Having produced the X-rays, we will suppose that it is desired to examine the bones of the hand. For this purpose a fluorescent screen is required. This consists of a sheet of white cardboard coated usually with crystals of barium platino-cyanide. In order to shut out all light but that produced by the rays, the cardboard is placed at the larger end of a box or bellows shaped like a pyramid. This pyramid is brought close to the X-ray tube, with its smaller end held close to the eyes, and the hand is placed against the outer side of the cardboard sheet. The outline of the hand is then seen as a light shadow, and the very much blacker shadow of the bones is clearly visible. For screen work it is necessary to darken the room almost entirely, on account of the feebleness of the illumination of the screen. If a radiograph of the bones of the hand is to be taken, a very sensitive photographic plate is necessary. An ordinary extra-rapid plate will do fairly well, but for the best work plates made specially for the purpose are used. The emulsion of an ordinary photographic plate is only partially opaque to the X-rays, so that while some of the rays are stopped by it, others pass straight through. The silver bromide in the emulsion is affected only by those rays which are stopped, so that the energy of the rays which pass through the emulsion is wasted. If a plate is coated with a very thick film, a larger proportion of the rays can be stopped, and many X-ray plates differ from photographic plates only in the thickness of the emulsion. A thick film however is undesirable because it makes the after processes of developing, fixing, and washing very prolonged. In the “Wratten” X-ray plate the emulsion is made highly opaque to the rays in a different and ingenious manner. Salts of certain metals have the power of stopping the X-rays, and in this plate a metallic salt of this kind is contained in the emulsion. The film produced in this way stops a far larger proportion of the rays than any ordinary film, and consequently the plate is more sensitive to the rays, so that shorter exposures can be given.