ness of the bones is more or less shown. This specific absorption is of great scientific interest as well as of practical importance.
Now, these X rays will penetrate several inches of wood, with varying amount of absorption, but they are almost entirely cut off by glass as thick as a window pane. They pass through thin layers of aluminum, even layers as thick as a silver ten-cent piece, while the silver coin almost entirely intercepts them.
It therefore immediately occurs to one. Why not return to Lenard's tube, provide a Crookes tube with an aluminum window, and thus save the great absorption of the glass walls of the tube? There are certain practical difficulties in the way. The aluminum must be very thin. Lenard used a window which was about one eight-thousandth of an inch thick, and it was necessarily very small, in order to stand the atmospheric pressure. An aluminum window one eighth of an inch thick, or as thick as a ten-cent piece, would absorb nearly as much as the glass walls of the present forms of Crookes tubes, which are not more than one sixtieth of an inch thick. Glass vessels seem at present to be more practical than any composite form, in which aluminum is glued to a glass-supporting vessel: first, because it can be blown very thin, and in a shape strong enough to withstand the atmospheric pressure; secondly, because the occluded air can be more effectively driven off the inner walls of the vessels by heating it while it is being exhausted than it can be expelled from a vessel of any other material.
To obtain successful photographs, the exhaustion of the air must be pushed to a high degree; and this is also interesting from the scientific point of view. Moreover, a high electro-motive force is necessary. Pictures can be taken in less than one minute of the skeleton of the human hand by means of high vacua tubes excited by high electro-motive force. Even in this bare recital of the present limits of the application of the X rays to photography, we perceive great possibilities in the application of the method to the surgery of the human extremities. There is no doubt that small foreign bodies, like shot and pieces of glass, can be detected in the fleshy tissues of the hand. Certain accessible regions of the body, like the mouth, can possibly be examined by placing a sensitive film inside the mouth and the cathode outside of the cheek; and it does not seem improbable that a suitable cathode vessel can be inserted into certain abdominal regions and a photograph be obtained by placing a sensitive plate on the outside of the body. By employing two cathodes, at the proper distance apart, stereoscopic representations of the bones can be obtained, and an estimate formed of the position of foreign bodies.
Let us now turn to some of the interesting scientific questions
