PHOTOGRAPHY, OR RADIO GRAPHY (Fr., Radiographic ; Ger., Rontgenphotographie) There being no method of bringing X-rays to a focus, the images produced on the photo graphic emulsion are merely shadows of the objects. The illustration shows the outfit for radiography, A being a metal-lined box con taining the X-ray tube ; B, couch for patient ; c, plumb for centring the tube ; 13, induction coil ; z, switchboard ; and F, ampere meter. X-ray photographs are produced by means of a high-voltage electrical apparatus and a Crookes' tube. The high pressure current necessary to produce fluorescence in the vacuum tube is obtained from a Wimshurst machine or an induction coil. The Wimshurst machine is self-contained, the current being generated by revolving glass or vulcanite plates in opposite directions, but these machines have serious drawbacks, and the induction coil is now almost invariably used. The positive pole of the wires is connected to the anode, and the negative is connected to the cathode end of the tube. If the current flows in a reverse direction, the tube is quickly ruined. The half of the tube opposite the anticathode gives out a bright green fluorescence when the current is flowing from anode to cathode ; but if the current is passing the reverse way, a flickering bluish green fluorescence appears all over the tube.
A coil giving a 4-in, spark is sufficiently powerful for experimental radiography, but an 8-in. or 10-in. spark is the lowest that can be used for practical work.
The X-rays are not visible to the eye, and for the visual examination of objects a fluorescent screen of barium platino-cyanide must be used in a darkened room. Radiography, however, need not be conducted in a dark-room. The dry plates, packed in light-tight envelopes, are placed behind the object to be photographed, the film facing the tube. If only a 4-in. coil is used, the object must be as close as possible to the tube, but with a powerful coil the distance should be increased to about 3o in. to ensure a sharp image with but little distortion. In medical X-ray work, the patient is placed upon a couch consisting of a wooden frame covered with canvas. A box containing the tube moves on wheels and rails beneath the couch ; it is lined with metal to shield the operator from the X-rays. The time of exposure depends upon the strength of current used, the power of the coil, and the condition of the tube. A " hard " tube—that is, a tube with an extremely high vacuum—requires less exposure than a " soft" or low-vacuum tube. The condition of the tube is ascertained by finding its " equivalent spark gap." While the coil and tube are working, the terminal points of the induction coil are slowly brought together. If a spark passes between the points while they are 6 in. or more apart, the vacuum is too high. If no sparking takes place between the terminals till they are within 3 in. of each other, the tube is low. A good working spark gap distance
is 41 in. A soft, or low-vacuum, tube gives better definition than a hard, or high-vacuum, tube, as the rays pass less easily through dense substances and show greater differentiation of tissue. A very high-vacuum tube may show but little difference between the bones and flesh, while a soft tube should give the minute structure of the bones. Tubes are now fitted with regulators for lowering the vacuum.
With a current of 5 amperes at loo volts passing through the primary winding of a io-in. coil, the exposure for a hand or foot would be from 3 to 15 seconds. The exposure for the thicker portions of the body would be from 20 seconds to 2 minutes. If an electrolytic break is used, about half the exposure would be required. Dry plates with extra thick sensitive films are specially prepared for radio graphy, the development and fixation being the same as in ordinary photography. The image is sometimes barely visible on the surface of the plate during development, but when fixed the negative may give good density and definition owing to the penetration into the film of the X-rays.
Several forms of " intensifying screens " have been introduced for the purpose of shortening exposure. The screens are coated with a sub stance giving a white or violet fluorescence. The coated side is placed in contact with the sensitive film of the dry plate which is exposed with the glass side facing the tube. Intensifying screens greatly reduce the exposure, but the quality of the negative is somewhat inferior owing to the grain of the screen being reproduced.
An energetic developer, such as metol-hydro quinone, is most suitable for radiography, and development should not be hurried.
The comparative resistance to X-rays of the following list of substances will give some idea of the penetrating power of the rays. The figures are only approximate, as the results vary with the hardness of the tube, etc. :— The unit is a sheet of cardboard, and the other substances are supposed to be of equal thick nesses. Cardboard, i ; wood, ; linen, 2 ; rubber, ; iron, i,000 ; glass, 40 ; lead, 3,000.
Dry plates must not be stored in an X-ray room except in metal chests, as the paper boxes and envelopes give no protection to the sensitive film.
In X-ray work the operator often has to find the position and depth of some foreign object, such as a needle in the human body. This can be ascertained by visual examination with the fluorescent screen from both vertical and lateral points of view. The best method for finding the depth of a foreign body is to take two radiographs on the same plate, moving the tube to one side before the second exposure is given. Two images will appear on the plate, and the position of the foreign body is cal culated from the separation of the two images and the distance of the tube from the plate.
J. I. P.