X-ray plates are sold usually wrapped up separately in light-tight envelopes of black paper, upon which the film side of the plate is marked. If there is no such wrapping the plate must be placed in a light-tight envelope, with its film facing that side of the envelope which has no folds. The ordinary photographic double envelopes, the inner one of yellow paper and the outer one of black paper, are very convenient for this purpose. The plate in its envelope is then laid flat on the table, film side upwards, and the X-ray tube is clamped in a horizontal position so that the anti-cathode is over and pointing towards the plate. The hand is laid flat on the envelope, and the coil or machine is set working. The exposure required varies so much with the size of the machine or coil, the distance between the tube and the plate, the condition of the tube, and the nature of the object, that it is impossible to give any definite times, and these have to be found by experiment. The hand requires a shorter exposure than any other part of the body. If we call the correct exposure for the hand 1, then the exposures for other parts of the body would be approximately 3 for the foot and the elbow, 6 for the shoulder, 8 for the thorax, 10 for the spine and the hip, and about 12 for the head. The exposures for such objects as coins in a box are much less than for the hand. After exposure, the plate is developed, fixed, and washed just as in ordinary photography. Plate XIV. shows a Röntgen ray photograph of a number of fountain pens, British and foreign. Prolonged exposure to the X-rays gives rise to a painful and serious disease known as X-ray dermatitis. This danger was not realized by the early experimenters, and many of them contracted the disease, with fatal results in one or two cases. Operators now take ample precautions to protect themselves from the rays. The tubes are screened by substances opaque to the rays, so that these emerge only where they are required, and impenetrable gloves or hand-shields, aprons, and face-masks made of rubber impregnated with lead-salts are worn. X-ray work is a most fascinating pursuit, and it can be recommended strongly to amateurs interested in electricity. There is nothing particularly difficult about it, and complete outfits can be obtained at extremely low prices, although it is best to get the most powerful Wimshurst machine or induction coil that can be afforded. As radiography is most likely to be taken up by photographers, it may be well to state here that any photographic plates or papers left in their usual wrappings in the room in which X-rays are being produced are almost certain to be spoiled, and they should be placed in a tightly fitting metal box or be taken into the next room. It is not necessary for the amateur doing only occasional X-ray work with small apparatus to take any of the precautions mentioned in the previous paragraph, for there is not the slightest danger in such work.