69. PROPAGATION. TURBIDITY OF AIR. Upon studying the shadows on the phosphorescent screen, it was noticed that the rays were bent around the edges of the object. Again, when the object had a slit, diffusion could be noticed by the shape (as in Crookes Ex., Fig. 15, p. 17,) of the luminous portion of the phosphorescent screen. In Fig. B, at beginning of this chapter, the spatter work represents the shape of the luminous portion, the darker part representing the most luminous surface of the screen, the latter being held at right angles to the thick plate, having the slit and opposite the aluminum window. By varying these experiments, especially by changing the angle of the screen he found that not the all rays were diffused, but as in the passage of light through milk, some were transmitted in rectilinear lines. 70. PHOTOGRAPHIC ACTION.—He performed with sensitive silver compound papers, an experiment somewhat similar to those with phosphorescent bodies and also others. Behind a rather thick opaque plate the chemical film was not acted upon, but the rate of blackening near the aluminum window without obstruction of intermediate bodies was about the same as that with befogged sunlight. The former, moreover, was acted upon at a much greater distance than that at which phosphorescence was exhibited and beyond the atmospheric luminosity. By means of shadow pictures or sciagraphs, he compared the shadows produced by the external cathode rays with those which would have been obtained by light. Referring to Fig. C, beginning of this chapter, the sensitive plate was half covered with a plate of quartz, Q, and half with a plate of aluminum, A´ overlapping the quartz. With light, the shadows would have appeared as in said figure, that is, one-half black as produced by aluminum, a quarter rather light as produced by quartz, and the other quarter bright, or a similar arrangement, according to whether the negative or the positive photograph is considered; but with the cathode rays, the appearance of the developed plate was as in Fig. D., beginning of this chapter. The quartz cast the black shadow, while the aluminum, the lighter one. Furthermore, the luminosity of the air produced a variable light on the other quarters. A similar appearance was produced by casting shadows of such plates upon the phosphorescent screen; but, of course, the picture was not a permanent one. The photographic plate served to accumulate the power, for the cardboard which cast a faint shadow upon the phosphorescent screen, showed a black shadow upon the photographic paper by sufficiently long exposure. At the same time, strips of thin metal were placed side by side between the chemical paper and the cardboard, and they showed different degrees of shading. The cardboard was quite thick, being .3 mm. Prof. Slaby (see _Elect. Rev._, Lon., Feb. 7, ’96), _after_ Röntgen’s discovery, produced sciagraphs of the bones of the hand at the window of the Lenard tube. Lenard doubted