X-RAY, The, a ray proceeding from a glass bulb from which air has been highly hich an electric charge is being discovered list Rontgen in Wurtzburg, 1895. Hundreds of others had ex perimented with Crookes tubes, glass bulbs ex hausted to one millionth of an atmosphere and enclosing the terminals of high tension electric wires. These had all produced X-rays now called Rontgen rays, But kontgen was the first to detect their presence. They are invisi ble and are supposed to be vibrations of a similar character to those of light but much more rapid than even the invisible ultra violet rays in sunlight. The extremely short wave length accounts for the fact that every sub stance regardless of its color is more or less transparent to the X-rays; and that the X-rays are not ordinarily reflected or refracted. In fact it was about 23 years after their discovery that it was found out that the cleavage surfaces of crystals, smoother than any artifically pol ished surface, would produce these two effects upon the X-ray which are so easily produced upon ordinary visible light. Excellent pictures are often taken through aluminum one-quarter inch thick, while denser substances like lead and gold arrest most of the X-rays even in much thinner layers. The modern X-ray tube contains a cathode or negative terminal from which the cathode stream of negative particles or electrons traveling at an average velocity of 20,000 miles a second starts under the influence of from 50,000 to 100,000 volts; also a target or anticathode upon which these particles im pinge, setting up the extremely rapid vibrations called the X-ray or Rontgen ray. There is a positive terminal, the anode, which may or may not be used as the target. The gas filled X-ray tube contains air or some gas exhausted upon an air pump to a vacuum of about one-millionth of an atmosphere. It is provided with a regu lator by which the degree of vacuum may be raised or lowered.
The electron discharge X-ray tube, such as the Coolidge tube, is exhausted to as complete a degree of vacuum as possible and even the highest voltage fails to send any electricity through it unless a low tension current is first sent through a tungsen filament forming part of the cathode. During the time that this filament is
incandescent, it liberates electrons which are ready to form the cathode stream when the high tension current is turned on. Arrange ments are necessary for regulating the strength of the low tension current so as to vary the incandescence of the tungsten filament.
The rays are made perceptible by looking through a fluoroscope or dark box enclosing a screen of some fluorescent substance like barium platiuo c)ailide which becomes brilliantly illumi nated. If the whole room is darkened the fluoroscope may consist simply of the flat screen. Anything held between the X-rav tube and the screen casts a shadow upon this illumi nated surface and parts of the object which have different densities become distinguishable in the image upon the screen by reason of dif ferences in the darkness of the shadow. Look ing at a person's chest, the ribs look moder ately dark and the lungs very light, while the heart is seen as a pale shadow expanding and contracting. • X-ray pictures are very often made with out ever looking at the patient with the fluoro scope and not really seeing the part under ex amination, such as the bones of the leg. until the plate or film is developed The latter is in a cassette or plate-holder made of aluminum which is opaque to ordinary light, but very transparent to the X-ray. Thin wood or thin hard rubber serve equally well. Or the plate may le simply wrapped in black and orange paper to protect it from ordinary light. No camera is required and the plate is not usually uncovered during the X-ray exposure. It is possible, however, to make an X-ray picture upon a completely uncovered plate providing the room is absolutely dark and the X-ray tube is surrounded by a covering opaque to ordinary light hut transparent to the X-ray; it being desirable to protect the uncovered plate from srdivary light rays produced by the tube in addition to the X-rays.