ASTRONOMICAL DISCOVERIES. 79 



the latter, the interior planetary comets of short periods of 

 revolution, together with many other phenomena which like- 

 wise escape the naked eye. While our own solar system, 

 which so long seemed limited to six planets and one moon, 

 has been enriched in the space of 240 years with the dis- 

 coveries to which we have alluded; our knowledge regarding 

 successive strata of the region of the fixed stars has unexpect- 

 edly been still more increased. Thousands of nebula?, stellar 

 swarms, and double stars, have been observed. The changing 

 position of the double stars which revolve round one common 

 centre of gravity has proved, like the proper motion of all 

 fixed stars, that forces of gravitation are operating in those 

 distant regions of space, as in our own limited mutually- 

 disturbing planetary spheres. Since Morin and Gascoigne 

 (not indeed till twenty-five or thirty years after the invention 

 of the telescope,) combined optical arrangements with mea- 

 suring instruments, we have been enabled to obtain more 

 accurate observations of the change of position of the stars. 

 By this means we are enabled to calculate, with the greatest 

 precision, every change in the position of the planetary bodies, 

 the ellipses of aberration of the fixed stars and their parallaxes, 

 and to measure the relative distances of the double stars even 

 when amounting to only a few tenths of a seconds-arc. The 

 astronomical knowledge of the solar system has gradually ex- 

 tended to that of a system of the universe. 



We know that Galileo made his discoveries of Jupiter's 

 satellites with an instrument that magnified only seven 

 diameters, and that he never could have used one of a higher 

 power than thirty-two. One hundred and seventy years later, 

 we find Sir William Herschel, in his investigations on the 

 magnitude of the apparent diameters of Arcturus (0"-2 within 

 the nebula) and of Vega Lyrae, using a power of 6500. Since 

 the middle of the seventeenth century, constant attempts have 

 been made to increase the focal length of the telescope. 



