PORTION OF THE COSMOS. VISUAL POWER. 59 



fixed stars, show that gravitating forces prevail in those dis- 

 tant regions of space, as well as in the narrower sphere of the 

 mutually perturbing orbits of the planets of our system. 

 From the time that Morin and Gascoigne combined optical 

 powers with measuring apparatus (which was not> how- 

 ever, until twenty-five or thirty years after the invention of 

 the telescope), it has been possible to obtain more delicate 

 and precise determinations of the alterations of place of the 

 heavenly bodies. In this manner it has become possible to 

 measure with the greatest precision the present position of a 

 heavenly body, the aberration-ellipses of the fixed stars and 

 their parallaxes, and the distances apart of the double stars, 

 though only amounting to a few tenths of a second of arc. 

 The astronomical knowledge of the solar system has gra- 

 dually expanded into that of the system of the Universe. 



We know that Galileo made his discoveries of Jupiter's 

 satellites with a magnifying power of 7, and that he was never 

 able to employ a higher power than 32. One hundred and 

 seventy years afterwards, we see Sir William Herschel em- 

 ploy, in his investigations on the magnitude of the apparent 

 diameters of Arcturus and a Lyra3, magnifying powers of 

 6500. From the middle of the 17th century men vied 

 with each other in attempting telescopes of great length. 

 Although as late as 1655 Christian Huygens discovered the 

 first of Saturn's satellites, (Titan, the sixth in distance from 

 the centre of the planet), with a telescope of only 1 2 feet, 

 he subsequently employed in astronomical observations tele- 

 scopes of 122 feet; but the three of 123, 170, and 210 

 feet focal distance, in the possession of the Royal Society of 

 London, which had been made by his brother Crnstantine 

 Huygens, were tried by Christian, as he himself expressly 



