| Pyro | 30 | grains |
| Potassium metabisulphite | 30 | „ |
| Ammonium bromide | 30 | „ |
| Water | 10 | ozs. |
But if much work was anticipated the solution was made up in a more concentrated form, and diluted to this strength of 3 grains of pyro per ounce for actual use.
The ammonia solution is prepared by mixing 3 drams ammonia fortiss. with 20 ozs. of water.
In developing it is necessary to remember that our object is to make the most of a very small difference in effect. The plate is first flowed over with a mixture of sufficient developer, with not more than a quarter of its bulk of the ammonia. If the cloud should flash out in a few seconds add more of the pyro solution, but unless the exposure has been much overdone this will not happen. If the image begins to appear after from thirty to forty seconds it is probable that the best result will be reached by leaving it alone, but if there is any hanging back of the detail another quarter bulk of ammonia should be put into the glass, the developer mixed with it, and the whole returned to the developing dish.
If no image appears after about forty seconds, add more ammonia as above described, and leave for another forty seconds, and so on, until by this method of trial the right quantity of alkali for the particular exposure has been ascertained. The development must never be hurried, or the background of sky will blacken too soon, and in some cases it may take a quarter of an hour or more to get enough density on the cloud. But as a general rule the image is fully out in about two minutes, and the plate is then washed and fixed in the usual way.
If a black mirror is used there will seldom be any necessity for intensification, but if not, it may frequently be required, especially for the more delicate kinds of cirrus. Indeed, the image may sometimes be so thin that the common process of intensification by mercury and ammonia does not give density enough. If that seems at all likely to be the case, it is wiser to use the formula known as Monckhoven’s, since that simply adds silver to silver instead of replacing the silver image by some other body, and the process can consequently be repeated more than once, if sufficient density is not secured by the first application. The formula does not seem to be very often used, so it may be best to quote it.
| A. | Potassium bromide | 100 | grains |
| Mercuric chloride | 100 | „ | |
| Water | 10 | ozs. | |
| B. | Potassium cyanide (pure) | 100 | grains |
| Silver nitrate | 100 | „ | |
| Water | 10 | ozs. |
Place the washed negative in A until it has gone white, then rinse it well and transfer to B, in which the image turns to a velvety black. After washing, the process can be repeated.
Intensification is, however, only a way of saving photographs which cannot be secured again. If the first photograph of a particular variety of cloud is not satisfactory, it ought at least to tell the operator where he had gone wrong, and a second attempt should produce a better result than any image built up by chemical action on an imperfect base.