After it would appear that all action of the developer has ceased, which will probably be after a minute or two, we have to do what is the most difficult thing in connection with development, or indeed with the whole of dry-plate photography — we have to judge whether or not the density of the negative is sufficient. It will readily be understood that to get a print of the desirable brilliancy from our negative the most opaque parts of it must have just a certain definite amount of opacity, not much more and not much less. We can, of course, only tell this by looking through the negative. To do so we take it out of the dish and hold it between our eyes and the ruby light. To judge of the correctness or otherwise of density requires some practice, as there is a certain loss during the after processes. We may say here that the very densest portions should appear nearly if not quite opaque. If they do not the plate is returned for a time to the solution, and is again examined after the lapse of half a minute or so. When the density is sufficient development is complete.
The plate is now taken from the dish and is well washed for a minute or two under the tap, or by pouring water on it from a jug. It is then placed in the alum solution, where it remains for five minutes. It is taken out and is again thoroughly washed. If the back of the plate be observed at this or any previous stage it will be found that it is still quite white. After the second washing the plate is placed in the dish containing the fixing solution. The whiteness will gradually disappear. When it is quite gone light may freely be admitted, but the plate must still be kept for a few minutes in the solution. In fact, a very long time in either the alum or fixing solution will in no way hurt it.
The negative is now complete, and requires only to be washed and dried. The washing should be very thorough. The plate should be kept in running water, in one of the zinc tanks made for the purpose, for at least an hour. It is then reared on edge in a plate-rack in any dry place to dry. Heat must on no account be applied.
If the beginner be able to secure a good negative to make comparison with, it will be well for him to notice whether the density of his is equal to that of the other, and be guided by the result in developing the next plate.
The plate which has received the longest exposure should be developed immediately after the first one, and whilst the action of the developer is fresh in the memory of the operator. A new quantity of solution is used. The image will come up more quickly than the first.
If the stop used with the lens has been as small as the smallest usually sent out with landscape lenses it is probable that the first plate will have been considerably under-exposed,—that is to say, there will remain after development considerable portions which have not been blackened. The second plate will very likely show signs of over-exposure,—that is to say, no portions of the film will remain quite, or nearly quite, white. It is, however, to be remembered that unless the deepest shadows darken or "veil over" very much indeed, this latter fault is not so grave as the other. In fact, the best result is produced when no portion of the plate remains quite white.
It should be noted that it is best to mix a fresh developer for every plate. The alum and hypo solutions may be used several times, but when they begin to be discoloured they should be replaced by fresh.
The object of the alum solution is to harden the gelatine film on the plate, and to improve the colour of the negative. It is quite possible to dispense with the use of it, but it is not desirable.
The object of the fixing-bath is to dissolve away such bromide of silver as is not reduced or blackened by the combined action of light and the developer.
We have described the process of development as taking place immediately after exposure. It must be borne in mind, however, that there is no need for this. It may be delayed for a very considerable length of time ; how long is not known with certainty, but at any rate for some months. Thus the amateur may, if he be away from home, keep all his plates for develop ment till he returns.