We have next to consider the question of the wave-detecting devices which enable us to appreciate and record the impact of a wave or wave train against the aerial. At the very outset it will be necessary to coin a new word to apply generally to these appliances. Most readers are probably familiar with the term "coherer," which was applied by Sir Oliver Lodge, in the first instance, to an electric wave-detecting device of one particular kind--viz., that in which a metal point was lightly pressed against another metal surface and caused to stick to it when an electric wave fell upon it. As our knowledge increased, it was found that there were many cases in which the effect of the electric radiation was to cause a severance and not a coherence, and hence such clumsy phrases as "anticoherer" and "self-decohering coherer" have come into use. Moreover, we have now many kinds of electric wave detectors based on quite different physical principles. At the risk of incurring reprobation for adding to scientific nomenclature, the author ventures to think that the time has arrived when a simple and inclusive term will be found useful to describe all the devices, whatever their nature, which are employed for detecting the presence of an electric wave. For this purpose the term _kumascope_, from the Greek [Greek: kuma] (a wave), is suggested. The scientific study of waves has already been called _kumatology_, and in view of our familiarity with such terms as _microscope_, _electroscope_ and _hygroscope_, there does not seem to be any objection to enlarging our vocabulary by calling a wave-detecting appliance a _kumascope_. We are then able to look at the subject broadly and to classify kumascopes of different kinds. We may, in the first place, arrange them according to the principle on which they act. Thus, we may have electric, magnetic, thermal, chemical and physiological operations involved; and finally, we may divide them into those which are self-restoring, in the sense that after the passage or action of a wave upon them they return to their original sensitive condition; and those which are non-restoring, in that they must be subjected to some treatment to bring them back again to a condition in which they are fit to respond again to the action of a wave.