the paper. Even the best photographs by the 

 silver process, fade slightly under the influence of light. Some 

 pictures, taken on platinised instead of silvered paper, were ex- 

 hibited; they had a pleasant "tone," and were believed to be much 

 more permanent than those produced by the silver process. 



Some of the ingenious methods of striking off pictures from 

 gelatine plates by the ordinaiy lithographic printing press, were then 

 explained at length. Of these, the Collotype and the Woodburytype 

 are the most useful and satisfactory. They depend upon the 

 principle that those parts of a gelatine film containing bichromate 

 of potash, which have been exposed to the action of light, are 

 insoluble in water, while those which have not been exposed are 



