THE EXPOSURE.
The best results are obtained by keeping to one source of light only. The of light varies inversely as the square of the distance, therefore if the distance from the source of light be doubled, the ex posure duration must be quadrupled. Hence, to obtain evenly printed proofs from the same negative, its distance from the light must be exactly the same at each exposure. The nearer the negative to the light the more important this becomes. Thus one edge of a negative may receive a much greater exposure by tilting towards the light, and this is a method often re sorted to where the density of the negative or certain reasons is unequal. The factors which govern exposure may be summed up as follows :—(a) The intensity and colour of light ; (b) the distance from the source of light to the frame ; (r) the position of the frame ; the colour of the negative ; (e) the contrast of the negative ; (f) the density of the negative : (q) the rapidity of the paper ; (h,) the kind of developer used ; and (i) the depth, colour, and contrast or gradation desired in the print. This It first seems formidable, but some of the factors may be eliminated. Thus, by using always the same light and distance, the first two disappear, whilst until some considerable experience has been obtained, it is well to keep to one make and speed of paper, and one de veloper, thus eliminating (q) and (h). That the negative should be free from yellow stain is especially necessary in this process, for as the light-obstructing power of the negative has usually to be gauged in arti ficial light, different degrees of yellowness, which make so much difference to ex posure, will pass unnoticed. To remove
yellowness, pass the negative before it dries through a 5 per cent. solution of hydrochloric acid. Of the remaining points (e) is connected closely with (i)—that is, if a weak effect is desired from a strong nega tive, it is possible to obtain it by under exposing, and (c) will come into play on rare occasions, so that only the density of the negative and the depth of printing need he considered in first. attempts. The best method of finding the exposure is to try a strip of paper on the negative in three sec tions. The makers of every brand of paper issue a rough guide with their instructions, such as " Exposure for average negative, 10 sec. at 18 in. from ordinary fish-tail burner." This is intended as the roughest guide only, for, as most amateurs' nega tives are either too dense or too thin, it is seldom correct. An excellent device for testing the density of any negative, and ascertaining the relative exposure as com pared with other negatives, is now obtain able. It is known as Dawson's Densito meter, and full directions are supplied with the apparatus.