146 PHOTOGRAPHIC FACTS AND FORMULAS 
the silver into silver ferrocyanide, which dissolves in the 
hypo. 
Homolka suggested as a stable reducer: 
Potassium ferricyanide 50 g 384 gr. 
Sodium amido-acetate 200g 30z., 224 gr. 
Water 1000 ccm 16 oz. 
For use dilute with from 5 to 10 parts of water, and, when 
the desired reduction is attained, immerse the negative in an 
acid fixing bath. 
Another modification of Farmer’s reducer is the following 
(Haddon), which has the advantage of keeping well: 
Potassium ferricyanide 10g ve ie 
Ammonium sulphocyanide 20 g 154 gr. 
Water 1000 ccm 16 oz. 
The negative should be soaked in water before immersion. 
AMMONIUM PERSULPHATE.—The particular value of this 
reducer is that it attacks the densest parts of a negative more 
than the shadows, thus considerably reducing contrasts; but 
its action is much complicated by intentional or accidental 
additions, such as acid, chlorides, and iron salts, which may 
occur by keeping the solution or the use of ordinary tap 
water. A 5 per cent solution in distilled water should be 
made, and allowed to stand for 3 to 4 hours, or 15 ccm 
(105 min.) of 10 per cent solution of sulphuric acid, or 
1 ccm (8 min.) of 5 per cent solution of ammonia iron alum 
added per liter (16 oz.). The following solution has been 
advised (Bennett), and can be kept for a considerable time 
in stock: 
Ammonium persulphate 125 ¢ 2 oz. 
Sodium sulphite, dry 10g 77 gr. 
Sulphuric acid 10 g 77 min. 
Water 1000 ccm 16 oz. 
For use dilute with from 4 to 8 parts of water, according to 
