FIXING AND CLEARING 129 
Dichroic, red or green fog, is practically unknown at the 
present day, but it was very prevalent in the early days of 
dry plate work, and showed itself mainly in the shadows as a 
more or less deep red colour by transmitted light, which was 
green by reflected light. The remedy for this (Abney) is 
to bleach the negative in: 
Ferric chloride 24¢ 184 gr. 
Potassium bromide 18g 138 gr. 
Water 1000 ccm 16 oz. 
Wash well and then redevelop. 
ALUMS AS HARDENING AGENTS.— 
Potash alum 6 parts 
Ammonia alum 5.6 parts 
Aluminum sulphate 4.2 parts 
Aluminum chloride, anhydrous 1.6 parts 
Aluminum nitrate 4.5 parts 
Chrome alum 2 parts 
The above quantities will render 100 parts of dry gelatine 
insoluble in hot water. Basic chrome alum is the most 
effective agent. This is made as follows: 
Chrome alum 100 g 768 gr. 
Hot water 800 ccm 12 oz: 
When dissolved, add: 
Ammonia quant. suff. 
to form a slight permanent precipitate after stirring well; 
then filter, and make bulk to 1000 ccm, or 16 oz. 
