ARTIFICIAL LIGHT, PHOTOGRAPHY BY Photography by artificial light presents no serious difficulty if proper precautions are taken, and d correct exposure given. It is imperative that the source of light should be in such a position that no direct rays reach the lens. When it is unavoidable that the source of light is in front of the camera, the lens may have fitted to it a sky-shade or a large temporary hood, so as to protect it from the light. 'Whenever pos sible, the source of light should be behind the camera, not directly at the back, but at one side or the other. Where the arrangement of the light is under the photographer's control, and the light is of such a nature that it can be divided, the larger part should be placed at one side and behind the camera, and the smaller part at the other side. Even where the light is fixed, but divided, as with two or more electric arc lamps, a similar result may be obtained by placing the camera so that one light is almost directly behind it, slightly to one side, and the other one considerably towards the other side. When practicable, the light should be diffused by a screen of tissue paper. This softens the light, and destroys the harshness of the cast shadows, which would otherwise be very strongly marked.
When photographing an interior, or a small object in a building, by electric arc lamps, the strength or actinic value of the light may be tested by means of an actinometer while the camera and subject are being arranged. For a room about 4o ft. by 25 ft. or 3o ft., with the camera and a single arc light of about 2,000 candle power at one end, the exposure should be ten minutes, using a lens aperture of f/16 and a plate of a speed of 200 or 250 H. & D. For a small object photographed at a distance of 6 ft. or 7 ft., illuminated by a single arc light of 2,000 candle power at a distance of io ft. or 12 ft., the exposure should be five or six minutes for a light-coloured object, up to twelve or fifteen minutes for one of dark colour.
Where arc lamps are not available, magnesium ribbon or powder, or a mixture of magnesium and aluminium, may be employed. The powder is frequently burned in the form of a flash, and though this may be desirable when groups of persons have to be included, the methods in which the powder is burned more slowly are preferable. Magnesium alone produces consider able, smoke in burning, and though this smoke is only in evidence after the exposure is com pleted, it is a serious objection in many cases, and quite prohibits making a second exposure. (See " Flashlight Powders.") A convenient form of artificial light easily obtainable is that sold under the name of " Flash Candle." These candles consist of a
celluloid tube filled with a perfectly safe mixture which burns for a few seconds and produces a light of great intensity. They are obtainable in various sizes, called two-second, four-second, and seven-second candles respectively. Two, three, or four of these may be placed in suitable posi tions for lighting an interior, and they may be lighted in rapid succession by applying a lighted taper to the touch-paper with which each candle is provided. Focusing and arranging must be done before lighting the candles. A room or space about 25 ft. by 16 ft. would be sufficiently illuminated if the camera and lights were at one end and two four-second candles were burned, using a lens aperture of 1116 and a plate having a speed of zoo or 25o H. & D. As in the expo sures given for electric light, it is assumed that there is a wall near the lights, or an equally well-lighted space beyond. If these candles or an electric light were used to illuminate a small space which formed part of 6 large room or open space, in such a manner that there was a large open dark space beyond the light, at least half the illumination would be lost by diffusion.
Still life and flower photography may be carried out very successfully by artificial light, using either magnesium ribbon or the flash candles. Oue four-second candle at a distance of 3 ft. should be sufficient, using the lens at f/i6 and a 200 H. & D. plate ; a screen of ground glass or tissue paper for diffusing the light is imperative, and a white reflector behind it.
Copying may be done in a similar manner, or by means of the ordinary gas or electric house lighting. Two lights should be used if possible, one placed at each side of the camera, so as to light the work to be copied as evenly as possible ; or half the exposure may be made with a single light at one side, and then the light placed at the other side for the remainder of the time. When copying a print with a glossy surface, care must be taken to avoid the sheen that may be produced by an improperly placed light. For copying a photographic print, the same size as the original, using a 5o candle-power gas or electric light placed about 2 ft. from the print, the exposure required for f/16 and a plate 200 H. & D. would be about five minutes. For copying a black print on plain white paper—a line engraving, for example—two minutes would be sufficient.
For rapid work by artificial light, see under the headings " Flashlight Photography " and " Flashlight Powders." For the application of artificial light in the production of prints, see under the headings " Bromide Paper " and " Gaslight Papers and Lantern Slides."