774 POPULAR SCIENCE MONTHLY. 



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-mo- 

 tive 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 acces- 

 sible regions of the body, like the mouth, can possibly be ex- 

 amined 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 ab- 

 dominal regions and a photograph be obtained by placing a sensi- 

 tive 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 



