FLASHLIGHT PHOTOGRAPHY The taking of photographs by means of flashes of artificial light, the light being generally produced by burning magnesium or flashlight mixtures. Magnesium was made known in 18o8 by Sir Humphry Davy, but for half a century or more it was regarded as a curiosity. In 1859 Bunsen, of Heidelberg, and H. E. Roscoe, of Manchester, pointed out the value of magnesium as a source of light for photographic exposures. Improvements in the manufacture of the metal took place (a company being formed in Man chester for the purpose in 1863), but its expensive ness (magnesium in the form of ribbon then cost half a crown per foot) kept its use restricted. A. Brothers, of Manchester, has been credited with taking the first successful photographs with magnesium, for early in 1864 he obtained a stereoscopic negative of a Derbyshire mine, and in May of the same year a portrait of Prof. Faraday, at the Royal Institution. In the following year the interior of the great Pyramid was taken by Prof. P. Smith by magnesium light, since when the metal, in the form of both ribbon and powder, has become cheaper and consequently widely used. The light given off is of intense brilliancy and of high actinic power.
Magnesium for photographic illumination pur poses may be obtained in three different forms— pure magnesium powder ; flattened wire, known as ribbon ; and a combination of magnesium and other substances, in powder. The ribbon is the safest, but does not allow of such short exposures as the powders ; however, it is commonly employed in photographing dark corners of rooms, caverns, cellars, etc., where the length of the exposure is not of much consequence, and it is sometimes useful as an accessory to day light, as a means of illuminating objects in deep shadow, since, on account of its burning slowly, it may be moved about while the exposure is being made, and so give diffused lighting. Another advantage of using ribbon is that the actinic value of the light obtained by burning a definite quantity remains constant under the same conditions, so that by measuring the length —or, preferably, weighing the quantity—of the ribbon before burning, the photographer can easily obtain data that will assist him in making the results of later exposures sure.
For flashlight work proper—that is, for what are known as " instantaneous exposures " the powders must be used. It is not necessary to employ a shutter as in making instantaneous exposures by daylight ; in most cases the ordinary lights of an apartment are too feeble in actinic rays to affect the plate, and the lens is left open until after the flashlight. Magnesium flashes are open to objections : the quantity of smoke produced, and the difficulty of diffusing or spreading the light over a sufficiently wide area to obviate harsh shadows and hard contrasts. Again, there is the risk of explosion, but such an accident can scarcely occur with careful, proper firing. Pure magnesium powder gives a bright and highly actinic flash, but it must be blown through a flame. Flashlight powders containing substances in addition to magnesium are really explosive mixtures, and they must be ignited by applying a light, for should they be blown through a flame a dangerous explosion will result. A flashlight compound must not be used in a closed storage reservoir or magazine lamp, because in these the flame would travel to the bulk and explode it. Only the pure magnesium powder should be used in such lamps. For the beginner, the ribbon is the safest and the pure powder the next safe.
As regards the amount of powder to be used, this depends upon nearness of subject, stop, plate, etc., but the table given below (compiled by J. H. Crabtree) may be taken as a guide :— The conditions assume a lens at fir f and a fairly rapid plate, the exposure being so short as to be regarded as instantaneous. It is also important to bear in mind that the quantities specified must be completely burned in the flame with an ample supply of air, and not half consumed or wasted.