ROENTGEN, or RoNTGEN, RAYS, certain invisible non-refractable rays emanating from the surface of an elec trically excited vacuum tube opposite the cathode electrode, having power (1) of permeating objects impervious to light or heat rays, (2) of discharging electri fied bodies or surfaces exposed to them, (3) of exciting fluorescence in fluorescent salts, and (4) of affecting sensitized pho tographic plates in a manner similar to light rays. They were discovered by William Conrad Roentgen, Professor of Physics at the Royal University of Wiirz burg, in Germany, toward the close of the year 1895. Not being certain as to the nature of the rays, Professor Roent gen provisionally termed them the X-rays, and they are still commonly known by that name, though the name Roentgen rays is also common. At the beginning of 1894, Prof. P. E. A. Lenard, at Bonn, announced the discovery that by using a Crookes tube in which the cathode rays were made to impinge on a thin sheet of aluminum a screen covered with a phosphorescent substance outside the tube could be made to phosphoresce by their action. That, further, it was possible by means of these cathode rays, as he supposed, to obtain "shadows" of objects through optically opaque substances and to produce an impress of these "shadows" on photographic plates, which could after ward be developed and fixed by ordinary photographic processes. Working on this line of investigation Professor Roentgen inclosed an excited vacuum tube in black ened cardboard treated with barium platino-cyanide, and discovered that the cathode beam is accompanied by certain rays not before known, which, though of phosphorescent and photographic quality, differ from any known form of light in not being susceptible of refraction.
These were the wonderful X-rays, which have opened up to the world a new region of scientific exploration. Besides obtaining radiographs of the bones in the living human hand, Professor Roent gen radiographed a compass card com pletely inclosed in a metallic box. From
these and similar experiments he inferred that these newly discovered rays gen erated in the neighborhood of the Crookes tube by the electric disturbance set up by the passage of a current possessed the property of passing through all bodies in their path, and that some bodies, being less permeable than others, cast a shadow. Subsequent experiments have established the fact that the transparency of a body to the X-rays is proportional to its den sity. As to the real nature of the X-rays eminent physicists differ, but all agree that they must be regarded as of a na ture essentially different from ordinary light. They cast an invisible life-size shadow of the objects that obstruct their passage, which invisible shadow if re ceived on a surface which phosphoresces or glows under their action becomes a visible shadow, which makes the wonder ful revelations of the fluoroscope possible. If this invisible shadow is received on a sensitive plate, the plate is impressed, and on subsequent development the represen tation of the obstructing object is per petuated on the photographic plate.
The Roentgen rays pass very freely through the various tissues and fluids of the body, but are obstructed by the bones; hence it is possible to take a per fect shadow-picture, or radiograph, as it is now generally called, of the bones of a living person or animal. By far the most important result of the discovery has been the application of the new rays to surgery. Needles, bullets and other foreign objects in various parts of the body have been successfully located, and the invention of the FLUOROSCOPE (q. v.) has made it possible to use the Roentgen rays, not only in surgical cases, in search ing for fractures, etc., but to undertake anatomical studies and make the diag nosis of internal diseases. The full physiological effects of the X-rays are not yet clearly understood. Experiments show that long exposure to the rays causes acute maladies of the skin and also 'baldness.