submitted to the rays. This sheet should be connected to the earth. This fact should be borne in mind when we come to speak of the electrical region outside a Crooke's tube.
Many investigators, reflecting upon the singular fact that the rays pass so freely through thin aluminum and that, on the contrary, glass absorbs such a large percentage, concluded that Crooke's tubes provided with aluminum windows would be an improvement upon the thin incandescent lamplike bulbs now used. The glass of these bulbs is very thin, not more than one thousandth of an inch in thickness, where the rays emerge, not thicker than a sheet of ordinary note paper, and the absorption of such a sheet of glass is so small that it can not be detected by photography. Thus a sliver of glass of this thickness in the hand would not appear on the X-ray photograph of this member, and would not cast a shadow in the fluoroscope. There does not seem, therefore, any advantage in supplying a Crooke's tube with an aluminum window. The mechanical difficulties, too, in accomplishing this are very great. There is no way of joining the thin aluminum disk to the glass so that an air-tight joint can be made. In the process of exhausting the Crooke's tube, the tube must be heated to a comparatively high temperature in order to drive off the air which clings to the inside of the glass. The rise of temperature would soften or melt any current which might be used to make the aluminum adhere to the glass.
We can not expect, therefore, any improvement in the direction of aluminum windows. At one time, I suppose that the rays were highly absorbed in passing through atmospheric air, and that it would be an improvement in the application of the rays to surgery to interpose, so to speak, a vacuum chamber between the body and the source of the X rays. The experiment led to some interesting results, but not in the direction anticipated.
The vacuum chamber consisted of a glass cylinder three feet long and about eight inches in diameter. The two ends were closed by sheets of aluminum, and it could be exhausted through a side tube. The reader will immediately ask, in view of what has been said. How could the glass tube be hermetically closed with sheets of aluminum? This was indeed a difficult matter, but less difficult than in the case of the Crooke's tube, for the ends of the glass cylinder were provided with heavy brass flanges, which were perfectly flat, and the sheets of aluminum lying smoothly could be confined by many bolts between the flange and suitable brass heads. This cylinder, having been exhausted, was placed between the Crooke's tube and the arm, for instance, in the hope that a greater depth of human flesh and tissue might be penetrated