The third scientist says: "The knowledge we gain by experiment brings home to us what a miserably imperfect piece of mechanism our bodies are. The ear can detect the slow-footed sound vibrations that come to us at the rate of between 40 and 40,000 a second. But the whole of space may be quivering and palpitating with waves at all sorts of varying speeds, and our senses will tell us nothing of them until we get them coming to us at the inconceivable speed of 400,000,000,000,000 a second, when again we respond to them and appreciate them in the form of light." The fourth scientist says: "The first indications of warmth come to us when the vibrations reach the rate of 35,000,000,000,000 per second. When the vibrations reach 450,000,000,000,000 the lowest visible light rays manifest. Then come the orange rays, the golden yellow, the pure yellow, the greenish yellow, the pure green, the greenish blue, the ocean blue, the cyanic blue, the indigo, and finally the violet, the highest degree of light which the human eye can register, and which occurs when the vibrations reach the rate of 750,000,000,000 per second. Then come the ultra-violet rays, invisible to human sight but registered by chemical media. In this ultra-violet region lie the X-Rays, and the other recently discovered high degree rays; also the actinic rays which, while invisible to the eye, register on the photographic plate, sunburn one's face, blister one's nose, and even cause violent explosions in chemical substances exposed to them, as well as act upon the green leaves of plants, causing the chemical transformation of carbonic acid and water into sugar and starches. These forms of 'dark light,' that is, light too high in degree to be perceived by the human eye, are but faint indications of the existence of still higher and still finer vibrations of substance and energy." The Human Wireless Telegraph Instrument.