(12 c.c.) (6o c.c.) (30 c.c.) Potassium bromide (zo% solution) 20 min. 64 Cr.
(2 ex.) (40 ex.)Caustic soda solution) . 31 Cr.
If the image makes its appearance in less, than ro minutes, the negative has been consider ably over-exposed and must be transferred to a developer containing a considerable amount of bromide ; in this solution at a temperature of 5o° F. development is complete in less than an hour.
If no trace of the image (except, perhaps, the sky) appears in 30 minutes, the negative has been under-exposed, and must be quickly placed in a bath accelerated by addition of a caustic alkali ; the temperature of this bath should be about 77° F. and development is then complete in It) or 15 minutes.
In cases where there is less uncertainty as regards the exposure, this method of working may be very much simplified by employing two developers only, both at room temperature.
Whilst the two developers for this purpose may be prepared from different substances, as a general rule the two baths are made up from the same developing agents but in different proportions ; one of them (the soft bath) must be such as to give the best result with under exposed negatives, whilst the other (the hard bath) will give the best results with over-exposed negatives.
The following formulae (A Calvet, 1911) are well suited to this method of working— Soft Bath Hard Bath Soda sulphite, 350 gr. (40 grm.) 130 gr. (I5 grm.) anhydrous Amidol . x8 gr. (2 grin.) 53 gr. (6 grin.) Potassium bromide 5o min_ (5 c.c.) I oz. 3f dr. (70 c.c.) (io% solution) Water, to make 20 OZ. (I,000 C.C.) 20 01. (I,000 C.C.) When it is noticed during development in a strong bath that the high-lights tend to acquire a considerable density, before the details of the shadows have acquired sufficient, the negative may be put into a dish of water ; the dish is rocked two or three times and then left undis turbed (§ 341). It has been suggested (A. Knapp, 1933) to alternate immersions in the developer and immersions in plain water in order to obi ain the best results from under exposed negatives.
Industrial Applications of Two-bath Develop ment. To use up as fully as possible the de velopers, which are the most expensive con stituent of developing baths, the various components may be divided into two baths, the first containing the developers and a part of the sulphite. while the second contains the
remainder of the sulphite, the alkali, and, possibly, the soluble bromide.
In the first bath the emulsion absorbs a quan tity of liquid depending on the swelling of the gelatine, a swelling which, for a given emulsion and a given bath, has a limit fixed for each temperature. No development occurs in this bath, so that its composition remains unchanged if we disregard atmospheric oxidation, which is very slow in the absence of an alkali.' Placed, without intermediate rinsing, in the second bath, the emulsion develops, first rapidly and then more and more slowly, owing to the diffu sion of the developer in the bath. Any variation in the times of immersion in the baths beyond the minimum times easily ascertained by trials, has therefore no appreciable effect on the value of Owing to the limited amount of developer involved, the straight portion of the density curve tends to curve in its upper part.
The first bath should be kept at a constant level by addition of stock bath. The second. bath, in which the waste products of develop ment accumulate, must be replaced fairly often, or kept at a constant composition by pouring away a given portion of the total volume and replacing this with new bath.
Very interesting applications of this method have been made, especially in aerial photography and in cinematography (A. de Odencrants, 1922 ; L. Lobel, 1926 ; J. I. Crabtree, H. Parker and H. D. Russell, 1933).
Two-bath development can also be effected by means of two complete developing baths : the first bath acting as a stock for maintaining the second, this contribution being automatic ally proportional to the surface developed, or the first bath may have the same composition as the second, but as the plate only remains a short time in the first bath, this remains almost new. The short immersion in this new bath suffices, however, to avoid a decrease of gamma and speed, as would occur if development took place entirely in the used bath. When the second bath is exhausted it is replaced by the first, which in turn is replaced by a fresh solu Lion (Crabtree, Parker, and Russell, 1933).