The Milky Way shows to the naked eye as a vast, zoneshaped nebula; but is resolved, with very slight optical assistance, into innumerable small stars. Its stellar constitution, already conjectured by Democritus, was, in fact, one of Galileo's earliest telescopic discoveries. The general course of the formation, however, can only be traced through the perception of the cloudy effect impaired by the application even of an opera-glass. Rendered the more arduous by this very circumstance, its detailed study demands exceptional eyesight, improved by assiduous practice in catching fine gradations of light. Our situation, too, in the galactic plane is the most disadvantageous possible for purposes of survey. Groups behind groups, systems upon systems, streams, sheets, lines, knots of stars, indefinitely far apart in space, may all be projected without distinction upon the same sky-ground. Unawares, our visual ray sounds endless depths, and brings back only simultaneous information about the successive objects met with. We are thus presented with a flat picture totally devoid of perspective-indications. Only by a long series of inductions (if at all) can we hope to arrange the features of the landscape according to their proper relations.

To the uncritical imagination, the Milky Way represents a sort of glorified track through the skies—