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The spectroscope consists essentially of three tubes joined in the form of the letter Y, one of which is a small telescope, in the focus of which a narrow slit is placed to admit the ray of light that is to be examined; a prism, or a ruled grating that disperses the light, so as to form a spectrum; and a view telescope, with which to observe the various parts of the spectrum.

By using a small telescope to view the spectrum of the sun, Fraunhofer, a German optician, in 1814, discovered that the whole length of the spectrum was crowded with dark lines, very narrow, indeed, but scattered all through the seven hues. He found that sunlight, whether taken directly or reflected from clouds or from the moon or planets, invariably gave the same spectrum; but in no case did light from the stars give a spectrum of the same sort as that from the sun.

YERKES TELESCOPE, UNIVERSITY OF CHICAGO.

Largest in the World.

Dr. Kirchhoff, of Heidelberg, in 1859, explained the origin of the dark lines, and showed that there are three kinds of spectra: first, that of an incandescent solid or liquid, which is always perfectly continuous, showing neither dark lines nor bright; second, the spectrum of a glowing gas, which consists of bright lines or bands separated by dark spaces. These lines are characteristic of the chemical elements that cause them; and so, from the composition of the bright lines in a spectrum, it is possible to tell their origin. Third, a spectrum crossed by dark lines; which occurs when an incandescent solid is viewed through absorbent vapors.

In the solar eclipse of 1868, M. Janssen first noticed that the solar prominences gave a spectrum of the second kind, and thus proved that the prominences consist of glowing gas. Since that time the march of discovery has been exceedingly rapid.

This simple instrument has thus led the way to a knowledge of the elements composing every heavenly body, no matter what its distance, provided only it is giving out light intense enough to reach our gaze. For the perfection both of the telescope and spectroscope we owe much to the optical skill and mechanical dexterity of the Clarks and Rowland, Hastings and Brashear, all Americans.

About forty chemical elements have now been recognized in the sun. The most prominent are iron, calcium, hydrogen, nickel, and sodium. A distortion, or displacement, of some of the lines in the spectrum enables us to calculate the speed at which the gases are rushing toward or from us. A given line in the spectrum of Aldebaran is displaced toward the violet in such a way as to show that the star is approaching the sun at the rate of thirty miles a second; while a similar line, in the case of Altair, so deviates toward the red end of the spectrum as to prove that it is receding from the solar system at a velocity of twenty-four miles a second. By this principle, recognized by Doppler in 1842, the motions of about one hundred stars toward or from the solar system have been ascertained.

There is no question but that the solar system, as a whole, is steadily moving away from Sirius, and toward the constellation of Hercules; whether faster than at a rate of twelve miles every second is still scarcely decided; but this rate would be about a million miles a day, or three hundred and seventy million miles a year.