HARD WICKE'S SCIENCE-GOSSIP. 



73 



RECENT ASTRONOMICAL DISCOVERIES. 



By F. W. LEVANDER, F.R.A.S. 



HE date of the inven- 

 tion of the telescope 

 is involved in some 

 obscurity, but we 

 shall, probably, be 

 not far wrong if ,we 

 say that the instru- 

 ment was first used 

 about three hundred 

 years ago, rather 

 less than more. In 

 its original form it 

 consisted of two 

 pieces of glass, with 

 curved surfaces, one 

 placed at either end 

 of a tube. The re- 

 fracting telescope 

 of the present day 

 consists, essentially, 

 also of two similar glasses, similarly placed, but 

 whereas a spectacle lens probably constituted the 

 first object-glass, the instrument has increased so 

 enormously in size that the object-glass of the large 

 equatorial in the Lick Observatory at Mount Hamil- 

 ton, in California, has a diameter of thirty-six inches. 

 Even this will shortly have to yield the palm to 

 another of forty inches, to be erected in the Yerkes 

 Observatory, near Chicago. 



In 1610, Galileo, the philosopher of Padua, was 

 viewing the planet Jupiter through one of these 

 newly-invented instruments, when he was struck by 

 the appearance of three small but very bright stars 

 in its immediate vicinity. He at first thought they 

 were merely some of the fixed stars, though his 

 attention was drawn to a peculiar resemblance they 

 seemed to bear to one another, and to the fact that 

 they were all in nearly a straight line. He was still 

 No. 340.— April 1893. 



more astonished to find, in the course of the next few 

 nights, that their positions, relatively to Jupiter and 

 to one another, were not always the same. More 

 than that, he sometimes saw a fourth. Occasionally 

 only one of these bright little bodies was visible, at 

 other times his telescope showed him two, or three, 

 or four, but never more than four. Galileo had 

 discovered the satellites of Jupiter. This discovery 

 led to another, that light occupies an appreciable time 

 in passing through space. For upwards of two 

 hundred and eighty years these four small bodies, 

 the diameters of which are respectively, beginning 

 with satellite I., that is, the one nearest Jupiter, 

 2390, 2120, 3480, and 2970 miles, were considered 

 the only companions the giant planet had in his 

 journey round the sun. The possessor of even the 

 smallest telescope is able to see these moons ; indeed 

 under certain peculiar circumstances they have been 

 seen by the unaided eye. 



The light-grasping power of a telescope is quite 

 distinct from magnification, the latter being de- 

 pendent on the relative focal lengths of the object- 

 glass and the eye-piece, whereas the former is 

 determined by the size of the object-glass. The 

 light-grasping powers of two telecopes bear the 

 same relation to each other as the squares of the 

 diameters of their object-glasses. The light received 

 by, say, a four-inch telescope is four times as much 

 as that received by a two-inch. We can thus form 

 some idea of the enormous amount of light gathered 

 at the focus of the great Lick telescope. 



It was on the night of September 9, 1892, that a 

 keen-eyed observer at Mount Hamilton was scrutiniz- 

 ing the immediate neighbourhood of Jupiter with 

 this glass, when he detected a minute speck of light 

 near one of the satellites. As the planets pursue 

 their respective paths in the heavens, they sometimes 

 appear to pass close to various stars. These can be 



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