Pyro Chlorhydrochinon H & D speed 150 180 Gamma infinity 1.45 2.12 Fog (at maximum gamma) .32 .60 Time of appearance 5 seconds 5 seconds Watkins factor 25 10 Velocity factor “κ” .320 .400 Temperature coefficient 1.40 2.0 Temperature limits 4° to 32° C 4° to 32° C Keeping power 45 minutes 8 days Exhaustion (100 c.c.) 30 sq. in. 300 sq. inches Dilution coefficient 2 2 Color coefficient 1.50 1.00 Resolving power 47 53 Owing to the difficulty of securing pure chlor-hydrochinon a metol hydrochinon of very similar properties has been worked out. Its composition is Metol 16 grams Hydrochinon 16 grams Sodium sulphite 60 grams Sodium hydroxide 10 grams Potassium bromide 10 grams Water to 1 litre To keep the ingredients in solution in cold weather, 50 c.c. of alcohol should be included in every litre of solution. All things considered this is probably the most practical and satisfactory developer for aerial negatives. =Developers for Papers.=—The following formula has been found very satisfactory for papers: Metol .9 gram Hydrochinon 3.6 gram Sodium carbonate 20.0 gram Sodium sulphite 14.0 gram Potassium bromide .5 to 1.0 gram Water to 1 litre =Fixing Baths.=—For plates the following fixing and hardening bath is recommended: Sodium thiosulphate (hypo) 350 grams Potassium chrome alum 6 grams Sodium bisulphite 10 grams Water to 1000 c.c. During hot weather, the above quantities of chrome alum and bisulphite are doubled. For papers the following: Hypo, 35 per cent. 100 volumes Acid hardener 5 volumes The acid hardener is constituted as follows: Alum 50 grams Acid acetic 28° 400 c.c. Sodium sulphite 100 grams Water to 1 litre