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fended as some mystic idol. Seldom do two exposure meters give the same reading under the same conditions of light; but the error in this instance, except in rare cases, is less than the variance possible by differences in manner of using the meters. Some photoelectric type meters become less sensitive with continued use. Others may be affected by heat and moisture. However, these sources of error can be controlled by care of the meter and by periodic checking. Or better still, instead of using the meter as an absolute instrument, it should be used as a guide in judging exposure, which judgment may be tempered by experience. A meter which consistenly indicates exposures which result in negatives of too high density or too low density does not have to be discarded, but rather, the values it indicates simply have to be modified by a factor which experience will dictate.
Processing Factors In the discussion concerning the factors which influence the character of the H. and D. curve it was pointed out that a great many of them center about the developing procedure. The ideal conditions which are met in laboratories (conditions which are rigorously controlled when speed ratings are determined) are seldom possible to duplicate in practice. Temperature control of processing solutions requires elaborate equipment installations, costing far more than the average photographer can hope to pay. The best solution to this problem is, perhaps, the constant use of a thermometer and simple methods of temperature control, such as a waterbath. However, in many cases these things are even avoided because of the
extra trouble involved. Still other laboratory methods are not adopted in practice, for example, the use of a fresh developer for each development. For economic reasons, it can hardly be expected that this practice will be adopted; therefore, the deterioration of a developer with use, and the accumulation of development byproducts certainly introduce serious obstacles to the use of fixed speed numbers. As we have previously pointed out, film speed is lost with increases of soluble bromide, and since free bromides are released during development, little can be hoped for if a developer is used over too long a period and no adjustments for changes in film speed are made. The latter would be indeed difficult to do, anyway, since the amount of bromide which accumulates not only depends on the amount of film development, but also depends upon the exposure level and degree of development. Furthermore, with some developers of low reduction potential (usually finegrain type developers) the effect of bromide is so much greater than with developers of high reduction potential that no fixed rule on this point can be set down.
The progress of development, it has been shown, is greatly affected by agitation, and so it is highly recommended that a standard method of agitation be worked out and adhered to.
Inherent Film Factors
Most of the inherent film factors of importance in speed determination have been discussed. Some minor considerations— latent image decay, slight variations in emulsions in manufacture, and variations from age — are
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