A Manual Of Photography

THE COLLODION PROCESS. 267 The plan of proceeding to obtain this extreme sensibility, as recommended by Mr. Fry, is to obtain a thick and strongly charged collodio-iodide, and to two parts of this add one of a saturated ethereal solution of gutta percha, allowing it to stand a day or two to clear itself, previous to being used. The plate is then coated in the usual manner. As the ether evaporates a peculiar white film comes over, at which time it is ready for immersion in the bath. This must be conducted as previously described, and, from its extreme sensibility, with, if possible, greater precaution than before. For the development of negative pictures, Mr. Fry recom- mends the pyrogallic solution, rather stronger than that pre- viously given, about one grain to the ounce, with the addition of an extra portion of acetic acid, and the plate re-dipped in the nitrate bath, in preference to adding silver solution to the pyrogallic acid. In fixing the image after development it is necessary to keep the hvposulphite on longer than with the ordinary collodion, as the iodide is held with greater tenacity. In other respects the method of proceeding is precisely the same. Having, by the foregoing means, obtained and fixed a negative photographic image on glass, and which is capable of producing positives upon paper by the ordinary photographic means, it is as well, previous to obtaining these, to render the tender film of collodion less liable to injury. This can be accomplished by means of a varnish, of which there are different kinds that may be used. By far the best kind of varnish which can be employed is one for which we are indebted to Dr. Diamond, of the Surrey Lunatic Asylum. This varnish is made by powdering some amber raid putting it into chloroform. In a few days a perfect solution takes place. This varnish flows readily over the plate, and dries in a few minutes, leaving a beautifully transparent hard glaze upon the picture. It was shown by Mr. Horne in the early days of collodion that the negative images could be converted into positive ones by mixing with the pyrogallic solution a very small quantity of nitric acid; but it has since been shown by Mr.1ry, and others, that a better result may be obtained by the use of proto-sulphate and proto-nitrate of iron. The former salt is readily obtained, and in a very pure form. It should be used as follows:— Proto-sulphate of iron .... 10 grains. Distilled water .............................1 oz. Nitric acid .................................2 drops.