The exposure must consequently be prolonged. in proportion to the extent to which develop ment is shortened for tones more or less red, so that details in the lights may appear with suffi cient strength before development is stopped.
According to the plate used, the time of ex posure necessary for obtaining red tones will be from io to 20 times that which is necessary for black tones, the same illumination being used. No general rule can be given, and, for a first trial, it is best to follow the instructions given by the makers of the plates. These preliminary tests can be made on pieces cut from one of the plates, the determination of the time of exposure being carried out under the conditions already des scribed (§ 553). The same test will furnish experimental proof of the fact that the colour of the image is independent of the time of exposure. The parts of the test piece, though exposed for different times, will all be identical in colour after development, and will differ from each other only in the density of the image and the rendering of the details in the lights.
576. Development. It is almost impossible to judge the colour and the density of the image properly in coloured light during development. A yellow light weakens considerably the con trasts of a sepia or red-chalk image, while a green light exaggerates them. In addition, both the colour and the density of an image change considerably on drying, as may be easily ascer tained by examining a plate which is partially dry. An image which is pale yellow when wet can become an intense red-chalk after drying.' Thus, for uniform results as regards colour, plates should always be developed in solutions identical in composition, and used at the same temperature and for the same time, or else the times of development should be worked out by the Watkins system. Although, as has been previously shown (§ 569), this method is in applicable in the case of transparencies of black tone, for which development is done by judging the light tones, its use is perfectly sound in the case of plates of warm tones when a certain colour is desired. It has been shown that the colour of the image depends essentially on the value of the contrast factor, and it must be understood that the character of the negative is suitable for the colour desired.
A few preliminary tests will soon indicate the " factor " by which to multiply the time of appearance of the first details for obtaining the total time of development corresponding with a particular colour. This may be either in the same developer diluted in different proportions for obtaining tones varying in warmth to different degrees, as the result of longer ex posures, in which case the " factor " decreases progressively in relation to the colour from black to red, or else in developers of different com position for obtaining various colours.
When transparencies are intended for projec tion in the lantern, it is well to suspend all judgment until they are seen on the screen. The appearance of the projected image is frequently very different from that which it presents when simply held up to the light.
577. A widespread belief exists that hydro quinone is the only developer which can be satisfactorily employed for warm tones. As a matter of fact, a large number of developers can be used, provided that a " factor " suitable for that particular developer is employed.
Formulae for developers are here given which are suitable for a large variety of commercial plates- Hydroquirtone- Soda sulphite, anhydrous 300 gr. (35 grm.) Hydroquinone . . 90 gr. (so grm.) Soda carbonate anhydrous 260 gr. (30 grm.) Ammonium bromide . i8 gr. (2 grin.) Potassium bromide . 26 gr. (3 grm.) Water, to make . . 20 OZ. (1,000 c.c.) To be used without dilution for black, and diluted about 10 times for red tones.
Glycin (Vannier, 1920)— Soda sulphite, anhydrous i oz. (5o grm.) Potassium carbonate . 350 gr. (40 grm.) Glycin • s 75 gr. (20 grm.) Potassium bromide . 130 gr. (15 grm.) Water, to make . . 20 oz. (1,000 C.C.) For use, to be diluted from 5 to 20 times, according to the colour desired.
Acid Amidol (G. Balagny, 1906)— Soda sulphite, anhydrous . 45 gr. (5 grin.) Amidol . . . . 22 gr. (2.5 grin.) Sodium bisulphite, lye . 6i dr. (40 c.c.) Ammonium bromide . . 45 gr. (5 grim) Water, to make . . . 20 OZ. (1,000 C.C.) Only sufficient for immediate use should be prepared. The action is very slow, and it is not necessary to dilute the solution for obtaining red tones.