_Temperature coefficient._ This is the factor by which the time of development at normal temperature (20 Cent.) must be increased or decreased in order to obtain the same quality negative, for a change of seven degrees either side of normal. _Temperature limits_ are the temperatures between which development can be carried out with any degree of control or without serious damage to the negative. These factors are of great importance where climatic or seasonal changes have to be endured. _Dilution coefficient._ This is the factor by which the development time is increased in order to maintain a given quality negative in different dilutions of the developer. It is useful in tank development. _Keeping power._ The keeping power of a developer, mixed ready for use, is determined by its ability to resist aerial oxidation. A developer of poor keeping power, which must be made up immediately before use, causes delay and waste of time whenever emergency work has to be done, whereas a developer of good keeping power may be left in its tank ready for instant use. _Exhaustion_ of a developer is the rate at which it becomes useless for developing, due both to aerial oxidation and to the using up of its reducing power by the work done in developing plates. It is conveniently measured by the area of plate surface developable before the solution must be renewed. _Length of rinsing._ The time required for rinsing between development and fixing bath plays a not unimportant part in total development time. Dichroic fog is caused with some developers if, due to insufficient rinsing, any of the caustic alkali is carried over to the fixing bath. Stains develop also if the fixing bath is old, or if light falls on the unfixed plate while any developer remains in the film. _Color coefficient._ The function of the sulphite, which forms a constituent of all developing solutions, is two-fold. It acts partly as a preservative, and partly to prevent the occurrence of a yellow color in the deposit. The yellow color, if present, increases the photographic contrast. This phenomenon has been purposely utilized, particularly in the British service, to give “stain” to negatives which otherwise would show insufficient printing density. The color index or coefficient of a negative (with a given printing medium) is the ratio of photographic to visual density. If we take a pyro developer containing five parts of pyro per thousand and ten parts of sodium carbonate, and then vary the amount of sulphite from none to fifty parts per thousand, the color index varies as follows: