ASTRONOMY. SIR WILLIAM HERSCHEL. 



[Diss. VI. 



the atmosphere, especially in these climates, impose 

 a speedy limit to the increase of magnifying power, 

 unless in exceptional circumstances. The same ob- 

 jections do not apply to the increased aperture 

 and illuminating power of telescopes. This con- 

 sideration induced Herschel to construct his forty- 

 feet reflector, the funds being advanced by the king, 

 on the recommendation of the Royal Society. The 

 speculum had a useful surface precisely four feet in 

 diameter, the thickness three and a half inches, the 

 weight when cast above 2000 pounds. The second or 

 plane mirror having been dispensed with, the image 

 was thrown to one side of the object-end of the tube 

 by means of a very slight inclination of the speculum, 

 and was there observed directly by a common eye-piece 

 or single lens, the observer being thus stationed with 

 his back to the object he is viewing. This vast in- 

 strument collected a cone of rays from a distant point 

 between six and seven times larger than the twenty- 

 feet instrument commonly employed by Sir W. Her- 

 schel, of which the aperture was 18'8 inches. This 

 telescope was not frequently used, partly from the 

 rarity of sufficiently steady weather, and from the 

 difficulty of preserving the figure of the mirror under 

 changes of temperature. 1 Its figure was also affected 

 by flexure under its own enormous weight, and it 

 has been found one of the greatest difficulties by those 

 who have followed Herschel's steps to avoid this 

 source of error. The forty-feet speculum has been 

 religiously preserved by the filial care of Sir John Her- 

 schel, in whose possession the writer of these pages 

 had the pleasure of seeing it, now many years since, 

 in a state of high polish. It remains in a deposit 

 hermetically closed, at Slough. 2 



(182.) Herschel made an ingenious determination of what 



neTnfthu:- ^ e ca ll e< ^ ^6 space-penetrating-power of telescopes, by 

 power of which he discriminated the relative distances at which 

 telescopes. a given fixed star would become invisible in his 

 several instruments. It was grounded on the as- 

 sumption that the visibility of the object depends, 

 first, on the density of its light reaching the eye, and 

 secondly, on the number of rays of that density 

 concentrated in the image. The former quantity 

 varies inversely as the square of the given distance, 

 the latter directly as the square of the aperture of 

 the telescope ; the limiting visible distance therefore 

 of the radiant body will be simply as the aperture. 

 To this he applied corrections depending on the loss 

 of light by reflection at the specula, and by trans- 

 mission through one or more eye-glasses. On this 

 scale the space-penetrating-power of the seven-feet 

 Newtonian being twenty, that of the twenty-feet tele- 

 scope (front view) was seventy-five, and the forty-feet 

 telescope 192. These estimations, as we shall see, 

 form an important step in Herschel's generalizations. 

 (183.) Of new bodies belonging to our system he dis- 



covered Uranus and his six satellites, the two satel- 

 lites of Saturn next to the ring, and several comets. 

 He did not, so far as we know, even suspect the 

 planetary character of Uranus, which he believed to 

 be a comet. The honour of this appears to be due 

 partly to Saron of Paris, partly to Lexell of St 

 Petersburg. Arago supposes that Laplace had 

 also a share, but the evidence on the whole matter 

 is somewhat obscure. Unquestionably it was an ex- 

 ceedingly difficult matter to fix upon a correct orbit 

 for a body moving with such extreme slowness within 

 the limits of the first few weeks after its discovery, 

 and its distance was far beyond what any one calcu- 

 lating a comet's path would readily assume. The 

 original discovery was also a fortunate one, for but 

 eleven days previously the planet was in a position 

 apparently stationary. It has led to results of still 

 higher interest by the prediction of the existence and 

 place of Neptune from the irregularities of Uranus. 

 The satellites move almost perpendicularly to the 

 ecliptic. Strange to say, they remained for, I be- 

 lieve, more than half a century, unseen by any eye but 

 that of their discoverer and his son : and two of them 

 still stand recorded on the single authority of Sir W. 

 Herschel. They are the least specks of light which 

 optical power has ever made visible. 



Herschel made many observations on the physical 

 appearance of the sun, moon, and nearer planets, and 

 the times of their rotation. Into these details we 

 cannot now minutely enter. The papers which record 

 them are characterized (like all those of the same 

 author) by the faithful minuteness with which the ob- 

 servations are detailed, and by the mention of every in- 

 fluential circumstance by which the results might be 

 affected. In the case of observations so exceedingly 

 delicate as those of the planets, the astronomer has to 

 keep up an incessant struggle between the anxiety to 

 record all that he sees, and that of recording nothing 

 more than he really and perfectly sees. One or two 

 glimpses are not sufficient. Herschel at one time 

 imagined that the new planet had one or more rings 

 which unquestionably do not exist. His elaborate in- 

 vestigation of the figure, belts, and rings of Saturn, the 

 incredible tenuity of the latter, and the fact of their ro- 

 tation in 10 h 32 m , were amongst the most interesting 

 of his observations, and were illustrated by drawings 

 made with remarkable care. The edge of the ring was 

 seen in 1789, when it had totally disappeared in all 

 but his forty-feet telescope, and he estimated that its 

 thickness could not exceed 100 miles. How astonish- 

 ing the magnitude and thinness of this stupendous 

 circular plane, or rather series of flat rings ! Some 

 idea of it may be deduced from the fact that if the 

 thickness of the paper on which this is printed be 

 taken to represent that of the rings, their greatest 

 diameter would correspond to nine inches. Observa- 



Herschel's 

 discovery 

 of Uranus 

 (or Oeorgi- 



um Sidus] 



(184.) 

 observa- 

 tions on the 

 planets ; 



1 The first object viewed by Sir W. Herschel with his great telescope in 1789 was the nebula in Orion ; possibly it was also 

 the last, for he again observed it in 1811 (See Phil. Tr. 1811, p. 322). 



2 See Weld's History of the Royal Society, ii., 193. 



