Waves of light and radiant heat originate in ether disturbances produced by molecular vibration, and have impressed upon them all of the important qualities of that vibration. Molecules of different substances differ in their modes of vibration, each producing a wave peculiar to and characteristic of itself. A useful analogy may be found in the fact that when one listens to the music of an orchestra without seeing it it is easy to recognize the tones that come from each of the several instruments, the characteristic vibrations of each being impressed upon the waves in air which carry the sound to the ear. So delicate and so sure is this impression of vibration peculiarities that it is even possible to know the maker of a violin, for instance, by a characteristic _timbre_ which must have its physical expression in the sound wave. The ear, more perfect than the eye, analyzes the resultant disturbance into its component parts so that each element may be attributed to its proper source. Unaided, the eye cannot do this with light, but the spectroscope separates the various modes of vibration which make up the confused whole, so that varieties of molecular activity are recognizable. The speed at which a source of sound is approaching or receding from the ear can be ascertained by noting the rise or fall in pitch due to the crowding together or stretching out of the sound waves, and in the same way the motion of a luminous body is known from the increase or decrease of the refrangibility of the elements of its spectrum. Indeed, had nineteenth-century science accomplished nothing else than the discovery of spectrum analysis, it would have marked the beginning of a new epoch. By this device man is put in communication with every considerable body in the universe, including even the invisible. The “goings on” of Sirius and Algol, of Orion and the Pleiads are reported to him across enormous stretches of millions of millions of miles of space, empty save of the ethereal medium itself, by this most wonderful “wireless telegraphy.” And it is by the vibratory motion of the invisibly small that all of this is revealed; the infinitely little has enabled us to conquer the inconceivably big.