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As the prepared side of the tissue is placed in contact with the negative, it is manifest, as we have just seen, that if it retained the slightest adhesiveness of surface, it would be dangerous to bring them together. Care must always be taken, therefore, not to use damp tissue.

For the exposure it is not necessary to use pressure frames with hinged backs, as the print is not, of course, examined in progress, the sole guide as to time being afforded by the photometer. The pressure of the back should be comparatively light, and the backing should be smooth and level. Fine cloth forms an excellent backing. Where the padding of the back is coarse, a piece of smooth cardboard may be placed at the back of the tissue. Too heavy pressure causes a kind of mottle of dark patches at points which have been pressed into absolute contact with the negative. If the tissue be quite dry, there can be no objection to sun printing; but if the slightest moisture were left in the gelatinous film, prolonged exposure to a hot sun with a dense negative would soften the film, and cause it to adhere. As, however, this tissue is much more sensitive than albumenized paper, printing in diffused light will generally