that the propagation of these waves would be enormously faster than the propagation of ordinary light waves.” Notice that the above was written twelve years prior to Roentgen’s discovery. 94. Prof. Schuster, in _Nature_, Lon., Jan. ’96, stated that the great argument against the supposition of waves of very small length lies in the absence of refraction, but questioned whether this objection is conclusive. He further stated: “The properties of the ether may remain unaltered within the greater part of the sphere of action of a molecule. The number of molecules lying within a wave length of ordinary light is not greater than the number of motes which lie within a sound wave, but, as far as I know, the velocity of sound is not materially affected by the presence of dust in the air. Hence there seems nothing impossible in the supposition that light waves, smaller than those we know of, may traverse solids with the same velocity as a vacuum. We know that absorption bands greatly affect the refractive index in neighboring regions; and as probably the whole question of refraction resolves itself into one of resonance effects, the rate of propagation of waves of very small lengths does not seem to me to be prejudged by our present knowledge. If Roentgen rays contain waves of very small length, the vibrations in the molecule which respond to them, would seem to be of a different order of magnitude from those so far known. Possibly, we have here the vibration of the electron with the molecule, instead of the molecule carrying with it that of the electron.” 95. Prof. J. J. Thomson showed how it was possible that “longitudinal waves can exist in a medium containing moving charged ions, and in any medium, provided the wave length is so small as to be compared with molecular dimensions, and provided the ether in the medium is in motion. It follows from the equation of the electro-magnetic field that the ether is set in motion in a varying electric field. These short waves would not be refracted, but in this respect they do not differ from transverse waves, which on the electro-magnetic theory would not be refracted if the wave length were comparable with molecular distances.” From _Elect. Eng._, N.Y., Mar. 18, ’96, p. 286, in reference to a paper before the _Cam. Phil. So._