In wave propagation, each particle of the medium suffers a small displacement from its equilibrium position and performs a periodic motion about that position. This displacement may be in the line of propagation—longitudinal vibration—or it may be in a plane at right angles to that line—transverse vibration. All the phenomena mentioned above, diffraction, interference, refraction, and also reflection, belong equally to either mode of wave propagation. Other phenomena must be made use of to distinguish between these. When the vibrations are transverse they may all be brought into one plane through the line of propagation. They may be _polarized_, when the ray will present different phenomena upon different sides. When the vibrations are longitudinal, no such phenomena can be produced. Polarization, then, serves to distinguish between longitudinal and transverse vibrations. Now let us consider briefly the Roentgen ray phenomena that bear upon the question of the nature of the propagation. [Illustration: FROM SCIAGRAPH OF NORMAL ELBOW-JOINT; STRAIGHT, IN POSITION OF SUPINATION. By A. W. Goodspeed. _Phot. Times_, July, ’96. Copyright, 1896, by William Beverley Harrison, Publisher of “X-ray” Pictures, New York. ] It seems to be settled beyond question that the origin of the Roentgen rays is the fluorescent spot in the discharge tube. § § 107, 108, 111. The evidence seems overwhelming that within the tube, the phenomena are the result of streams of electrified particles of the residual matter, shot off from the cathode in straight lines, perpendicular to its surface. § 57. This was Crookes’ original theory, § 53, _near centre_, and it seems to have stood well the test of scientific criticism. These flying particles falling upon anything in their path, give rise to X-rays. It is preferable, but not essential, that the bombarded surface should be connected electrically with the anode. § § 113, and 116. The best results are obtained by using a concave cathode, and placing at its centre the surface which is to receive the bombardment, thereby concentrating the effect upon a small area. Nearly all experimenters agree in locating the origin of the X-rays at this bombarded spot. The energy here undergoes a transformation, and the X-rays represent one of the forms of energy developed. What are the characteristics of this particular form of radiant energy?