The nitric acid should be added as stated, and not after the iron salt as some peroxidation of the iron might then occur, which would occasion a browning of the solution, and be injurious.
The mode of developing the picture is quite different from that in the negative process. It must be done almost at a blow, and is completed as soon as there is a visible indication of the details in the shadows when the plate is laid upon a black ground. These details come out very quickly, and the development generally occupies only a few seconds. If it be carried too far the boldness and vigour of the contrasts is destroyed. Over-exposure produces a blue solarized appearance, as in the Daguerreotype process.
The picture is to be washed, and fixed with cyanide of potassium, then washed again, dried and varnished in the same way as a nega tive ; after which, the back of the plate is to be coated with black varnish. Some operators varnish the picture itself with black varnish because then it is non-reversed on looking at it through the glass ; but this is a bad plan, because the varnish injures the tone of the whites, and is very liable to crack and destroy the picture. When applied to the back of the plate, it can easily be rubbed off and re newed if it cracks, and no harm is done to the picture. See " Var nish." Positives should be taken in a non-reversing slide whenever the reversion of the image is objectionable.
Some operators take positives on purple glass, which forms an ex cellent background to the picture, and renders the use of black var nish unnecessary. This is an excellent plan.
Positives may be taken upon a variety of substances, such as black varnished paper and card, black-enamelled iron tablets, black patent leather, black glazed canvas &c., &c., Some of these processes are
very ingenious and useful, and deserve a particular description To take Positives upon Paper, or Card. The paper or card must first be gelatinized by floating the side on which the picture is to be taken on a warm strong solution of gelatine, and drying it. It is then cut a little smaller every way than a glass plate, to which it is to be attached in the following manner :— Lay the back of the paper upon the glass, heat the blade of a penknife in a spirit lamp, rub it upon a lump of wax, and then pass it quickly all round the edge of the paper so as to fasten it to the glass plate by an edging of wax, with out allowing any liquid to get behind it. Next coat the paper with black varnish, and let it dry. Then take the positive in the ordinary way, and when finished cut off the edges of the paper and detach it from the glass. Positives taken on paper or card in this way may be safely transmitted by post. The object of the gelatine is to pre vent the black varnish from soaking into the paper.
To take Positives on Patent Leather, Glazed Canvas, Enamelled Iron Tablets &c., The leather or canvass is to be attached to the glass by an edging of wax in the manner described for paper, and the positive taken on the glazed surface at once, without black-var nishing it. Enamelled iron tablets may be treated in precisely the same way as a plate of purple glass. The back of the tablet should be varnished so as to prevent its injuring the nitrate bath.
Collodion positives may be transferred to a great variety of tablets by methods which are described in the article on TRANSFERRING. See "Transfening."