ASTRONOMY OF EIGHTEENTH /CENTURY. 263 



7 l Ii t* 



Est via sublimis, coelo manifesto, sereno, 

 Lactea nomen habet ; candore notables ipsp. 

 Hac iter est superis ad magni tecta TonantA ' / _, 

 Regalemque domum. / I \ r . 



Our English ancestors gave it such fanciful names ^s tfctcoVs Ladder, 

 The Way to St. James's, etc. Aristotle supposed it to be caused by 

 exhalations set on fire, and Theophrastus imagined it to be the course 

 of the conjunction of the two hemispheres. Democritus and Pytha- 

 goras anticipated Herschel's discovery when they declared the Milky 

 Way to be nothing but a vast assemblage of stars. Such was indeed 

 the spectacle that the telescope at Slough revealed. It would be im- 

 possible to give any adequate idea of the vast numbers of stars in the 

 Milky Way. Herschel calculated that in forty minutes of time 2 5 8,000 

 stars passed across the field of his telescope. Herschel's theory of 

 the Milky Way was that the stars are so placed in space that they form 

 a stratum, so to speak ; that is, that vast as may be numbers of stars 

 gauged in one direction (which may be called the thickness of the 

 layer), these are inconsiderable compared with the length and breadth. 

 He considered that our system may be placed towards the centre of 

 the stratum, and not far from a place where the stratum separates into 

 two layers, producing that cleft in the Galaxy which extends from 

 Cygnus far into the Southern Hemisphere. 



When Herschel was in possession of his first telescope, he began to 

 systematically make observations of all the stars visible in his horizon. 

 This led to a field of discovery almost new namely, a multitude of 

 stars, which up to that time had been considered single, were found 

 to be really two, three, or four very closely grouped, and usually of dif- 

 ferent colours. In 1782 he gave a catalogue of 269 such stars, in- 

 cluding all particulars as to the size, colour, and position of the com- 

 ponents. These compound stars he arranged in classes according to 

 the angular distance between the components, ranging from i" or 2" 

 to 120". Before Herschel commenced his observations, only four such 

 stars were known, but he observed and described nearly five hundred, 

 and subsequent observers have so increased the number that the cata- 

 logues of multiple stars now include nearly six thousand of these ob- 

 jects. Herschel at first supposed that multiple stars were the results 

 of two or more stars lying in the line of vision, and it was chiefly with a 

 hope of determining their parallax that he undertook to form his cata- 

 logue. But, instead of finding any annual fluctuation, he observed a 

 regular progressive change, in some cases of the angular distance, in 

 others of the position ; so that a real motion of the stars was indicated, 

 or a movement of translation through space of the solar system as a 

 whole. Finally, Herschel announced the existence of several systems 

 of double stars, each component of which revolves in an elliptical orbit 

 about a common point. To such stars he proposed to give the name 

 of binary, to distinguish them from stars which appeared double only 



