To go back to our disc. The octave above this lowest musical note is obtained by doubling the rate of puffs, namely, by revolving the disc twice in one second, and the next octave by revolving four times in a second, and so on, doubling each time, until, at about the thirteenth octave, the sound has become so high that the majority of listeners cannot hear it, and fancy it must have stopped, whereas a few will still be saying: "How shrill it is!" At last, at about the fourteenth octave, when there are 20,000 beats to the second and each wave is about half an inch long, it passes beyond human audition, and, although we can show that the air is still vibrating, all is silent, the human ear being incapable of hearing so many beats in a second even as a continuous sound, though I have evidence to show that many insects can hear probably considerably beyond this limit. It is, however, possible to make these higher vibrations perceptible to our senses by means of what are called sensitive flames: we can actually, by these, measure the length of these silent waves, and as we know the rate at which they travel, we can at once compute the number which occur in a second of time, and thus ascertain their pitch. By this means we can follow for about three more octaves above the audible limit, namely, up to 160,000 pulsations per second, with a length of wave of one-twelfth of an inch. Two and a half octaves above these numerically, _i.e._ at about the twentieth octave, we reach the frequency of Electro-Magnetic Rills, used by the Marconi System of wireless telegraphy, which pulsate at about 950,000 per second, and have a wave-length of something like 1000 feet. The reason for this great increase in length of wave is caused by these frequencies being propagated in the Ether at the rate of 186,000 miles per second, instead of, as with sound waves, in the air, at only 1130 feet per second. We can trace these particular frequencies, called, after their discoverer, Hertzian waves, for about fifteen octaves, when we arrive at the frequency of 32,000,000,000 in a second, with a wave-length decreased to a quarter of an inch; we can render the effect of these waves visible, but have no physical organ by which we can feel these pulsations. After this, however, we get into the region of frequencies which, though still of exactly the same kind, we know and can feel as Radiant heat; these are situated in the next fourteen octaves, and bring us up to those subtle frequencies which affect another of our sense organs, and which we appreciate as light; these we have already seen have the enormous frequency of 530,000,000,000,000 pulsations per second for red light, up to 930,000,000,000,000 per second for violet, and having wave-lengths so small that it takes 40,000 and 70,000 of them respectively to cover one inch in length. There is only a little over half an octave that the eye can appreciate as light, and then all is darkness; but we can