=Difference between Light Waves, Heat Waves, and Sound Waves.= The light and the heat waves of the ether are infinitesimal ripples as compared to the backward and forward pulsations that constitute the sound waves of the air. Within a space of one inch there are sixty-six thousand of the violet waves of light, which are the shortest etheric vibrations to which the human eye responds, and over thirty thousand of the red waves, the longest that affect the eye; while the sound waves of the air vary from about one foot for the shrill notes of the human voice to four feet for the middle C of the pianoforte. A shrill whistle produces waves of about one half inch. There are twenty-two thousand of certain heat waves to the inch, and these, like some of the light waves of the ether, are invisible. There is also a vast difference between the velocity of vibration of the air waves and those of the ether. The human ear is sensitive to sound waves of somewhere between twenty-nine per second to thirty-eight thousand per second; while the eye responds to light waves of from five hundred million to one billion per second. Some ears are better adjusted to the low vibrations and some to the high, and the ears of no one hear any but a small part of the melody of a great symphony. Tyndall could hear the sharp chirp of thousands of insects that were inaudible to his guide as the two climbed the Alps, but the guide’s ears responded to the long, slow waves that came from the dull tread of the donkey’s hoofs farther up the mountain, which waves the scientist was unable to hear. Likewise some eyes are able to penetrate far into the violet, or the red, or both, and some are unable to distinguish between certain colors. =Chemical Rays of Light.= The chemical or photographic rays have still shorter waves than the violet. They produce special physiological effects in vegetable and animal tissues, and, acting upon particular kinds of matter, they cause fluorescence, which is the property possessed by some bodies of giving off, when illuminated, light of a color different from their own and from that of the light that illuminates them. These chemical rays are sometimes called ultra-violet rays.