coated with a very thick film apparently impregnated with some substance that fluoresces under the X rays. The time of exposure of such plates is less than with ordinary ones, though not much less than is required for a quick plate covered by the fluorescent screen, but the latter will not give the detail and differentiation of parts which are unequally penetrable by the rays that can be got from the X-ray plates.
The rays also affect sensitive paper, especially bromide paper, and now so-called X-ray paper is in use requiring even briefer exposures than plates. The picture on such paper is a negative—that is, shadows are light and parts affected by the rays are dark. Fig. 2 is an example of a picture taken on such paper, the objects being such as were greatly in vogue for the early pictures—a purse, a pincushion, etc. In the early efforts such a picture required
Fig. 2.—An Early Picture Repeated. Exposure, two and a half seconds.
fully twenty minutes' exposure to the rays; the example here was produced in two and a half seconds, or about one five-hundredth part of the former time. The writer has obtained a perfectly distinct picture of the same kind by a single fluorescent flash in the tube. That is practically instantaneous.
Fig. 3 shows the principal changes in style of tubes that have been approved. Nos. (1), (2), and (3) are forms that were to be found in most collections of Crookes's tubes in physical laboratories when the X rays were first made known. No. (1) was one of the earliest to give satisfactory results; then (2) was found to be preferable, and this "pear shape" was recommended as the most suitable form. Almost at the same time No. (3) was found to be particularly efficient. In this the cathode rays converge from a concave terminal upon a platinum plate used as an anode, such plate becoming the source of Röntgen rays proper. This
