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Lens Story: 24 of 28

THE STORY OF THE LENS The Spectroscope
ONE of the most remarkable adaptations of magnifying glass to the unraveling of the secrets of the universe is the spectroscope. The world's original and largest spectroscope is to be found in nature itself, for every rainbow that paints the sky is an immense spectrum produced by sunlight shining through falling drops
of water. Those bright, flashing pieces of glass called "prisms" are also spectroscopic.
    A spectroscope consist essentially of four parts: (1) A very narrow slit, through which passes the beam of light, (2) a small telescope called a collimator, at the focus of which the slit is placed, (3) a prism, or a very closely ruled glass plate to disperse the light into its component colors, and (4) an observation telescope to produce a magnified image of the spectrum.
    Light is an electromagnetic wave motion in the ether of space. It differs from the vibrations that produce heat, wireless and chemical effects only in its wave length. Wireless waves are very long, frequently a mile or more, while light waves are measured in millionths of an inch. Like musical tones, one is of a very low pitch, the other very high. Color is simply pitch, and, within the very short range of the ether waves visible to the human eye, there is a whole scale of color, starting with the red and ending with the violet. The red waves are longest, the violet shortest. When light waves fall obliquely upon one of the faces of a glass prism that portion which enters the glass is retarded most. Therefore when white light passes through a prism the red waves are retarded least and the violet length most with the colors of other wave lengths lying between. The result of this unequal retardation, or refraction as it is called, is to separate the light into its component colors.
    In 1815 Fraunhofer, an eminent optician, using a spectroscope of higher magnifying power than his predecessors, mapped certain dark lines crossing the sun's spectrum. For many years the meaning of these lines was a mystery. Then, in 1858, Kirchhoff and Bunsen, bringing this instrument to a much higher degree of perfection, discovered the following principles of spectrum analysis: (1) Incandescent solids and liquids and also gases under high pressure give a continuous spectrum, or solid band of color. (2) Gases under low pressure give a series of bright lines whose number and position depend upon the elements present. (3) When white light passes through a gas of lower temperature than its source, this gas will absorb from the white light those colors which it would produce, if viewed by itself in the incandescent state.
    At once the meaning of the Fraunhofer
lines became apparent. These dark bands were due to elements in the state of incandescent vapor in the sun's atmosphere and they neutralized in the sun's spectrum the colors which they themselves would have emitted. Now every element gives its own characteristic bright line spectrum, the lines being of absolutely definite color and position. Therefore by devising a spectroscope with a comparison prism so that the solar spectrum or that of a star might be viewed side by side with the spectra of terrestrial elements, it at once became possible to determine the chemical composition of any heavenly body whose light would reach our telescopes. And more, this instrument reveals the physical state of stars, nebulæ and comets, for a continuous spectrum means an incandescent solid or a gas under great pressure, while a discontinuous, or bright line, spectrum proves the presence of a light, vaporous firemist. Most of the nebulæ, those worlds in the process of formation, have been shown to be of the latter composition. By replacing the eyepiece of the spectroscope with a photographic plate these spectra may be photographed and studied at leisure. Every great telescope carries a spectroscopic attachment and the light gathered by the great lens or mirror is dispersed by the prism.
    The spectroscope discloses the motion of a distant star. When a star is approaching our solar system its spectrum is shifted from the normal position it would occupy if the star were stationary toward the violet. When the star is receding from us the spectrum is shifted toward the red. By the amount of the shifting the velocity may be determined with an accuracy of within two to three miles per second. This shifting of the spectral lines has led to the discovery of twin stars, revolving about a common center of gravity and periodically eclipsing each other. Each star gives its own spectrum and as the lines alternately shift first toward the red and then the violet there can be but one conclusion, that is, companion stars in mutual revolution.
    In the hands of the chemist the spectroscope has proved the most delicate means of detecting minute quantities of chemical elements. So small a quantity as one two-hundred-thousandth of a grain of sodium may be detected by this means.
    A marvelous instrument is the spectroscope, and its possibilities are not yet exhausted.