Leaving for the present a further discussion of the question What are the X rays? let us examine what the actual condition of the art of using the rays is. Many attempts have been made to improve the Crooke’s tube, in which the rays are produced, but, like the hand telephone, its form has remained substantially unaltered since the first flush of discovery. Its present form consists of a bulb of thin glass, exhausted of air, containing a little concave mirror of aluminum, and opposite to this, separated by a gap of several inches, is an inclined sheet of thin platinum, called the focus plane, or anticathode. The electrical discharge passes between this plane and the mirror, and the X rays are thrown off from the inclined sheet of platinum. They are not reflected in the ordinary sense of the term, but the electric rays converge from the mirror to a spot on the platinum which glows with a red heat, and the X rays emanate from the heated spot as if it were their source. Thousands of investigators have endeavored to improve the form of tube, but, with several important minor appendages, it still maintains the principal features of an aluminum concave mirror and an inclined plane of platinum. Aluminum is found to be the best metal for the mirror from which the rays are generated, largely because its metallic particles are not torn off by the discharge, as would be the case if it were made of platinum. It is also light, and can be easily fixed to a platinum wire. Among the important modifications of the tube are those which enable the operator to control the degree of vacuum in the tube. This is accomplished by sealing to the main tube an appendage containing certain chemicals which, on being heated, give off a small amount of vapor, and which take it up again on cooling. This modification is made necessary by the singular fact that after a Crooke’s tube is submitted to an electrical discharge for some time the vacuum becomes more and more complete, and a higher and higher electro-motive force or pressure is needed to produce the discharge in the tube. It prefers in time to jump over the surface. Thus, at the very beginning of our use of the X rays we meet with a mystery. Where do the remaining particles of air go? It is surmised that they disappear in the platinum terminals. [Illustration: FIG. 1.--The evolution of the Crooke’s tube.]