One of those extraordinary discoveries which startle the whole world came nearly at the end of the nineteenth century, in the winter of 1895–96. At that time a modest professor in the University of Würzburg announced that he could readily see the skeleton inside the body through the flesh! Naturally, the first announcement was received with almost absolute incredulity; but very soon his discovery was confirmed from all sides, and it has now taken its place among the recognized phenomena of science. By means of certain rays, which, being of unknown nature, were called “X”-rays, after the well-known mathematical X, or unknown quantity, Professor Röntgen has shown us that not only can the bones be seen, but that almost every substance in the body can be seen and reproduced in pictures. The reason for this is because they are all obstacles to the passage of these X-rays and so produce shadows on a sensitized photographic plate. If the exposure is sufficiently prolonged the rays penetrate even through the bones and act upon the photographic plate, so that no shadow remains. If the rays are allowed to penetrate for a shorter time the bones show dense shadows, and one can get a light shadow of the soft parts. If the exposure is still shorter, then we can recognize the dense shadow of the bone, the much less dense shadows of the muscles, and the still lighter shadows of the layer of fat immediately under the skin. The heart can be seen beating, and its shadow is now a well-recognized feature in skiagraphs of the chest. At first it was thought impossible to discover anything inside the bony skull, but there are now on record nearly a score of instances in which bullets have been detected within the skull, and after trephining have been found and removed exactly at the location indicated. It is a very common thing now to locate a piece of steel or other similar foreign bodies within the eyeball by the method of Dr. Sweet, or some similar method, within one or two millimetres (a millimetre is one-twenty-fifth of an inch). It is now well recognized that even stones in the kidney will throw shadows sufficiently strong for them to be recognized, and by noting their level in relation to the vertebræ we can tell precisely in what part of the kidney to make the incision in order to find and remove them. It has happened to myself and many other surgeons in the past to cut down upon a kidney, believing that there was a stone in the kidney, only to find that we had been misled by the apparently clear symptoms of such a foreign body. In future no such mistake should be made by any surgeon within reach of a skilful skiagrapher. Unfortunately, gall stones and numerous other foreign bodies, vegetable substances such as beans, corn, wood, etc., being as transparent to the X-rays as are the soft parts, are not revealed by means of this new method of investigation; but cavities in the lung, abscesses in bone, and similar diseases which produce