116 Professor B. Grant [May 5, 



propounded by Copernicus the earth was regarded as the body of 

 paramount importance in the universe. 



It was the invention of the telescope and its application to the 

 purposes of astronomical observation which first revealed to the 

 human mind the marvellous extent of the physical universe, and 

 suggested the idea that the earth might be a mere atom in comparison 

 with the vastness of the material system beyond. When it was dis- 

 covered that the planets are round dark bodies like the earth, shining 

 only by the reflected light of the sun, and that they presented ap- 

 parent diameters of sensible magnitude when viewed through the 

 telescope, no doubt was henceforward entertained that the planets are 

 bodies comparable with the earth in magnitude, and that the earth is 

 merely one of a family of similar bodies, which revolve in orbits of 

 difierent magnitudes around the sun. It is worthy of remark that 

 Galileo, to whom is due the telescopic discoveries which first dis- 

 closed the vast extent of the material universe, has nowhere expressed 

 any opinion respecting the nature of the stars. His mind was pro- 

 bably too much occupied with the more immediate consequences of 

 his discoveries to indulge in speculations leading to more remote 

 conclusions ; and a similar remark is generally applicable to his 

 successors in the field of telcsco2)ic exploration, who flourished during 

 the seventeenth century. It was reserved for Huyghens to propound 

 the doctrine that the stars are suns. This he did in a work on 

 Cosmical Astronomy, which was published in 1G99, shortly after his 

 death. Henceforward the stars have been regarded by astronomers 

 as self-luminous bodies, comparable in magnitude and splendour 

 with the sun. 



While more correct ideas were being formed respecting the nature 

 of the stars, the method for ascertaining the exact j^osition of an 

 object in the celestial sphere underwent at the same time a complete 

 revolution. The telescope in its original form was not suited for 

 aiding the observer in fixing the precise position of a star in the 

 heavens, but the subsequent form of the telescope, consisting of a 

 combination of two convex lenses, suggested the admirable invention 

 of telescopic sights, which may be said to constitute the foundation 

 of all exact astronomy. The places of the stars were now determined 

 with a vastly greater degree of precision, and the way was thus pre- 

 pared for the consideration of the important question whether the 

 epithet Jixed is strictly applicable to those bodies, or whether they 

 might be rather endued with a movement, so extremely slow as to 

 have hitherto eluded detection. 



To Halley is due the discovery of the important fact that some of 

 the stars have a proper motion. In 1717 he communicated a paper 

 to the Royal Society, in which he showed that a comparison of the 

 places of Sirius, Arcturus, and Aldebaran, as determined by Hip- 

 parchus about the year 130 B.C., with corresponding observations of 

 the same stars made by himself, clearly indicated that during the 

 intermediate interval the stars had sensibly moved southwards with 



