NATURE 



573 



THURSDAY, APRIL 21, 1S81 



SIR WILLIAM HERSCHEL"- 

 III. 



IN the concluding chapter of his Memoir Prof. Holden 

 presents a Review of the scientific labours of WiUiam 

 Herschel designed to enable the general reader to follow 

 the course of his work and discoveries, astronomical and 

 physical, referring to the Analyse de la Vie el des Travaux 

 de Sir William Herschel, published by Arago in 1842 for 

 a more detailed and precise account suited to the pro- 

 fessional astronomer; also to "A Subject-Index and 

 a Synopsis of the Scientific Writings of Sir William 

 Herschel," prepared by himself and Dr. Hastings, and 

 forming one of the publications of the Smithsonian 

 Institution. 



Prof. Holden naturally commences his review with 

 the improvements in optical instruments and apparatus 

 effected by Herschel. Up to his time the principal aids 

 to observation were the Newtonian telescopes of Short 

 and the small achromatics of DoUond, the six-foot New- 

 tonians of the former maker, aperture 9'4 inches, and the 

 forty-six-inch achromatics of DoUond, aperture 3'6 inches, 

 were much esteemed, and one of each class was in use at 

 the Royal Observatory, Greenwich, in 1765. Herschel 

 gives us some account of the progress of his manufacture 

 of telescopes in his description of the forty-feet reflector 

 presented to the Royal Society in 1795. When he resided 

 at Bath, he tells us, he had long been acquainted with the 

 theory of optics and mechanics, and wanted only that 

 experience so essential in the practice of these sciences. 

 This he gradually acquired by way of amusement in his 

 leisure hours (we have seen that he was closely occupied 

 in his profession as a teacher of music), and thus he made 

 ' ' several two-foot, five-foot, seven-foot, ten-foot, and 

 twenty-foot Newtonian telescopes, besides others, of the 

 Gregorian form of eight, twelve, and eighteen inches, and 

 two, three, five, and ten feet focal length," in all, as 

 already stated, he made not less than 200 seven-feet, 100 

 ten-feet, and about 80 twenty-feet mirrors, in addition to 

 the Gregorian telescopes. The number of stands he 

 invented for these instruments he states it would not be 

 easy to assign. Proceeding further, as early as 1781 he 

 had designed and commenced the construction of what 

 he terms " a 30-feet aerial reflector," and invented and 

 executed a stand for it ; he cast the mirror, " which was 

 moulded up so as to come out 36 inches in diameter," but 

 " the composition of the metal being a little too brittle, it 

 cracked in the cooling." It was cast a second time, but 

 here the furnace gave way and the metal ran into the fire. 

 These accidents and the discovery of Uranus, which 

 introduced Herschel to the patronage of the king, put a 

 temporary stop to the construction of a great telescope. 

 In 1783 he finished "a very good twenty-feet reflector 

 with a large aperture," and after two years observation 

 with it, became so convinced of the advantages of such 

 apertures, that he recurred to his previous intention of 

 increasing them still further. Soon afterwards, by the 

 representations of Sir Joseph Banks, president of the 

 Royal Society, Herschel, as his sister relates, obtained 



' Continued from p. 455. 



Vol. xxiii. — No. 599 



" the promise that 2000/. would be granted for enabling 

 him to make himself an instrument." 



The forty-feet reflector, the c/tcf d'ceuvre of Herschel's 

 optical and mechanical efforts, was commenced about the 

 latter end of 1785, and, as Prof. Holden remarks, the 

 history of the instrument extends from this date until the 

 year iSi i. The work was carried on assiduously with no 

 further interruption than was occasioned by the removal 

 from Clay Hall to Slough, where, soon after arrival, 

 Herschel began to lay the foundation of the whole struc- 

 ture, and the highly-polished speculum was put into the 

 tube, and the first view through it was obtained on 

 February 19, 1787. But he dates the completion of the 

 instrument from a much later period, for the first speculum 

 came out thinner than was intended, and from its weak- 

 r.ess did not permit of a good figure being given to it ; a 

 second mirror, cast in January, 1788, cracked in cooling ; 

 but in the next month it was re-cast and proved of the 

 proper degree of strength. In October following a pretty 

 good figure and polish had been assured, and Herschel 

 says he observed the planet Saturn with it ; he continued 

 to work upon it till August 27, 1789, when upon trial on 

 the fixed stars it gave a pretty sharp image, and on the 

 following night he records, " Having brought the telescope 

 to the parallel of Saturn, I discovered a sixth satellite of 

 that planet, and also saw the spots upon Saturn, better 

 than I had ever seen them before, so that I may date the 

 finishing of the forty-feet telescope from that time." The 

 diameter of the polished surface of the great mirror was 

 48 inches. In proof of the efficiency of the mechanism 

 for giving horizontal and vertical motions to so large an 

 instrument he mentions that in the year 1789 he had 

 many times taken up Saturn two or three hours before 

 meridian passage and kept the planet in view with the 

 greatest facility till two or three hours after the passage. 

 On the 17th of September a seventh satellite of Saturn, 

 the minute object now called Mimas, was discovered with 

 the forty-feet telescope, and though the instrument was 

 used for various purposes till 1 811, these discoveries of 

 satellites constitute its most prominent additions to our 

 knowledge. Sir John Herschel has stated that the entire 

 cost of construction, including the apparatus for casting, 

 grinding, and figuring the mirrors, of which two were 

 constructed, amounted to 4000/., which sum was provided 

 by King George III. His father observed the great 

 nebula of Orion with the forty-feet telescope on January 

 19, 181 1, and this was one of his latest observations. In 

 1839 the wood- work had so far decayed as to be 

 dangerous, and Sir John Herschel pulled it down, but 

 piers were erected upon which the tube was placed. 

 Writing in March, 1847, he remarks that it was so well 

 preserved that "although not more than one-twentieth of 

 an inch thick, when in the horizontal position it sustained 

 within it all my family, and continues to sustain inclosed 

 within it, to this day, not only the heavier of the two 

 reflectors, but also all the more important portions of the 

 machinery." 



As Prof. Holden remarks, and a similar opinion has 

 been expressed previously, it is probable that the general 

 public expected more from the forty-feet telescope than it 

 actually performed ; but Herschel gave valid reasons why 

 he did not make more extended use of the instrument : 

 the time required to get it into proper working order and 



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