A Manual Of Photography

198 PRACTICE OE PHOTOGRAPHY. roots or buds, it will be best to divide them with a sharp knife, for the purpose of equalizing the thickness in all parts, and en- suring close contact. Engravings are to be placed with their faces to the prepared side of the photographic paper, and laid very smoothly on the glass, and then with the cushion and back pressed into the closest contact possible : the least difference in the contact, by permitting dispersion, occasions a cloudiness and want of sharp- ness in the photograph. Of course, a copy of anything taken by means of the rays which have passed through it, must present all the defects as well as all the beauties of the article, whatever it may be. Thus, in copying a print, we have, besides the lines of the engraving, all the imperfections of the paper : this renders it necessary that those engravings should be selected which are on tolerably perfect paper. If the preservation of the engraving is not a matter of much moment, by washing it over the back with a varnish of Canada balsam and spirits of turpentine it is rendered highly transparent, and the resulting impression is much improved. Care must, however, be taken to use the var- nish very thin, that it may not impart any yellow tinge to the paper. An exposure of a few minutes only is sufficient to produce strong and faithful copies during sunshine ; but in diffused daylight a longer period is necessary. Some kind of copying frame is an indispensable requisite to the photographer : it is used for copying al1 small objects by transmission, and multiplying the original pictures obtained by means of the camera obscura from nature : it is, indeed, the printing-press of the artist. Some prefer two plates of stout plate-glass pressed very closely together with clamps and screws; but, as the intention is to bring the object to be copied and the sensitive paper into the closest possible contact, numerous mechanical contrivances will suggest themselves for this pur- pose. A great number of experiments should be made with the copying frame before there is any attempt at using the camera obscura. The Camera Obscura, or Darkened Chamber, was the invention of Baptista Porta, of Padua. Its principle will be best understood by the very simple experiment of darkening a room by closing the window-shutters and admitting a pencil of light througli a small hole in them. If a piece of paper is held at a little distance from this hole, the figures of external objects will be seen delineated upon it ; and, by putting a small lens over the hole, they are rendered much more evident, from the condensation of the rays by the spherical glass. If, instead of a darkened room, we substitute a darkened box