PHOTOGRAPHY. 15 



I should state that sometimes the avidity of the pyro- 

 gallic acid in the presence of ammonia for bromine is so 

 great that it is found necessary to give it a soluble bromide 

 wherewith to satisfy it. Thus potassium bromide is usually 

 added to the alkaline developer. We will expose a plate 

 prepared with silver bromide behind a negative and develop 

 it by this alkaline method, throwing the resulting picture 

 on the screen; There is no noticeable difference between 

 this image and that developed with the ferrous sulphate in the 

 wet plate. 



Now it is not necessary to prepare a sensitive bromide 

 film by immersing the collodionized plate in a solution of 

 silver nitrate ; by an artifice we can have the sensitive salts 

 held in suspension in the viscid collodion. 



I will very briefly cany you through the operations 

 necessary to produce this emulsion, as it is technically called, 

 of silver bromide. In collodion are dissolved soluble bromides. 

 Silver nitrate dissolved in weak alcohol is added in sufficient 

 quantity to convert it into silver bromide. If rightly 

 carried out, this solid bromide remains in suspension in a very 

 finely divided state in the collodion. The collodion is next 

 poured out into a dish and allowed to become gelatinous by 

 the evaporation of the ether, after which it is well washed 

 (to eliminate all the soluble salts present), and dried. 

 The pellicle is next redissolved in a mixture of ether and 

 alcohol, and we have as a result such a viscous fluid as in 

 this bottle. To prepare a film for exposure in the camera, 

 all that is necessary is to pour it over the surface of a glass or 

 other plate as if it were ordinary collodion. It is equally 

 well acted on when dried as when still moist with the 

 solvents. It is usual to add some bromine absorber to the 

 collodion, but this is not absolutely necessary, though it is 

 generally considered that sensitiveness is increased by so 

 doing. 



