A YEAR OF THE X RAYS. 659 



coated with a very tliick film apparently impregnated with some 

 substance that fluoresces under the X rays. The time of expos- 

 ure of such plates is less than with ordinary ones, though not 

 much less than is required for a quick plate covered by the fluo- 

 rescent screen, but the latter will not give the detail and differen- 

 tiation 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 re- 



FiG. 2. An Early Picture Kepeated. Ex^wsure, two and a half seconds. 



quired 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 labora- 

 tories 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 Rontgen rays proper. This 



