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. IX. WHAT IS DONE IN A LARGE OBSERVATORY; ITS WORK. A visitor who wants to know what is done in a great observatory might go to Harvard some evening. He would probably find the large refractor pointed toward the satellites of Jupiter, Uranus, or Neptune, with a view of noting their precise places, so as to compute tables of their exact motions; or he might find a laborious observer watching such double stars as have considerable proper motion, and making drawings of conspicuous nebulæ, so that future astronomers may be able to decide whether time has wrought any changes in their constitution or figure. The great glass at Princeton, under the charge of Professor Charles A. Young, is largely used for spectroscopic work, examining the sun’s photosphere by day, and noting the spectra of the stars at night. Spectral observation is an important part of the routine at the Yerkes Observatory in Wisconsin. Many faint comets have been successfully photographed at the Lick Observatory, on Mount Hamilton, California, and elsewhere by the use of very sensitive plates and a long exposure.