374 HANDBOOK OF PHOTOGRAPHY 



have not been established. A procedure recently recommended for physical develop- 

 ment before fixation involves the following steps: 



1. Treatment with potassium iodide bath. 



2. Rinse. 



3. Development in silver-salt bearing physical developer. 



4. Fixation. 



5. Washing and drjang. 

 The baths suggested are: 



For Step 1: 



Potassium iodide 10 g. 1 oz. 145 gr. 



Sodium sulphite (anhydrous) 25 g. 3 oz. 145 gr. 



Water to 11. 1 gal. 



For Step 3; 



No. 24 



Stock Silver Solution 



Sodium thiosulphate (hypo) (crystal) 160 g. 1 lb. 5 oz. 145 gr. 



Sodium sulphite (anhydrous) 60 g. 8 oz. 



Silver nitrate (crystal) 16 g. 2 oz. 58 gr. 



Water to 11. 1 gaL 



For use, add 1 part stock silver solution to 4 parts water, and add reducing 

 agent as directed below. 



To make up this stock silver solution, dissolve the sodium sulphite in 300 cc. 

 (40 oz.) of water, then dissolve the silver nitrate in 100 cc. (16 oz.) of water and add 

 to the sulphite solution, stirring until the white curdy precipitate dissolves. Dilute 

 the whole to 950 cc. (120 oz.) with water, add the thiosulphate, and stir until complete 

 solution is obtained, after which the volume of the solution should be 1 1. (1 gal.). 

 Filter through cotton. The solution is fairlj^ stable and keeps well. 



At the time of use of the developer, for each liter of diluted solution add 1.7 g. 

 of Amidol, or for each 15 oz. of diluted solution add 12 gr. of Amidol, and stir until 

 completely dissolved. The Ainidol should not be added to the solution more than 

 10 min. before the development is to start. 



With tray development, 35 min. to 1 hr. has been recommended. Metals other 

 than stainless steel or chromium plate are to be avoided and hard rubber or bakelite 

 is preferred. 



The special silver-bearing developer just described maj' be used for physical devel- 

 opment after fixation if that is attempted. Greatly increased exposures are required 

 over those needed with ordinary chemical development. Fixation should be carried 

 on in the dark, with rather dilute neutral or alkaline hj'po, such as represented by 

 the formula below: 



Sodium thiosulphate (hypo) (crystal) 45 g. 6 oz. 



Ammonium hydroxide (sp. gr. 0.90) 2 cc. 2 fl. dram. 



Water to 11. 1 gal. 



The plate is to be removed from the hypo as .soon as cleared and washed very 

 thoroughly in water after which it may be developed immediately or dried and devel- 

 oped later. 



Developers with Special Properties. — During the years of study of developers and 

 development, various special effects have been discovered beyond the basic process of 

 development. 



Low-sulphite Pyro. — Pyro developers containing little sulphite produce two effects 

 aside from the actual development, i.e.. the reduction of the exposed halide of the 

 emulsion to silver. They harden the gelatin locally, in the region where the silver 



