In practice, daylight enlarging is fairly simple. The negative is fixed in position at A and focussed to the required size on the ground glass at B. The ground glass will afterwards give place to a dark slide containing the bromide paper. We need hardly remind the operator that the larger the size of picture required, the longer the distance must be between B and c, and the shorter the distance between C and A ; in fact, from 13 to C will equal A to C multiplied by the number of times the enlargement is to exceed the original. A negative is not a handy thing to focus, as it provides no clear standard of comparison ; and in getting accurate focus it is usual to substitute a glass plate ruled in parallel lines and squares. Focus with a full aperture and then stop down, remembering that enlarging work is a very severe test of the virtues of a lens, and that the smaller the stops the better for the outlines, but, on the other hand, the longer the exposure.
When using films or plates of smaller size than the full capacity of the negative holder, take care that the margins are well masked, either by the suitable plate-carriers or black paper. Marginal light is as fatal almost to good enlarging as pinholes in the bellows.
Exposure.—Choose a window having a north aspect or, at any rate, such that the direct rays of the sun are not within the field of view ; tilt the camera as shown in the figure in order to secure even illumination, which is not interfered with by the shadow of the earth, intrusive trees, buildings, etc. If the situation is very confined, take it on the roof or out-of doors and tilt nearly upright.
Now we come to the crucial point in daylight enlarging— time of exposure. Fixed tables are useless as a guide. We must create a standard for our own lens with the actinometer. A test strip of bromide paper is given a series of exposures in the dark slide, by moving the shutter upwards or down wards at intervals of 20 seconds. On development the best result is obtained at 60 secs., when we will suppose the actinic value of the light is such that the full tint of the actinometer is reached at 30 secs. This may be taken as the normal exposure, from which all others can be calculated. If on the next occasion we require a similar enlargement, the full tint takes 4o secs. to print, the exposure necessary is found by simple rule-of-three : 30 : 40 : : 6o : 8o secs.
According to the numbers of times the original is to be enlarged, exposure is increased in proportion to the square of the distance that the camera is racked out. If for our normal exposure the distance between lens and bromide paper was 12 in., and in order to get greater enlargement this distance is
increased to 20 in., the exposure must be also increased in the ratio of and or as 144 is to 400. In altering the stops we must remind the reader that, for enlarging purposes, the equivalent focus varies with the degree of enlargement, and that the diaphragms are of proportionately diminished value. We can no longer rely safely on the rule of doubling ex posure with each diminution of stops as marked on the lens, although, in view of the latitude which bromide paper permits of in development, the error would not be a very serious one in a single change, e.g. from (nominal)f/22 to f132.
Enlarging by Artificial Light.—Daylight enlarging has its advantages, amongst others that of giving greater softness and better gradation. But the majority of workers prefer artificial light, with its constant exposures independent of weather, and its many opportunities for variation of effect.
For negatives not exceeding 3 in. in length the ordinary magic lantern will serve as a basis for the enlarging lantern. If it emits stray beams of light, as some patterns do, a box must be made to enclose the body. The projection lens is probably not of chemically correct focus, and a rectilinear lens, or that in the camera, must be used instead. Most of the hundred different varieties of enlarging lanterns are on the principle of the magic lantern, with improved methods for holding the negative. The most recent are provided with a swing, both for front and back, and sufficient extension to allow of a great variety of lenses, as well as for the use of the same instrument in lantern-slide making. The con denser should in all cases exceed in diameter the diagonal length of the plate to be enlarged. Thus, even illumination will be obtained over a quarter plate by a 52 in. condenser, while half plate will need about 8,1 in. Oblong-shaped con densers should be about i in. longer than the plate with which they are to be employed. Of illuminants, the most desirable are acetylene and incandescent gas, the latter being the most convenient, and giving exposures short enough for all ordinary purposes, particularly the latest inverted mantles. Oil lamps should be relegated to the backwoods, though no doubt, were there a sufficient demand, an incan descent oil lamp could be manufactured for lantern use nearly as serviceable as the gas lamp has proved itself. For commercial work, the arc light, Nernst, or mercury vapour lamp are adopted.