We do not feel called upon in this brief chapter to indulge in a philosophical treatise on the nature of the X-rays, nor even to describe very fully the apparatus by which they are produced. The details of these matters concern the electrician rather than the photographer. The latter is, however, often invited to assist in the purely photographic manipulations, and a few directions with regard to this department will therefore be of possible service to him.
Suffice it then to say that these rays are incidental to the passage of electricity at considerable electromotive force through highly rarified gases. Rays analogous in their properties are also emitted by certain radio-active minerals, such as some salts of uranium and thorium. The exposure, however, necessary to affect a photographic plate with these substances would occupy several hours, and the cost be a prohibitive one.
The primary source of electricity for a first-class X-ray installation should be an accumulator, a dynamo, or a bichromate battery which is capable of a continuous, unidirectional supply at high pressure, and the demands made upon it will be very considerable. The induction-coil must give at least a 3 in. spark, and a 5 in. or 6 in. coil will allow of the use of most ordinary tubes with moderately short exposures. A less inexpensive method is to employ a Holtz or Wimshurst machine, which will not require the coil. A 2 in. machine will produce the rays, while a 3 in. or 4 in. Wimshurst machine, with the usual Leyden jars and sparkregulator, needs only to be connected with the Crookes tube on a test-tube stand, by means of two spirals of fine copper wire. Full instructions how to make a Wimshurst machine of this capacity, with the various accessories, were given in the Year Book of Photography for 1902, by Mr. S. R. Bottone.
A great variety of tubes is available, termed soft or hard, according to the amount of resistance offered to the passage of the discharge. Very soft tubes produce rays having little penetrative power, and the radiographs taken with them will have the appearance of under-exposed plates, showing no structural detail in the substance of the subject. Hard tubes, on the contrary, require considerable E.M.F., and the radiographs will lack contrast; they are therefore only suitable when detail in bones, rather than surrounding flesh, is required. An ordinary focus-tube is the best type for the small-spark apparatus, but the choice of a tube is a matter requiring special knowledge and experience. The best way is to consult a responsible dealer, explaining the nature of the source of electrical supply and the peculiar purpose for which the tube will be employed.
Procure a piece of stiff cardboard about 10 in. by 12 in., or stretch a piece of good parchment on a frame, and coat well with celluloid varnish. Powder about 1 oz. of barium platino-cyanide in a mortar, and sprinkle evenly on the sticky surface by means of a sieve. When dry, another coat of varnish may be very carefully added. If the screen is not intended for demonstrations, but only to test the actual presence of the X-rays, crystallised calcium tungstate may be substituted for the expensive cyanide salt.
Many firms, including the llford Company, Edwards, Lumiere, etc., etc., have a special plate suitable for the X-rays, though, in case of need, any good extra rapid plate will do, subject to length of exposure. The plate must be enclosed in a pair of Tylar's light-tight bags, the inner one yellow, the outer one black; and up to the time of exposure should be kept in a metal case. No plates, films, or other sensitive surfaces, even in dark slides, are safe in the vicinity of an X-ray apparatus, unless they are isolated by a metal partition. Operations should never be conducted in an apartment closely adjoining the photographer's storeroom or dark room. The best shutter for controlling exposures is a piece of thin sheet lead.1