Presumably the electrical connections will be superintended by a practical electrician. There ought to be a considerable distance between the coil and the tube, and the spiral wires connecting the two must be securely fixed in such a way also that they will not be liable to touch the glass. Adjustments of apparatus, we need hardly remark, must not be made while the current is on.
Switch on the current for a moment, and observe the effect. If the light in the tube is weak, and assumes a violet glow, the sparking gap probably needs increasing; mottlings and patches, with fluorescence, opposite the anti-kathode show that the current must be reversed. A constant, bright, greenish-yellow light, filling at least half the tube, is an indication that the right condition of affairs is established; an orange glow, with sparks discharging outside the tube, shows that the spark gap is too wide.
Now take the fluorescent screen, and hold it, coated side towards the spectator, a few inches in front of the tube, and place the hand just behind it. The ligaments and bones should be plainly discernible, standing out as blacker in colour than the shadow of the surrounding flesh.
The plate, in its two light-tight bags, is now brought in and laid in position, film side uppermost (the film side must be noted when the plate is inserted in the bags). For experimental purposes the best distance is about 8 in., if the subject to be radiographed is not of too bulky character. Of all subjects the human hand is the favourite with beginners; it is in itself always an interesting one, and a friend will generally volunteer to provide it.
1 Plates are now treated with nonactinic dyes, so that they may be handled in daylight and are sensitive only to the X-rays.
Place the hand on the black band covering the sensitive plate, keeping it perfectly still, and switch on the current. At the expiration of, say 100 sec, cover the hand with the lead sheet, switch off, and remove the plate for development.
This is not easy to decide offhand. The nearer the tube the shorter the exposure; on the other hand, a certain distance from the tube is necessary to avoid great distortion of the resulting image. Much depends also on the nature and bulk of the subject. If a hand requires 90 sec., a foot will require 4 1/2 minutes, and the forearm about 5 minutes. The trunk and abdomen may take anything between 30 minutes and 1 hour. Trial ex-posures may be given by exposing the subject in sections, shifting the lead sheet 1 in. for each 15 sec. backwards or forwards, in the same manner as with trial exposures in ordinary photography.
Fig. 75. The Hand In Position.
No great difference will be found between the development of ordinary negatives and radiographs. As in photo-micrography, the adoption of a slow, detail-forming agent, such as glycin or rodinal, is indicated; prolonged development with either of these will often reward the operator's patience by producing detail in the shadows and internal structure which would otherwise be absent.
In conclusion, we must once more reiterate the suggestion that the production of X-rays is not an undertaking likely to turn out well unless qualified assistance is obtainable. The absolutely necessary directions can be given in a few pages; whole volumes would not suffice to warn against all the pitfalls which might possibly lead to failure or worse mishap. The tyro will find this branch of work no more difficult than any other, provided he is content to work under the superintendence of some one experienced in the management of electrical apparatus. As to the supposed dangers of the X-rays, they may be dismissed as non-existent when dealing with tubes, like the 3 in., of comparatively small penetrative powers.