There is a considerable gap of unexplored wavelengths intermediate between these of Hertzian waves and what is commonly known as heat. The shortest Hertzian waves which have heretofore been produced are of the order of one millimetre in length. In this communication Prof. Reginald A. Fessenden describes a method of obtaining waves of intermediate length. The method was discovered by the author some years ago, but he has been unable to carry on any work with it. If two copper rods are placed against the plane surfaces of two piano convex lenses having the convex surface directed toward each other and separated by a very short air-gap, and are charged from a source of high potential, discharges will take place across the gap between the lenses, and these discharges will have short wave-lengths. If the lenses be replaced by metallic bodies the capacity of the system will be considerably increased and the wavelengths considerably lengthened. But with glass lenses, as described, the wave-lengths of but a few ten-thousandths of an inch are obtained, and there appears to be no necessary limit to the frequency. Inert gases of the helium type seem to give the best results, but good results are obtained by using quartz lenses in air. Quartz is used because it does not seem to become conducting when heated by the passage of the discharge, as does gas. For this reason the wave-length remains more constant.-"Nature."