The black mirror could with some advantage be replaced by a Nicol’s prism mounted between the components of the lens, so that it could be turned in any position; but Nicol’s prisms are expensive, and such an arrangement would cost many times the sum sufficient for an excellent mirror, and then would narrow down the field of view in a very inconvenient way. With this apparatus exposures of a tenth to a fifth of a second were usually required for high clouds in bright daylight, while longer times, up to a second, might be required under less actively actinic conditions. The exposure having been made, the next step is development. Now, every practical photographer has his own pet formula, his own particular favourite among the numerous developing compounds now on the market. It is, therefore, rather a thankless task to offer advice as to which should be selected. In all probability as good results may be got by other methods and other formulæ, and the description which follows must be understood rather as an account of the process actually adopted, than advice as to that which should be chosen. The developer used has been always pyro and ammonia, made up in accordance with the formula-- 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.