The result of increasing ammonium bromide is to make the action of the developer much slower, to keep back detail, and to increase the density of the image.
Now we will see how we can alter the proportion of our developer so as to suit different circumstances. The solutions are so mixed that the bromide and the pyro gallic are together, as these are in some respects similar in their action.
If we have under-exposed we have merely to increase the quantity of No. 2—the ammonia solution, and we shall increase the energy of the developer and thus bring out more detail. The power of the ammonia is, how ever, limited. When increased beyond a certain amount the effect is, as we explained before, that even those portions of the plate which have received no impress of light are blackened, the result being to produce a foggy negative which is useless.
On the other hand, if we have over-exposed we have merely to increase the proportion of the pyrogallic solution (No. 3), when we prevent the appearance of detail on account of the bromide, whilst at the same time the increased pyrogallic acid enables sufficient density to be gained. Unlike the last case, there is scarcely any limit to the amount of compensation which is possible in the case of over-exposure. It will thus be seen that where we are at all doubtful of what exposure to give, we had better err on the side of over-exposure rather than under-exposure.
So far we have indicated the variations in the de veloper which it is necessary to make to compensate for errors of exposure, and it is evident that there is no difficulty involved if we know whether we have exposed too little, or too much, as may often be the case after we have developed the first of a series of plates which have been exposed at the same time.
It very frequently happens, however, that we have to develop plates concerning which we are quite uncertain as to whether the exposure has been too little, correct, or too much. We now describe the procedure to pursue when we are thus uncertain. First we will take the case which is common, for example, in studio work, where we know that our exposure cannot be much too little or too great. In this case we begin with the normal developer, but have in readiness the vessel con taining No. 3. We watch the process very closely to see the first symptom of over-exposure. When a plate is over-exposed this is indicated by the manner in which the details appear. The high lights are a little quicker in showing themselves, and after they do so, the rest of the details, instead of coming up gradually in the order of their brightness, appear rapidly one after another, so that in the case of a very much over-exposed plate the whole surface darkens within a few seconds of the time when the high lights appear.
Whenever the operator observes by this quick coming up of detail that his plate is over-exposed, he pours away the solution that covers it, and pours No. 3 into the dish in place of it. This will stop the further appearance of detail, but will allow density to continue increasing, and it is quite possible that the development may be thus completed. If, however, there appear to be insufficient detail the solution in the dish is emptied into a measure, and a little of No. 2 is added, and the solution repoured
over the plate.
If, on the other hand, the high lights have been slow of appearing, and shortly after they do come action ceases, leaving large patches of unblackened film, this indicates under-exposure, and a certain quantity of solution No. 2 must be added as described above. The addition of this may be continued until the desired effect is produced, or until there is two or three times as much of No. 2 in the developer as there is of No. 3. It is not safe to go beyond this or fog will probably result. If sufficient detail be not by this time gained it may be assumed that the plate has been so much under-exposed as to be useless.
We now take the case which is very common in land scape work, where we do not know but that our plates may be very much over-exposed indeed. It would now be dangerous to begin with the normal developer, as detail would flash out before we had time to pour off the one solution and pour on the other. We now start with a solution consisting of one part of No. 2 and three parts of No. 3. For example, if we are about to develop a quarter plate we will take half an ounce of No. 2 and one and a half ounce of No. 3. With this developer the image will appear hut slowly even if the plate has been over-exposed, and very slowly if it has been correctly exposed. Only practice enables us to be able to tell at once from the rapidity with which the image appears whether we have over-exposed, under-exposed, or correctly exposed. If the former be the case we may continue development with the solution as we began with it, or even may in crease No. 3 if it appears that the exposure has been very much too great. If the exposure appears to have been correct we add No. 2 till we have as much of it as there was of No. 3. If it appears to have been too short, we have to continue adding No. 2 as described before.
We have now only to explain how it is, at times, desir able to vary the developer so as to suit different subjects.
A consideration of what we said with regard to the properties of each of the constituents of the developer will show that, apart from considerations of exposure, the effect of increasing No. 3 in proportion to No. 2 is to increase density or contrast, whilst the effect of in creasing No. 2 in proportion to No. 3 is to reduce con trast. AU that we have to do then, if we have a subject in which contrast is excessive, and from which we expect to get a chalky print, is to increase the quantity of No. 2. If, on the other hand, our subject presents but weak contrasts we have to increase No. 3. We may thus at times, even if exposure has been correct, use the two solutions mentioned in proportions of three or four parts of one to one part of the other.
Some operators like to have in readiness, besides the solutions mentioned, a concentrated solution of bromide of ammonia and a strong mixture of ammonia and water. These may be useful in certain circumstances. One ounce of bromide of ammonia may he made up with water to ten ounces, and a mixture may be made of one ounce ammonia to nine ounces water ; these will then each be ten per cent solutions.