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 downwards 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 40 secs. to print, the exposure necessary is found by simple rule-of-three:

30 : 40 : : 60 : 80 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 122 and 202 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 exposure 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 f/32.

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 condenser 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 5 1/2 in. condenser, while half plate will need about 8 1/4 in. Oblong-shaped condensers should be about 1 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 incandescent 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.

Ordinary focussing cameras may be converted into fairly efficient enlarging lanterns with the expenditure of a few shillings. There is the ellipsoid reflecting lantern, or a lantern with condenser attached, either of which can be readily clamped on to the back of a camera of corresponding size of plate, and both systems are constantly undergoing improvements.

Enlarging Easel

A good easel consists of a board provided with a detachable hinged frame holding a sheet of plate-glass, behind which the bromide paper is kept perfectly flat. For convenience it should swing, in cases where, for the correction of lines, a vertical position is unsuitable. In practice we use a large printing-frame containing the bromide paper, and clamp it to the board with two adjustable slips, which slide up and down as required. This easel should be fixed at one end of the table devoted to enlarging purposes.

Focussing

Preliminary focussing is performed with the ruled screen, as in daylight enlarging, and the stop adjusted to produce even definition. A yellow glass cap is then placed on the lens, the negative is inserted, and the final focussing carried out with the bromide paper actually in position on the easel. For exposure simply remove the yellow cap. The rules governing exposure are in general very similar to those laid down for the daylight camera. Do not let the source of light approach too near the condenser until it has been thoroughly warmed, and see that a sharp image of the mantle does not appear focussed as part of the picture on the easel. Shift the burner backwards and forwards until it is quite out of focus. Another caution refers to the yellow glass cap. Sometimes it alters the focus of the lens, and this must be allowed for in the final adjustments. The ideal place for the yellow mask would be behind the negative.

Vignetting

Suspend a piece of cardboard in which a suitably shaped hole has been cut about midway between lantern and easel; the nearer the lantern the larger will be the vignette, and the softening of the edges is secured by keeping it constantly in motion. Masks for borders must be pinned immediately in front of the sensitive surface.