"If we come down to the other end of this great gamut, to very slow vibrations, we shall find a certain number so slow as to affect the heavy matter of the atmosphere to strike upon the tympanum of our ear and reach us as sound there may be, and there must be, an infinity of sounds which are too high or too low for the human ear to respond to them; and to all such sounds, of which there must be millions and millions, the human ear is absolutely deaf. If there be vibrations so slow that they appear to vs as sound, and other exceedingly rapid ones which appear as light, what are all the others? Assuredly there are vibrations of all intermediate rates We have them as electrical phenomena of various kinds; we have them as the Rontgen rays. In fact, the whole secret of the Rontgen rays, or X-rays, is simply the bringing within the capacity of our eyes and within the field of our vision a few more rays, a few of the finer rates of vibration, which normally would be out of our reach"