After each piece has been developed for the given time, it is placed directly in a fixing bath, so as to allow the various pieces to be compared in a bright light. It will be found, in the case of bromide paper, that the black developed for say ioo seconds has less density than that developed for 140 seconds, but that there is no appreciable difference between the blacks developed for 140 and 200 seconds. From this it follows that a time of development less than 140 seconds leads to an impairment of the black. Further, the half of each piece which has been protected from the action of the light should be perfectly pure on those pieces of which the time of development has not exceeded 200 seconds, though it may be grey and even stained yellow on the piece developed for 280 seconds.' From this it follows that the time of development necessary to produce the best black can be ex ceeded considerably before fog or stain make their appearance.
In order to determine that there is a minimum time of development independent of the ex posure given, the experiment should be re peated under identical conditions, excepting that the strip of paper should be exposed for a considerably longer time than in the preceding test, e.g. five times longer. It will then be found that although the over-expo sure has been considerable, a perfect black is obtained only after development for a similar time to that determined in the previous experi ment. From this it is concluded that insufficient development cannot be compensated by an increase in exposure.
As a general rule, a photographic paper of good quality and easy to manipulate may be developed in a suitable solution for twice the minimum time necessary for obtaining the maxi mum black without any staining or veiling.
557. Experiment also shows that all times of development comprised between the maxi mum and the minimum, determined as described in the preceding paragraphs, will yield perfect prints, provided that the exposure is adjusted to suit the time of development adopted. 3 The corresponding exposures are equally comprised between two limits, and, as a general rule, identical prints are obtained, provided that, within the limits specified, the time of exposure multiplied by the time of development is a constant number (B. T. J. Glover, 1922). For example, if the two extremes of the time of development are 2 and 4 minutes, and develop ment for 4 minutes corresponds with an ex posure of 15 seconds, under certain conditions, a print exposed for 20 seconds and developed for 3 minutes, or a print exposed for 30 seconds and developed for 2 minutes should be indis tinguishable from each other. But it would not
be possible to compensate by development for a shorter exposure than 15 seconds nor for a longer exposure than 30 seconds, as development could not be longer than 4 minutes nor less than 2 minutes in the example quoted, without the onset of fog or stain in the former case, and failure to produce a good black in the latter case.
558. As already shown in regard to the development of negatives (§ 342), the speed of development is greater in proportion as the temperature is raised. The proportionate times of development for different temperatures are not the same when one developer is substituted for another. The variation may be twice as much with one developer as with another for a difference in temperature of 18° F.
The time of development determined by the tests just described would therefore be quite valueless if development were carried out on another occasion at a different temperature. We shall see, however, that for certain types of paper, at any rate, development can be regulated by other means than by a constant time.
559. Developers for Positives. While the differences in colour of the silver reduced by various developers is of very limited importance in negatives, they assume considerable import ance in the case of positive prints. A developer consisting of metol and hydroquinone yields principally warm black tones ; metol without hydroquinone, a more neutral black ; while diamidophenol or amidol tends to produce bluish-blacks which are unsuited for cream tinted or chamois paper.
The formuhe recommended by the different manufacturers vary considerably, even when intended for use with emulsions of the same type. Beginners will do well to follow the makers' instructions, but it may be stated that any developer suitable for one type of emulsion will develop quite satisfactorily all other emulsions of the same type. There is only one exception to this ; plates or papers intended for yielding warm tones will often work more satisfactorily with one formula than with another.
Potassium bromide, the use of which is optional in developers for negatives, is a neces sary constituent of developers for positive images on plates, films, or papers. The proportion of bromide which is most suited to one emulsion. is not always the best for another. It is, there fore, best to make a few systematic trials to determine the most desirable proportion of bromide for a given paper.