The advance of photography : its history and modern applications (1911)

DRY PLATES, FILMS AND PAPERS 193 although it should be properly described as sodium thiosulphate. The plate is immersed in a solution of this salt until all traces of the unchanged silver bromide have disappeared, and then it should be thoroughly washed, in running water if possible, so as to remove all traces of the "Hypo." It is a very common practice with those who have a room set apart as a dark room, to keep a dish of hypo made up, and to use this over and over again even after it is somewhat discoloured. A. and L. Lumiere and Seyewitz x have shown that if it is desired to avoid staining negatives on gelatine bromide plates, not more than 100 plates (9x12 cm.) should be fixed in one litre of a 15 per cent, solution of sodium hyposulphite. [The plate can be easily compared with English makes by remembering that 1 inch = 254 cm.] If the bath contains 1'5 per cent, of sodium bisulphite, then not more than 50 such plates should be fixed. As a practical test for the exhaustion of the bath, a drop of it may be placed on a piece of paper, which should then be exposed to light and warm air, noting whether the spot turns brown or not. Printing of Positive. — When a satisfactory negative has been produced the next object of the photographer is to obtain positive prints from the negative, and it is often desirable to have means at hand for doing this under very diverse conditions of lighting, etc. Thanks to the enterprising character of those engaged in the manufacture of photographic appliances, there are at the present time papers on sale which will answer the purpose for all ordinary conditions, and there is no longer any need for the individual experimenter to indulge in the needless expense incurred by sensitizing papers at home. On the other hand, there are many processes which offer themselves to the more enterprising as a means of carrying 1 Photo. Journ., Feb. 1907. N