86 PHOTOGRAPHIC PROCESSES. 



all creaminess has disappeared from the back of the 

 film, and then washed in frequent changes of water 

 for not less than six hours. It is then placed in a 

 saturated solution of alum for fifteen minutes, and 

 again thoroughly washed. 



After this washing, if the plate on examination 

 appears to lack density and be deficient in vigour 

 owing to over-exposure, it is immersed in a saturated 

 solution of bichloride of mercury, until perfectly 

 white ; again thoroughly washed for several hours, 

 and finally placed in a solution consisting of — 



Liquor Ammonia, 3 iii. 

 Water, 3 x - 



which changes the white colour to a dense black. 



By modifying the strength of this solution, any 

 density between that produced by the mercuric 

 chloride alone, and its complete reduction to the 

 metallic state by the ammonia, may be obtained, if 

 the plate is washed immediately on the appearance 

 of the desired colour. 



The plate is put aside to dry spontaneously, as the 

 application of heat is liable to melt the film and 

 destroy the negative. 



Varnishing is unnecessary. 



If only one plate is developed at a time, or the 

 method of development described, considered too 

 complicated, Edwards' Developer may be used. 



Two stock solutions are prepared as follows : — 



