A Manual Of Photography

266 PRACTICE OF PHOTOGRAPHY. carry the hand forward; immediately blow upon the face of the plate, which has the effect not only of diffusing it over the sur- face, but causes the solution to combine more equally with the damp surface of the plate: it also has the effect of keeping any deposit that may form in motion, which, if allowed to settle, causes the picture to come out mottled. A piece of white paper may now be held under the plate, to observe the development of the picture, if the light of the room is adapted for viewing it in this manner well ; if not, a light must be held below, but in either case arrangements should be made to view the plate easily whilst under this operation, a successful result depending so much upon obtaining sufficient development without carrying it too far. As soon as the necessary development has been obtained, the liquor must be poured off, and the surface washed with a little water, which is easily done by holding the plate over a dish and pouring water on it, taking care, both in this and a subsequent part of the process, to hold the plate horizontally, and not ver- tically, so as to prevent the coating being torn by the force and weight of the water. Fixing of image.—This is simply the removal of iodine from the surface of the plate, and is effected by pouring over it, after the water, a solution of hyposulphite of soda, made of the strength of 4 oz. to a pint of water. At this point daylight may be admited into the room ; and, indeed, we cannot judge well of its removal without it. We then see, by tilting the plate to and fro, the iodide gradually dissolve away, and the different parts left more or less transparent, according to the action of light upon them. It then only remains to thoroughly wash away every trace of hyposulphite, for, should any of the salt be left, it gradually destroys the picture. The plate should, therefore, either be im- mersed with great care in a vessel of clean water, or, what is better, water poured gently and carefully over the surface. After this it must be stood up to dry, or held before a fire. We have now carried the operator carefully through every stage of the process, from the cleaning of plate to the fixing of image ; but our remarks have reference to collodio-iodide alone ; that is, gun-cotton dissolved in ether, charged with an iodide of silver. We cannot, however, consider our task finished without mentioning the addition of gutta percha to the collodion. This valuable discovery was made by Mr. P. W. Fry, to which gentle- man belongs some of the most important steps made in the art. The sensibility of the plates appears to be more materially in- creased by the addition of the gutta percha ; indeed, pictures by superposition may be obtained with absolute instantaneity, and in the camera obscura in less than a second of time.