Various Methods of Development 381

time, oz, negatives, factor, developer, normal, watkins and grin

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The table on page 260 gives some formulae of fine grain developers. Most of them are fairly quickly exhausted and can, therefore, be used advantageously only for developing a small number of negatives (about 7 sq. in. per fl. oz.). In these developers with a large content of sul phite a desensitizer would coagulate immedi ately ; desensitization is therefore possible only by means of a preliminary bath.

386. Semi-automatic Method based on the Time of Appearance of the First Details of the Image. (Watkins factorial system.) Develop ment for a total time, calculated by multiplying the time of appearance of the first details of the image (excepting the sky in landscapes) by the Watkins " factor " of the developing solution used (§ 34.1), if, as expressly assumed by the inventor of this method, it is applied only to correctly exposed negatives, has the advantage of taking into account automatically all varia but the range of luminosities of the sub ject, all the negatives being thus developed to the same development factor or gamma. But as it is possible to choose a Watkins "factor" (ascertained experimentally) for cads value of the development factor, it is easy to class the usual subjects into various groups ;iecording to their contrasts, and to use for each group a Watkins " factor" which will produce negatives best suited for the method of printing in view.

For example, with the developer given below (British journal Almanac formula, due to Welborne Piper, and one of the best formulae known for pyro, both from the point of view of the keeping quality of the stock solutions and from that of absence of any stain on the nega tives), any " factor " between ro and 14 will usually give satisfactory negatives. The lower " factor " should be chosen for very contrasty subjects and the higher one for subjects with little contrast.

(A) Pyre . . i oz. (so grin.) Soda sulphite cryst. $ oz. (400 grin.) or anhydrous . 4 oz. (200 grin.) Potassium metabisulphite i oz. (30 grin.) Water . . oz. (3,000 c.c.) (B) Soda carbonate, cryst. i 2 oz. (60o grin.) or anhydrous . 4 oz. (225 grin.)Water . . . io Oz. (3,00o c.c.) At the time of use, mix (A) i part, (B) i part water 2 parts.

In the case of all other developers, the average Watkins " factors " given in § 344 for various developers may be used, at least as an approxi mate guide.

387. Rational Automatic Development. The sure use of automatic development is possible only if the correct time of development has been experimentally ascertained with plates or films of the same batch of emulsion which is to be used (C. E. K. Mees, 1911). On this principle,

B. T. J Glover (1921) has worked out a rational method of automatic development, for a detailed description of which we refer the reader to his booklet, Perfect Negatives, and of which we here confine ourselves to the main outlines— Lay in a stock of plates or films of the various sizes required for a consumption of at least three months. This stock must be purchased on one order, with the express stipulation that the whole supply must come from one batch of emulsion.

Having chosen a developer (two of the sug gested formulae are given below) of which a given portion is used once only, and having divided all subjects into three classes according to their range of luminosities (§ r5), the following test is made : Any common subject with medium contrasts (such a.; a house illuminated sideways, half in sunshine and with a foregoumd object one side of which is in shadow) is photographed three time -, the exposure given to each plate be ng double that calculated by an actinonieter (§ 324). It is with this same subject (and if possible at the same time of day) that the tests should be repeated with each new batch of emulsion.

In order to ascertain the normal time of development (defined by Dr. Glover as the time of development which, with a subject of this class, gives a negative suitable for contact print ing on print-out paperor on normal rapid bromide paper), the three negatives are developed in the chosen developer at the fixed temperature of 65" F. for times respective:y equal to the average 1 .ngth of the normal duration of development (for that particular developer), as well as for six-tenths and for sixteen-tenths of this time (for instance, 3, 5, 8 minutes the metol hydroquinone developer, the average time for which is 5 minutes). After finishing these three negatives, they are printed on the chosen paper with every care to obtain the best result. Then the normal duration of development will be the time of development used for the negative that gives the best print. It may happen that none of the three prints is perfect. In this case, the normal time of development may be inter mediate between two of the times tested, or it max' be outside their range. In the former case it suffices to take an average time ; in the latter, a new set of development time tests becomes necessary.

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