The colour of the image depends on two factors, exposure and development. The mini mum exposure that will produce a well-graded image with sufficient strength in the shadows and full detail without any mass of clear glass in the high lights is that which must be given for pure neutral black tones. For a warm black, the exposure must be doubled, and for various tones of brown, deep or rich, from four to eight times the minimum exposure will be necessary. The exposure may be increased even beyond this with some plates, very warm brown and red brown colours being obtained by increasing the exposure up to sixteen times the minimum. For neutral black tones any of the developers given for bromide printing will give the best result. For warm black and brown, pyro and soda will give excellent transparency and quality. Potassium bromide will be found the best restrainer. Most of the additions suggested by some photographers for obtaining warm tones clog the shadows and produce a semi opaque deposit that spoils the effect of d slide when projected. Potassium bromide added liberally will produce the same degree of warmth combined with great transparency and fineness of quality. Other developers may be used in stead of pyro by adding potassium bromide in the same way.
The following are typical formulae, the deve loper being prepared from the usual stock solu tions : For deep brown tones Pyro15 grs. 3.5 g.
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Potassium bromide 7+ Sodium carbonate 6o-i2o 14-28 Sodium sulphite 6o-120 I p Water . . io oz. i,000 ces.
For rich brown tones Pyro15 grs. 3'5 g.
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Potassium bromide 15-3o 3-5-7 Sodium carbonate 6o-i 20 14-28 Sodium sulphite 6o-i 20 p p Water . 10 oz. i,000 ccs.
Begin with a small quantity of the sodium car bonate solution, increasing to the larger amount if necessary. Development will take from six
to twelve minutes. If the development is begun in the more highly restrained solution, and the exposure should prove to be insufficient, the plate may be transferred to the less restrained, and a good slide should result ; and, if development is begun with a small quantity of sodium carbonate, and this should prove insufficient, adding more will not detract from the quality of the slide, but will modify the colour. This method of working allows very great latitude in exposing, as the development may be modified, during its pro gress, to suit the exposure.
After development, the plate should be washed in two or three changes of water and then fixed, preferably in an acid " hypo " bath containing oz. of potassium metabisulphite to 1 lb. of " hypo," diluted so that i pint of solution will contain 3 oz. of " hypo." The plates should remain in the fixing bath for ten or twelve minutes. The developer for use with plates that are developed in weak gaslight is the same as that given for gaslight papers. Restraining by means of bromide may be adopted for secur ing warm tones. The same pyro developer may be used, or an equally large proportion of bromide may be added to an amidol or other developer. With these plates a longer range of tones may be secured, a good red and red-brown being easily obtained by increase of exposure and restraining. The image produced on these plates is exceptionally transparent and fine in grain. Even when strong, the image presents the appearance of a stain rather than a deposit.
The clearing of lantern slides should be unnecessary, if an acid fixing bath is used; otherwise proceed as explained for negatives under the heading " Clearing Solutions."