PROGRESS OP ASTRONOMY. lift 



the laborious observations of the Landgrave "William IV. at 

 Cassel. Tycho Brahe's catalogue, as revised and published 

 by Kepler, contains no more than 1000 stars, of which one 

 fourth at most belong to the sixth magnitude. This cata- 

 logue, and that of Hevelius, wh'ch was less frequently em- 

 ployed, and contained 1564 determinations of position for the 

 year 1660, were the last which were made by the unaided 

 eye, owing their compilation in this manner to the capricious 

 disinclination of the Dantzig astronomer to apply the telescope 

 to purposes of measurement. 



This combination of the telescope with measuring instru 

 ments — the union of telescopic vision and measurements — 

 at length enabled astronomers to determine the position of 

 stars below the sixth magnitude, and more especially between 

 the seventh and the twelfth. The region of the fixed stars 

 might now, for the first time, be said to be brought within 

 the reach of observers. Enumerations of the fainter tele- 

 scopic stars, and determinations of their position, have not 

 only yielded the advantage of making a larger portion of the 

 regions of space known to us by the extension of the sphere 

 of observation, but they have also (what is still more import- 

 ant) indirectly exercised an essential influence on our knowl- 

 edge of the structure and configuration of the universe, on 

 the discovery of new planets, and on the more rapid determ- 

 ination of their orbits. When William Herschel conceived 

 the happy idea of, as it were, casting a sounding line in the 

 depths of space, and of counting during his gaugings the stars 

 which passed through the field of his great telescope,* at 

 different distances from the Milky Way, the law was discov- 

 ered that the number of stars increased in proportion to their 

 vicinity to the Milky Way — a law which gave rise to the 

 idea of the existence of large concentric rings filled with 

 millions of stars which constitute the many-cleft Galaxy. 

 The knowledge of the number and the relative position of 

 the faintest stars facilitates (as was proved by GaflVs rapid 

 and felicitous discovery of Neptune, and by that of several 

 of the smaller planets) the recognition of planetary cosmical 

 bodies which change their positions, moving, as it were, be- 

 tween fixed boundaries. Another circumstance proves even 

 more distinctly the importance of very complete catalogues 

 of the stars. If a new planet be once discovered in the 

 vault of heaven, its notification in an older catalogue of po- 



* Cosmos, vol. i , p. 87-89 



