PHOTOGRAPHY. 13 



placed the silver leaf, whilst to B was applied the tannin. 

 The reason of the difference is obvious. It is only those 

 particles of the silver iodide in actual contact with the 

 iodine-absorber which can be affected by the light. In the 



FIG. 4. 



one case the silver leaf was only in contact with the surface 

 particles, in the other the tannin permeated the film. 

 Having thus prepared the ground, we are in a position to 

 understand the preparation of dry plates, as they are called, 

 that is, the preparation of sensitive films in collodion which 

 can be exposed in the dry state. 



If a surface of silver iodide were allowed to dry with the 

 free silver nitrate solution from the bath upon it, the latter 

 would crystallize and spoil the film. If we wash away 

 the free silver nitrate it would be insensitive, but if 



