upon it, and remain dark at the points where the rays are intercepted by a substance opaque to such rays, thus outlining a shadow. Not only do these light rays in passing through the body tissues (transparent to them) cast a shadow of the bones or any foreign objects, but by the application of photography to these shadow pictures a species of photograph, called a radiograph, or skiagraph, may be taken, and thus any foreign body, such as a bullet, may be definitely located in the human body and quickly extracted, without the element of doubt which beset the old method of diagnosis, which, at best, was only intelligent guessing. Not only are foreign bodies so located, but the fractures of the bones may also be accurately observed, studied and adjusted. Stone in the bladder may be discovered, and the condition and movements of the heart and lungs ascertained. This new kind of light ray was discovered November 8, 1895, by Prof. W. C. Roentgen, of the Royal University of Wurzburg, and was named by him the “X-Ray,” probably because the letter x in algebraic formula represents the unknown quantity, and the hitherto unknown and elusive quality of this light suggested to Prof. Roentgen this appropriate name. As before stated, a peculiar quality of the X-Rays is that they are not visible to the eye. A beam of X-Rays, thrown into a dark chamber through an aluminum window, would produce no illumination whatever in the room, but such rays would still penetrate the room, and if a fluorescing screen were placed in their path it would instantly light up. It is not surprising, therefore, that these subtle rays should have so long eluded the observation of the scientist. A brief sketch of the conditions leading up to the discovery of the rays is necessary to a proper understanding of the same. [Illustration: FIG. 215.--THE CATHODE RAY.]