168
THE PROCESSES OF PURE PHOTOGRAPHY.
Exposure varies directly- as "times" of enlargel!lent. Example: To enlarge a quarter-plate negative to whole plate (4 areas) the required exposure is found to be ten minutes. To enlarge the same negative under the same conditions of light, lens, sensitive surface, etc., to 13x17 inches (16 areas), the exposure required would be forty minutes: 4: 16 : : 10: 40. During exposure no actinic light must enter the apartment, and during focusing it is well to allow no light to enter the room at all, actinic or otherwise, except what passes through the negative and the projection lens. While the sensitive material is being put in position after focusing, the lens is to be capped, and the room may be illuminated by non-actinic light, for which provision was made. While the exposure is going on a great deal can be done in the way of " dodging." Vignettes of any shape can be produced by cutting an aperture of the required shape in a piece of cardboard or wood, and holding the mask so made between the lens and the sensitive surface, moving the mask constantly to and from the sensitive material. As the image is plainly seen projected on the sensitive surface, this operation is one of perfect simplicity. In like manner any part of the image that prints too dark can be shaded during part of the exposure. But in any case the mask should be kept on the move, else hard lines will result on the print. Whatever be the sensitive surface used, bromide paper, wet collodion, etc., development is conducted as already laid down under the severa't headings. When large sheets of paper are used special developing dishes may be required; dishes for larger sheets may be made with plate glass bottoms and varnished wooden sides. Enlarging by ths Optical Lantern with (1) an oil lamp (9) oxyhydrogen lime light. This is perhaps the favorite process with amateurs, and it is certainly a convenient and simple process for enlarging small originals. But the condenser must be of diameter equal to, and ought to be of diameter slightly greater, than the diagonal of the part to be enlarge.I. In the next place the lantern nozzle should be so constructL>d as to allow the objective to be racked to at least twice its own