Reference has been made in the first chapter to the fact that those who wish to make a photographic study of clouds must follow a special course of procedure. For every photographic purpose there is some particular process or some special kind of apparatus which is better fitted for the end in view than any other, and half the difficulty in attaining success is to find out the best tools and the best methods.

There is no difficulty whatever in securing excellent photographs of heavy grey clouds, or of clouds which stand out dark against a twilight sky. Any camera and any plate can be used, and in an experienced hand will ensure success after a few trials, but except under these special conditions, cirrus, in all its varieties, the alto clouds, and even many of the lower ones, present a real difficulty due to two causes. In the first place, they and their surroundings are so brilliant that a very short exposure is sufficient, far shorter than would be needed for a sunlit landscape; and in the second place, the actinic value of the light they reflect is very little greater than that received from the background of blue sky. When so minute a difference comes to be represented in the monochrome of the ordinary photograph, the eye fails to appreciate it, and all the finer details are lost.

Now, if proper care is taken in the development of a negative, satisfactory results may be attained even if the exposure is twice as great, or only half as great, as it should have been to get the best result. But if the exposure is four or more times the best duration, the negative will generally yield but poor contrasts, if any result at all can be coaxed out. Again, if the exposure is only a quarter or less of the ideal time, little or no image will come out. Suppose, now, we have a brilliant object, and the correct exposure for the plate and aperture of lens employed should be one-fiftieth of a second; if we make an error either in judging or in effecting the exposure, which amounts to one twenty-fifth of a second too much, we get the negative exposed three times as much as it should be. Suppose, again, the object is less brilliant, and the correct exposure should be one-fifth of a second, an equal error of one twenty-fifth will make little difference. But in photographing cirrus and such clouds, if we used the same plates and the same lens apertures as we employ for ordinary landscape work, we should want exposures of the order of those given by a focal plane shutter, and a mistake either in judging or in making the exposure, of even the hundredth part of a second, would be fatal to good results, and would probably completely spoil the plate. Evidently one of our first steps must be to lengthen the correct exposure.

There are four ways in which this can be done—by using a slow-acting plate, by lessening the aperture of the lens, by putting some transparent screen in front of the lens to shut off some of the light, and, finally, by pointing the camera, not at the cloud itself, but at its image in a black mirror.

Of these, of course the slow plate and small aperture are the simplest to adopt, and all the cloud studies shown in the illustrations to these pages have been taken on plates prepared for photo-mechanical purposes or for transparencies. There seems to be nothing to choose between these two brands. Orthochromatic, isochromatic, double-coated, and many other special types of plate had previously been tried, both with coloured filters in front of the lens and without them, without showing any marked superiority over an ordinary plate of low rapidity. At last the photo-mechanical plates were tried, and the efforts made to get satisfactory cloud portraits, which had previously been marked only now and then with satisfactory results, became uniformly and continuously successful.

If the slow plates are exposed in the camera without either a screen or the black mirror, the diaphragm should be reduced to a small size and the exposure suitably adjusted. The length of exposure may generally be judged by looking at the image on the focusing screen, and reducing the aperture until the picture shows its detail easily. Then, regarding the picture as that of a sunlit sea or distant landscape, judge the necessary exposure by the brightness of the image.

No definite rule can be given. The light varies enormously from day to day, and hour to hour, and especially with the position occupied by the cloud relative to the sun. Thus, working with a lens of six inches focus and an aperture of a quarter of an inch, the exposure may vary from the quickest snap of a Thornton-Pickard roller blind to as much as a quarter of a second, or even more. Again, using a lens of eighteen inches focus and an exposure of a fiftieth of a second, the necessary aperture might vary from an eighth of an inch up to an inch and a half. But if we suppose that we are dealing with an ordinary bright summer sky between 9 a.m. and 5 p.m., and that the clouds are cirrus or cirro-cumulus, an aperture of about one thirty-second of the focal length will probably give some sort of image with a snap-shot exposure. At first the failures will be many, but a little practice will soon enable very respectable pictures to be taken by varying either the diaphragm or the speed of shutter. Heavier clouds of the alto types will need rather longer exposure or larger aperture.

