In the case of horizontal development in a stagnant solution, the lateral diffusion of the waste products around the strongly-exposed parts tends to produce silhouette effects. Outside the strongly-exposed portion development is somewhat retarded, and the dense part of the image is thus surrounded by a border which is lighter than the ground (Mackie lines). When developing a plate on which there are patches uniformly exposed but of very different dimen sions, it will he noticed that the smallest patches have the greatest density, and that in each patch the density is greater at the edges than in the centre (Eberhard, 1912).
These various effects are avoided by prolonged development or by agitating the solution, or at any rate they are reduced to a point at which they are negligible unless subjected to critical examinat This agitation has a very marked effect on the speed of development, and to a certain extent on the character of the image. By eliminating the bromide formed in the course of development, agitation accelerates develop meat, and does so to a degree increasing with the susceptibility to the action of bromide of the particular developer employed. At the same time agitation prevents the falling-off in con trasts and in the energy of the developer which would tend to be produced by variations in the local concentration of 344. The Arithmetical Coefficient of a Devel oping Solution. Among the numerical laws of development one of the most important from the practical standpoint is certainly the approxi mate law formulated in 1894 by Alfred Watkins to connect the length of time required for the appearance of an image with the length of time necessary to achieve an arbitrarily chosen de velopment factor or degree of contrast (§ 202, footnote), viz., say the ratio between the con trasts of the negative and the contrasts of the subject, in the case of negatives of which the exposure has been within normal limits.' With a normal exposure and in a given develop ing solution, the time required for the negative to reach a given development factor is proportional to the time of appearance of the first details of the image (except the sky in landscapes, owing to its over-exposure) provided only that the energy of the bath be not modified during the course of development.
The multiplying factor (arithmetical factor of the developing solution or Watkins "factor ") varies for a given developing solution, according to the value adopted for the development factor, that is to say according whether it is wished to reproduce exactly the contrasts of the subject in the negative, or to increase or decrease them in a varying degree. Since the image becomes more contrasted the longer it is developed, the Watkins " factor " requires to be increased to enhance the contrasts, and decreased to reduce them. Unfortunately, no rule can be fixed regarding this matter, for a variation of the " factor," which, for example, increases the contrasts to a given degree, does not apply equally (neither absolutely nor relatively) to different developing solutions.
The average values for the Watkins " factors " for various developers, as given by different writers, correspond, according to the personal taste of each individual writer, to values of the development factor usually lying between a-8 and i (slightly reduced contrast or exact reproduction of the contrasts). These values
must therefore be regarded as an approximation only ; the " factor " to use must be worked out for each particular case.
While the Watkins " factor " varies consider ably for different developer substances, varia tions are practically negligible (except in the case of pyrogailol) so far as concerns the pro portions of the constituents of a given developing solution or the use or omission of bromide. Dilution and variations in temperature, at least within reasonable limits, do not occasion any appreciable difference, it being of course under stood that any modification in the developing bath is made before development begins, and that from this point the developing solution remains unaltered 1 (except for variations due to reactions in the course of development).
Developers of high reduction potential, with which the appearance of the image is always very rapid (the entire image appearing almost at once) have a much higher Watkins " factor " than developers of low reduction potential, with which the image appears only slowly and gradually (the image of the high-lights often appearing long before that of the shadows.) The following table gives the approximate values of the Watkins " factor " for various developers— The addition to the developing bath of an iodide or of a desensitizer considerably affects the " factor." The control of development by this factorial method is not applicable to sensi tive films the emulsions of which have been wetted before immersion in the developing solu tion (wetting with water or with a desensitizer.) In the case of developing solutions containing several developing agents, the Watkins " factor " can usually be calculated by applying the rule of mixture.' The determination of the total time of development by the method based on the measure of the time of appearance of the first details of the image is a very valuable one for the beginner, and will save him serious mistakes. It may be objected that it is not logical to develop for equivalent times the images of subjects with very weak contrasts and of subjects with very strong contrasts, the nega tives thus obtained not being suitable for the same types of positive printing paper. But the beginner easily makes far greater mistakes than this, and it is a simple matter to employ a variety of emulsion papers, enabling negatives of widely different character to be printed satisfactorily.
Objections based on the differences in con trast of the various classes of subject do not apply to the case of copying, where the aim is to obtain as far as possible a facsimile of the original, irrespective of its character.
Finally, this method is the only one which enables anyone not having a specially equipped, scientific laboratory to carry out the develop ment of different emulsions to the same development factor, thus allowing of satis factory comparisons being made between various emulsions.
Some makers of developing solutions sold ready for use, or in the form of powders, or tablets, etc., follow the useful practice, which might well become general, of giving the average Watkins " factor " for each developing prepara tion. It is highly desirable that the same information should be given for the developing formulae recommended for various plates and films by the makers of these materials.