ENLARGING BY ARTIFICIAL LIGHT The principal difference between enlarging by daylight and by artificial light consists in the method of illuminating the negative evenly and with sufficient strength to obtain an enlarge ment in a reasonable time. A necessary element is a condensing lens for collecting the light and presenting it in the form of an evenly and brilliantly illuminated disc immediately behind the negative from which the enlargement is being made. The source of light may be an oil lamp, a gas flame, or an electric lamp. The first-named is the least satisfactory by reason of its variable character and comparatively poor quality. An incandescent gas burner is one of the most satisfactory for general use. At times the light is projected in the form of an enlarged image of the mantle, and a similar difficulty may arise with an incandescent electric lamp. This may be entirely obviated by interposing a sheet of ground glass between the light and the condenser, as near the latter as possible. Other methods of lighting the negative are sometimes adopted, but in practice they are far from satisfactory. The diameter of the condensing lens must be fully equal to the diagonal of the plate that is being used—for example, 5i in, or 51- in. for a quarter-plate ; but there is no advantage in any size in excess of this. The usual arrangement is to enclose the light in an iron casing, one end of which holds the condenser. By this method the light is excluded from the room, so that the paper can be handled freely, and the enlargements developed without any risk of fogging, excepting when an exposure is being made. As close to the condenser as
possible there is a carrier for holding the nega tive ; this carrier forms the back of a camera body, the opposite end holding the lens for pro jecting the image. Beyond the lens an easel or board is required for holding the sensitive paper, and, as the distance between this board and the lens, and between the lens and the nega tive, must be varied according to the degree of enlargement, a method of extending the camera body, and one for sliding the easel, have to be provided. The easel should slide on guides, as it is imperative that it should be quite parallel with the negative. The arrangement will be more easily understood from the illustration, in, which G is an incandescent gaslight, c the con denser, N the negative, r, the lens, and p the paper on which the image is received.
The exposure will depend on the negative, the paper, and the degree of enlargement. In enlarging from quarter to whole-plate by incan descent gas, with lens aperture f/8, the exposure will vary from ten seconds for a thin negative up to forty for a strong one, using a commercial bromide paper. A test exposure on a small piece of paper should always be made before making the enlarged print. The test piece should include the strongest part of the nega tive.
Any negative that will give a good contact print on bromide paper will yield a good enlarge ment by incandescent gaslight with a con densing lens for concentrating the light.