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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.

S.W. Burnham, of Chicago, is famed for his acuteness of vision, tested in having detected and measured over one thousand double stars which to other eyes had appeared only as single stars. The discovery of these objects belongs wholly to the nineteenth century; for in 1803, Sir William Herschel first announced the existence of sidereal systems composed of two stars, one revolving around the other, or both moving about a common centre. Some of these binary systems have periods of as great a length as fifteen hundred years; and some are as brief as four, and even two days. Some of them afford curious instances of contrasted colors, the larger star red or orange, and the smaller star blue or green.