148 PHOTOGRAPHIC FACTS AND FORMULAS 
the use of the persulphate, but it is still valuable. The nega- 
tive should be immersed in: 
Hydrochloric acid 25 ccm 192 min. 
Potassium bichromate 8g 61 gr. 
Alum 40¢g 307 gr. 
Water 1000 ccm 16 oz. 
Leave until bleached through to the glass, then thoroughly 
wash until all traces of yellow disappear. lt should then be 
developed with a slow acting developer, such as hydrochinon, 
about one-fourth the usual strength, and development con- 
tinued until, on examination of the negative from the glass 
side, it is seen that the shadows and half tones are fully 
developed, while the high lights still show white silver chlo- 
ride. The negatives should then be fixed and washed. This 
method is also useful for halated negatives. The following 
modified bleacher may be used: 
Chromic acid 5g 38 gr. 
Potassium bromide 10g 77 gr. 
Water 1000 ccm 16 oz. 
The method of using is as above, or, to hasten the removal of 
the yellow stain, apply a 2.5 per cent solution of potassium 
metabisulphite or sodium bisulphite. 
MECHANICAL Repuction.—This is a method but little 
practiced and rather dangerous, except in expert hands. It 
consists in local attrition of the gelatine, preferably with a 
chamois leather moistened with denatured alcohol (methylated 
spirit). Another abrasive has also been suggested (Baskett), 
and consists of a mixture of equal parts of terebene, olive oil, 
and metal polish. This is also used with chamois leather, and 
acts more quickly than the alcohol. After its use, the negative 
should be rubbed two or three times with benzol to remove 
the grease. Some commercial metal polishes may be used 
without any admixture, but, after shaking well, should be 