The lens may be of any kind, as long as it gives a well-defined image, but there are many advantages in using one of the rectilinear type provided with an iris diaphragm. A rapid lens is not needed; indeed, it has been pointed out that slowness is a very great desideratum, and if the camera is provided with a rapid lens it must be ruthlessly stopped down. For general cloud purposes the best kind of lens is a wide-angle rectilinear, but many occasions will present themselves on which a lens of longer focus will be wanted in order to give more insight into the details of some specially delicate clouds. If the lenses are good, and the focusing is accurate, enlargements will go a long way towards revealing the minuter structures, but the results can never be quite so well defined as a direct photograph in a long camera.

A shutter will be essential, and it should be one which opens in the middle, or which travels across the lens. The shutters which are ingeniously contrived to give more exposure to the lower part of the picture than to its upper part are useless for the purpose in view. It should have some latitude of exposure, from about one-sixtieth of a second up to a full second or more.

Then as to the camera. Any light-tight camera will do, and, as the objects will all be at a great distance, it may very well be a fixed-focus one, or may be kept set up and fixed in focus for a distant object. If not, on setting it up it should be focused on the horizon or most distant object possible, and not on the cloud itself. As, however, the clouds present themselves at all heights above the horizon, even in the zenith, it becomes necessary to have some means of pointing the camera in such directions. To a certain extent the ordinary stand does allow of tilting, but a special support which will allow the camera to be fixed firmly in any position is of the greatest convenience.

If the study is meant to be at all prolonged, the best plan is to make a suitable camera, once for all, which can be left in fixed focus, so as to be always ready, and which can be directed with equal ease to any part of the sky, from the horizon to the zenith. If it is intended to use a black mirror, then a special mount becomes almost essential.

Many of the most delicate of the photographs reproduced here have been taken with a camera of peculiar pattern, the structure of which is shown in Plate 61. The lens is an ordinary rapid rectilinear, and the stop used was generally one-sixteenth of the focal length. The shutter is a light slip of aluminium, which can be drawn across from side to side at any desired pace. The body of the camera is mahogany, with a bellows part for getting correct focus, but when once this was obtained the back was clamped to the tail-board and a little varnish brushed over the clamping screws.

Supported in front of the lens by light brass-work is the black mirror, made of a very dark glass optically worked on the front face. It is a curious fact that, although bits of plate-glass blackened on the back seem to the naked eye to give a single image of sufficient truth, if such a mirror is placed in front of the camera the second faint image formed by reflection from the blackened surface is almost always to be detected. Moreover, the lens with its large aperture at once detects irregularities in the surface of the glass, which are quite imperceptible through the narrow limits of the pupil. Black glass, with a truly worked surface, is essential then, but the surface need not be of the high order of excellence required for mirrors used for telescopic work, since the first image is not, as a rule, intended to be highly magnified.

The mirror is held so that its surface makes an angle of about 33 degrees with the axis of the lens, and the block carrying shutter and mirror can be turned round into any position by slipping it round the lens mount as an axis. The mirror thus always retains the correct angle.

The action of the mirror is to a large extent due to mere diminution of brightness, but it also partly extinguishes the blue light of the sky without exerting any such influence on the white light from a cloud. This is due to the fact that the blue light of the sky is partly polarized, while that reflected from the cloud is not. Now, polarized light which falls upon a black mirror held in a particular position is not reflected by it. This position depends upon various circumstances, but one condition is that the reflected ray must make an angle of about 33 degrees with the surface of the glass. The amount of the polarized component of the blue light varies greatly, but is at a maximum at all points 90 degrees away from the sun. This, then, is the best possible position for photographing a cloud, as the whole of this polarized component may be suppressed by adjusting the mirror to the proper position, and then the most delicate cirrus fibres stand out brilliant on an almost black background.

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—

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.

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.

There is nothing novel in any of these methods, and there is no doubt that other formulÆ would be as good; but the one thing essential is to have a developer whose action can be held under control, and to apply that developer in such a way that very considerable over-exposure will not result in the ruin of the plate. If a number of photographs have been taken in about the same part of the sky, and within a short time of each other, then the correct proportions of developer and alkali will be nearly the same for all, but the first of such a batch will always have to be attacked in the cautious step-by-step method. Patience and perseverance, backed by a steady refusal to be discouraged by the failures which are at first inevitable, are as certain to be crowned by success as they are in other studies.

The workers are few, and there is much to be done; for it is mainly to those who will photograph the higher clouds, and so trace the stages of their growth and decay, that we must look for the data which will enable us to solve the problems they present, and so enlarge the narrow boundaries of our knowledge of some of the most beautiful things in Nature.