ADVANCE OF ASTRONOMY. 189 



the last quarter of the eighteenth century that Wil- 

 liam Herschel (1738-1822) began to realise his 

 ambition of obtaining " a knowledge of the construc- 

 tion of the heavens," and rapidly passed from being 

 " a star-gazing musician " to the post of royal astron- 

 omer. 



He made clear, what had been suspected by some, 

 that there were systems of stars, in some measure 

 comparable to the planetary system, but varying 

 greatly in the periods and forms of their revolutions. 

 A double star had been usually regarded as an opti- 

 cal phenomenon due to the fact that two stars which 

 might be very far apart, happened to be nearly in the 

 same line of sight from the earth; Herschel proved 

 that many double stars were real binary combina- 

 tions, " intimately held together by the bond of mu- 

 tual attraction." In the apparent motions of the 

 stars he distinguished one component due to a trans- 

 lation of our planetary system towards a point in 

 the constellation Hercules, and another component 

 due to a real movement of the stars themselves. In 

 his study of nebula? he was gradually forced to the 

 conclusion that there were nebulosities which could 

 not be resolved in stars, but consisted of a " shining 

 fluid " or " self-luminous matter " diffused in space, 

 and " more fit to produce a star by its condensation, 

 than to depend on the star for its existence." This 

 led him about 1791 to a theory of the nebular origin 

 of stars, apparently in complete independence of thie 

 nebular theory of Laplace (1796). 



Two main contributions, then, must ~be traced io 

 Herschel, that fie extended Neuionian methods to 

 the study of the stars, and that he made the whole 

 scientific picture of the heavens vividly kinetic. On 

 the one hand, he extended the range of precise 



