Sulphite Color Index Parts per Thousand 50 1.16 25 1.24 15 1.30 10 1.45 5 1.80 0 2.75 The color index is somewhat different with various kinds of printing media. This staining effect is a variable one, depending upon length of development, dilution of the developer, length of rinsing, temperature, the fixing bath used (plain hypo being necessary for a maximum effect), the length of washing after fixation and the properties of the water used. Standardization of these conditions in the field is difficult; hence any developer which will give the same effective contrast without resorting to stain is to be preferred. _Resolving power._ Some developing processes and conditions will introduce bad grain into the negative. Hence the resolving power which a developer brings up must be investigated among its other characteristics. =Practical Developers for Aerial Negatives.=—In the English service a pyro metol developer was generally used, producing stained negatives. The French, American and Italian practice was to use metol-hydrochinon, without staining. A special chlor-hydrochinon developer, worked out by the Eastman Research Laboratory for the United States Air Service, has probably the greatest merit of any yet tried. A comparison, given below, between it and a pyro metol formula used on a representative plate, illustrates the use of the various bases of study given above. PYRO FORMULA Solution A Solution B Pyro, 3.75 grams Sodium carbonate, 53 g Potassium metabisulphite, 3.75 g Metol, 3.05 g Potassium bromide, 1.5 g Water, 500 c.c. Water, 500 c.c. Use 1 part of A to 1 of B CHLORHYDROCHINON FORMULA Solution A Solution B Chlorhydrochinon, 25 g Sodium carbonate, 30 g Metol, 6 g Sodium hydrate, 10 g Sodium bisulphite, 2.5 g Potassium bromide, 3 g Sodium sulphite, 25 g Water to 670 c.c. Water to 330 c.c. Use 2 parts of A to 1 of B