Professor Trowbridge's experiments with the cathode rays at the Jefferson Physical Laboratory have shown that the quality of glass constituting the Crookes tubes has very little to do with the phenomena outside the tubes. A bulb that gives a blue fluorescence will produce as good cathode photographs as one that produces a yellow or green fluorescence. It is practically useless to endeavor to obtain photographs with tubes which are not exhausted to a high degree, perhaps one millionth of an atmosphere. When the anode throws a strong shadow of the cathode on the fluorescent walls of the tube one is sure of obtaining photographs and not before. It is a waste of time and dry plates to attempt to take them be means of Edison lamps.

It is also a waste of time to endeavor to make Crookes tubes largely of aluminum. Unless the aluminum window is very small and extremely thin the occluded air in the aluminum cannot be driven off in the process of exhaustion. When the window is made of very thin aluminum it cannot be larger than a ten cent piece; and there is no cement which is serviceable for any length of time. Glass is the only practical substance for Crookes tubes. The great difficulty at present in the application of the cathode photography to surgery lies in the expense of the method. In order to take a photograph of the hand to detect a bullet for instance, or a piece of glass, at least two photographs should be taken-in the obverse and reverse portions of the hand with respect to the sensitive plate. In doing this one is liable to break two tubes, which cost at present ten or twelve dollars apiece.

The future of the method seems to lie in the application of currents of high frequency and very great electromotive force, such as one obtains by the use of the Thomson or Tesla coil and by the use of only one terminal. In this way the breakage of the tubes can be prevented.