The avoidance of reflections generally neces sitates the illumination of a square much larger than that required by the largest dimension of the original, unless, of course, the latter is of very narrow shape. Rather than draw up a number of complicated rules, we would recom mend the following method. Place a mirror or a piece of glass flat on the original, and then, with the eye close to the lens, make sure that no reflection from the lamps can be seen.
If the orig nal is to be illuminated by tubes, the length of the tube should exceed the greatest dimension of the original, the difference in length being greater the farther the tube is removed. If, for example, lamps 4 ft. in length are placed at a distance of i8 in. from the original, the illumination, in the direction parallel to the lamps, will only be uniform over a distance of about 2 ft. To obtain uniform illumination at right angles to the lamps, the latter should be separated by a distance of about r7 times that from the original.
296. Illumination by Magnesium. The com bustion of metallic magnesium, in the form of thin ribbon, has been used since 1850 (Bunsen and Roscoe) to obtain a very actinic intense light. About 1865 Traill Taylor experimented with an explosive mixture allowing of the almost instantaneous combustion of an appreciable weight of magnesium shreds, but it was not until twenty years later that magnesium in the form of fine powder was first brought on to the market. At first the powder was ignited. by blowing it through a very hot flame (Arm strong, 1888), but later pyrotechnic mixtures, similar to the present-day flash powders, became obtainable with either rapid or slow combustion, the idea being derived, no doubt, from the earlier work of Train Taylor. In certain of the mixtures the magnesium has now been replaced either partly or entirely by other metals, or by combustible metallic compounds.
The table given below, taken from sensito metric experiments ( J. M. Eder, 1890; E. Huse, 1923), shows in visual candle-seconds (white light) the actinic values of I grin. of magnesium, burnt in three different forms, using three types of sensitive emulsion." The pyrotechnic mixture given is similar to the type now generally in use (magnesium metal and potassium perchlorate). The use of calcium,
strontium, and barium salts as combustion agents allows the activity towards orthochro matic and panchromatic plates to be increased, by correcting for the excess of violet and ultra violet, which is due, in an ordinary flash-powder, to the very high temperature of combustion.
The combustion of magnesium (or of the metals which can be substituted for it) is neces sarily accompanied by the formation of a corresponding amount of oxide (magnesia,' in the case of magnesium), together with, in the case of flash powders, the solid residue of the substance used as a combustion 2 agent. A part of this solid residue is deposited immediately, while the remainder, which is in a very finely divided condition, as smoke, is carried upwards by the current of warm gas, and only deposits slowly.
297. Burning of Magnesium in Ribbon or in Powder. Magnesium ribbon, usually about kin. wide, TO in. thick, and weighing about To gr. per can only be used for inanimate objects, firstly, owing to the time required for the combustion of a sufficient weight, and secondly because the very strong glare causes sitters to close their eyes. It can be burnt either in a lamp with a clockwork movement, which automatically releases the ribbon at the same speed as the combustion, in a small lamp where the ribbon is pushed out by hand, or as a kind of torch made by twisting round a piece of fine iron wire two or three strands of ribbon, the whole being held firmly in a horizontal' position at the end of an iron clamp.
Owing to hardness of the shadows produced by a point-source, it is necessary to keep the lamp or torch moving during the exposure, or to burn several torches in different positions. The working conditions should be arranged, however, so that the lighting predominates in one direction, taking care that the shadows from the auxiliary positions of the light-source cannot be seen. The operator should protect his eyes by means of a piece of deep red glass or film, or emery cloth in order to remove the magnesium oxide which would prevent the ignition. Mag nesium ribbon should always be kept in a dry place.