75 



is exposed behind a negative and is then devcslopcd I)v washing in 

 warm water. Where the bichromated gelatin lihn lias been exposed 

 to hght it has become insoluble, where it has not been exposed to 

 light it dissolves in warm water and the pigment is washed away. 

 Since the bottom of the film will have been protected from light by the 

 pigment it will dissolve in water and all the gelatm film will come 

 away fiom its support. It is therefore necessary to transfer the 

 gelatin layer on to another support and to develop the image by 

 washing away the pigment that was the bottom of the film. 



Photographic Emulsions. 



If, to a solution of potassium bromide we add an equivalent amount 

 of a solution of silver nitrate, we obtain a precipitate of silver bromide 

 which quicldy collects in the form of large flocks and settles to the 

 bottom of the vessel. If the precipitate in this state or after washing, 

 is shaken uj) with a solution of gelatin containing a trace of potassium 

 bromide, a colloidal solution of silver bromide is obtained^. Such 

 solutions are usually termed emulsions, though, strictly speaking, 

 this is a misnomer they are suspensions, in which gelatin acts as a 

 protective colloid. If the silver bromide is allowed to stand for 

 two or three days before treatment with the gelatin and bromide, it 

 win not form the emulsion. The usual method of preparing emulsions 

 is to precipitate the silver bromide in the presence of gelatin. The 

 particles in the emulsion as soon as they are prepared, are very fine 

 indeed, and such emulsions are sometimes called grainless. If the 

 emulsion is washed free from dissolved salts at any time and kept 

 at ordinary temperatures it becomes fairly stable, that is to say it 

 changes fairly slowly or not at all. If there are dissolved salts present, 

 which have a sUght solvent action on the silver bromide, an increase 

 in the size of grain takes place. The change is favoured by rise of 

 temperature. The grains become more sensitive to hght as they 

 grow. This growing of the grains is called " ripening." Under 

 suitable conditions the grains grow by capilliary forces until they are 

 2/i. to 10/x diameter". The smaller grains being more soluble than 

 the larger, dissolve and make the solution supersaturated with respect 

 to the larger ones M^hich therefore grow. Usually crystals will only 

 grow in this wa}' until their diameter is about 2/z., because above this 

 size the solubihty no longer diminishes as the size increases. The 

 crystals of silver bromide can certainly grow to lOjU, diameter during 

 ripening, and this may be due to the fact that such crystals are in 

 the form of very tliin plates (less than 1/a thick), or it may be caused 

 by unequal heatmg and the presence of convection currents in the 

 solution duruag ripening. 



The chloride and bromide of silver erystalHse in the cubic systems 

 at all temperatures used in the preparation of photographic emulsions. 

 The iodide crystalUses in the hexagonal sj^stem below 146° C, and 

 above that temperature in the cubic system. The three haUdes 

 form solid solutions with each other, so that in an emulsion containing 

 more than one haUde, we have only one land of mixed crystals present. 

 Although in many plates several per cent, of silver iodide is present 



