The sky of a landscape negative which has been taken on an ordinary plate is usually of such a density that, when printed, it leaves a uniform white expanse of paper. Taking into account the fact that, in the absence of clouds, the luminosity of the sky decreases the higher it is from the horizon, it is possible to improve such a picture considerably by slightly greying the sky with increasing intensity from the horizon to the upper edge. After making the print, it is exposed to light, and, at the same time, a piece of cardboard is moved in front so as to cover the picture of the landscape and expose the upper part of the paper for a short period of time (with development papers, mark out the position of the horizon and expose the paper at some distance from a weak source of light. This second printing can sometimes be done during development, immediately the image begins to appear).
Another method which is entirely disapproved of by some workers, but which has frequently been used by others with great success, consists in the introduction of the clouds of another negative into a photograph. 1 Film negatives of clouds may be obtained commercially, but the choice is extremely limited, while the use of such devices leads to undesirable repetitions. Further, such negatives have been made under entirely unknown conditions. Now, in order to introduce a sky into a landscape, it is not suffi cient that the lighting has the same direction in both parts of the composite picture, 2 but the endeavour should be made to combine only negatives which have been taken with the sun at about the same height. (The curves in Fig. 169, § 326, show the times when the sun is at the same height.) For this reason, landscape workers who make use of inserted skies nearly always form a col lection of cloud negatives for themselves, together with particulars of the conditions under which they were taken. It is advisable to varnish such negatives, so as to avoid any alterations which might occur during the various processes of manipulation to which they are subjected.
Other more punctiliouslandscape workers make two successive negatives of the same landscape, with the times of exposure suitably adjusted to render the sky and landscape correctly. If the landscape has a satisfactory sky, it is simpler and more economical to photograph it on an orthochromatic plate with a suitable yellow filter, even if the exaggerated contrast which sometimes occurs when a sky is photographed in this way has to be reduced when printing.
Before going into the method by which the two images are combined, attention is drawn to the frequent fault of giving too great a density to the image of the sky. Except in stormy weather, the darkest clouds are always very much lighter than the shadows in the landscape. The exposure under the cloud negative should therefore be kept fairly short, so as to produce only a grey image.
Before proceeding to combination printing, a mask must be prepared to limit printing of the clouds to the parts which form the sky of the landscape. The counterpart of this mask should be used to mask the image of the sky on the landscape negative, if the sky tends to print out on the picture. This mask should be made by exposing a sheet of print-out paper under the negative of the landscape until the line of the horizon is readily visible. With a fine pair of scissors (embroidery scissors) this sheet of paper is very carefully cut along the line of the horizon, and the two pieces of paper left in the light until completely blackened.
The mask of the sky is then fixed on the back of the landscape negative, and the mask of the landscape on the back of the cloud negative, the blackened surface of the mask being upper most. This work should preferably be carried out on a retouching desk. The masks are kept in position at several points with gum or rubber solution. Both printings should be carried out in diffused light.
If print-out paper is used, there is little diffi culty in adjusting the paper, which already has the image of the ground upon it, under the cloud negative.
If development paper is used for the process, register marks may be made on the negative, but it is simpler to proceed as follows : First expose the paper under the landscape negative and develop it. As soon as the image appears, stop development with a solution of sodium bisulphite before full density is attained ; place the print on a piece of thin celluloid (unless the sky negative is varnished), and fix the whole in a suitable position behind the cloud negative. Make the second exposure, and replace in the developer, where the sky will develop up while the development of the ground is being com pleted. The relative times of exposure can be determined by making a few preliminary tests on pieces of the same paper.