Up among the exceedingly rapid vibrations which affect the ether there is a certain small section — a very small section — to which the retina of the human eye is capable of responding, and these particular vibrations produce in us the sensation which we call light. That is to say, we are capable of seeing only those objects from which light of that particular kind can either Issue or be reﬂected. In exactly the same way the tympanum of the human ear is capable of responding to a certain very small range of comparatively slow vibrations — slow enough to affect the air which surrounds us and so the only sounds which we can hear are those made by objects which are able to vibrate at some rate within that particular range. In both cases it is a matter perfectly well known to science that there are large numbers of vibrations both above and below these two sections, and that consequently there is much light that we cannot see, and there are many sounds to which our ears are deaf. In the case of light the action of these higher and lower vibrations is easily perceptible in the effects produced by the actinic rays at one end of the spectrum and the heat rays at the other.