In making transparencies we must be as particular in keeping the hypo, solution at a respectful distance, as we must be during the operation of toning prints. And this is best achieved by not mixing any hypo. until all the transparencies which are in hand are ready for fixing. Let the operations be conducted as follows.
In the first place clear the operating table of everything not actually required for the business in hand. On the operator's right hand let the gas-lamp be in readiness turned down to its very lowest. At the back of the table should stand the red lamp supported at a convenient height upon some form or stand. Immediately in front of it should be placed the developing dish, and a half plate ebonite one is the best for the purpose. It has the merit of having a flat bottom, and it is large enough to hold two plates side by side. This is a great convenience in transparency work, for the reason that sometimes through injudicious exposure a plate will require much persuasion before it will yield up a good image. It may remain in the developer for ten minutes or more without detriment, while other more obliging plates are being finished in the same dish.
At the left hand side of the developing dish should stand a large dish capable of holding a dozen or two lantern slides. This should be filled with a weak solution of alum and water. As each plate is developed, it should be washed well, and transferred to the alum bath. In this bath the plates can remain for many hours if required, without suffering any harm; of course they must not see daylight for they are still in a sensitive condition, but an occasional gleam of gaslight such as they would get while other plates are being exposed, will do them no harm whatever. If one plate is being exposed to light while another remains in the developing dish, the latter should be covered over with a piece of card or the lid of a cardboard plate box.
After all the plates of a batch are developed and in the alum bath, we can proceed to fix them in hypo. solution. This should be mixed freshly for the purpose. Indeed, it should be a standard rule with photographers to mix this salt freshly for whatever purpose it may be required. The soda is so cheap now-a-days that economy in its use need not be thought about.
The dish used for the fixing solution should be large enough to contain at least four lantern slides at one time. As each is cleared it may be rinsed and put back in the alum dish until all are so replaced. Then take the large dish to the sink, carefully empty away the alum and water, and let the tap run into it for some minutes, every now and then tilting up the dish as the water accumulates. Finally, let the plates remain in water for at least a couple of hours, and then proceed to examine them. If the iron developer has been used, a slight deposit of oxalate of lime may possibly be noticeable on the surface of the film, making it somewhat milky in appearance. This is often of such a superficial nature that it can be wiped off with a tuft of cotton wool, while the water from the tap is allowed to run upon the plate. But if the disease is of a deeper nature and seems to attack the body of the film, more energetic measures must be resorted to for its cure, Mix the following: -
Saturated Solution of Alum ... 10 ounces. Sulphuric Acid ... ... ½ ounce.
Pour this mixture on and off the plate two or three times, and the milkiness will at once disappear. This strongly acid solution will have, with some plates, a tendency to cause them to frill, so that it must be used with caution. But in most cases the application of a tuft of cotton wool will do what is necessary. If the acid be used the plates must undergo another careful washing, but if not, they can be rinsed under a tap, and placed in a rack to dry spontaneously. On a fine day they will dry quickest in the open air, but should be placed in some situation where wind and dust cannot do them any injury.
In developing transparencies the all important point is to arrive at the proper amount of density. This amount must depend upon the purpose for which the transparency is required. For an ordinary lantern picture the development should be allowed to continue until the picture appears to be rather overdone. Its real density may be better judged by holding it up to the red lamp once or twice during development, and viewing it by transmitted light. If it does not appear deep enough, return it once more to the dish until the right amount of density is attained.
But the transparency may be wanted merely as a vehicle for obtaining another negative, or for enlarging purposes. In these cases the development must be carried to a much greater extent, and every bit of detail must be dragged out before the plate can be considered complete. The same rule must guide the operator, in his transparencies are intended for window decoration. These, it must be remembered, will be viewed when finished, by transmitted light, and can be therefore made very dense indeed. We may state in passing, that there is a very wide field open to the photographer in this direction. Good transparencies backed up with ground glass, and perhaps leaded together with a frame of tinted cathedral glass, have a splendid effect when used as door panels, fan-lights, or in ordinary windows.