It causes certain salts to fluoresce, § § 66, 84, and 132, and it affects the photographic plate. § § 70 and 84. In these respects, it is like the short wave length radiations from a luminous source. It is, however, totally unlike these in its power of penetrating numerous substances entirely opaque to light, such as wood, paper, hard rubber, flesh, etc. In passing through hard rubber and some other opaque insulators, X-rays are like the long wave length radiations from heated bodies, but X-rays penetrate many substances that are opaque to these long wave length radiations, and they are especially distinguished from all forms of radiant energy previously recognized, in their relative penetrating power for flesh and bones which makes it possible to obtain the remarkable shadow pictures which have become within three or four months, so familiar to all the world. But these phenomena, although they serve to distinguish the X-rays from all other forms of radiant energy, do not furnish any clew to the nature of the X-rays themselves. In attempting to formulate a theory of X-rays, the idea that first naturally presents itself is that they are due to some form of wave motion. [Illustration: FROM SCIAGRAPH OF KNEE-JOINT, STRAIGHT, SIDE VIEW, SHOWING PATELLA, OR KNEE-CAP. By Prof. Goodspeed. _Phot. Times_, July, ’96. ] The characteristics of wave motion are diffraction and interference phenomena. So far, no positive evidence of diffraction, § 110, nor interference, § 89, have been recognized, although experiments, have been tried that would have shown plainly, diffraction phenomena, had light been used in place of the Roentgen radiations. § 170. We must, therefore, conclude, either that the Roentgen radiations in the experiments were too feeble to produce a record of the diffraction effects, or, that they are not due to wave motion at all, unless of a wave length very small even when compared with waves of light. The absence of refraction is also opposed to any wave theory of the Roentgen radiations, for it is difficult to believe that waves of any kind could travel with the same velocity through all media, which they must do if they suffer no deviation. § 86.