574 



NATURE 



{April 2\, 1 88 1 



the number of assistants necessary were impediments in 

 the way of its being utilised for regular observation, and 

 he assures us he "made it a rule never to employ a larger 

 telescope when a smaller will answer the purpose." It is 

 certain that the mirror which was in the tube in October, 

 1789, the month following that in which Herschel dates 

 the completion of the telescope, was of excellent defini- 

 tion. On the i6th of that month he followed the sixth 

 and seventh satellites {Eiue/adus and Mimas) up to the 

 limb of the planet, and witnessed their occultation. 

 Holden writes : " I have never seen so good definition, 

 telescopic and atmospheric, as he must have had on these 

 occasions." 



Between the years 1796 and 1799 Herschel made an 

 elaborate classification of stars visible to the naked eye 

 according to their comparative brightness, which he com- 

 municated to the Royal Society in four papers published 

 in the P/ii/. Trans. It formed the first general catalogue 

 of the kind, exhibiting the exact state of the sky in his 

 time. A reduction of Herschel' s observations was under- 

 taken by Mr. C. S. Peirce, and the results appear in vol. 

 ix. of the Annals of the Observatory of Harvard College. 

 So far as we know, their reduction had not been previously 

 attempted. Instances of variability in the light of naked- 

 eye stars were detected during the progress of the classi- 

 fication, the most notable discovery in this direction being 

 perhaps that of the periodical fluctuations of a Herculis, 

 in about sixty days. Another star in the same constella- 

 tion he considered had totally disappeared in 1791, though 

 he had seen it distinctly in 17S1 and 1782. 



Herschel was led to his numerous discoveries of double 

 stars by his expectation of being able to determine the 

 parallaxes of stars from measures made at opposite sea- 

 sons of the year of the distances of pairs which appeared 

 near together, and in the search for such pairs, his first 

 catalogue of upwards of 200 double stars was formed 

 and presented to the Royal Society in 1782. Long had 

 previously measured stars upon a similar plan without 

 success, but Herschel pointed out that his stars were not 

 well chosen. 



For the successful application of the method it is 

 necessary that one of the pair of stars should really be 

 situated at a much greater distance from us than the 

 other, and as the most reasonable test of distance, 

 Herschel assumed their difference of brightness, so that 

 he sought for pairs where the components differed widely 

 in this respect. The view therefore which he adopted at 

 this time with respect to two stars seen in close proximity 

 to each other was that one was in nearly the same line 

 of sight as the other, but might be far more distant, thus 

 constituting together what we now term an optical Aov^At 

 star. From this beginning he was led to the discovery 

 of revolving double stars, stars changing their relative 

 position from year to year ; and in 1803 he communi- 

 cated to the Royal Society his memorable paper : " An 

 account of the changes which have happened during the 

 last twenty-five years in the relative situation of double 

 stars, with an investigation of the cause to which they 

 are owing." He was then satisfied that there were in the 

 heavens pairs of stars which were physically connected 

 with each other. The research for stellar parallax was 

 not successful, but in place of it he discovered the exist- 

 ence of binary systems. He could not in his day decide 



whether the motions of suns round suns was obedient to 

 the laws of gravitation, but five years after his death the 

 French astronomer Savary proved that one of these 

 revolving double stars, discovered by Herschel, | in 

 Ursa Major, really was subservient to that law, and as 

 every student of astronomy will be aware, the number of 

 physically connected systems where the elements of the 

 orbits have been determined, is now a large one, and is 

 gradually increasing. 



Following at present the order in which Prof. Holden 

 refers to the scientific labours of Herschel, we now arrive 

 at his researches on planets and satellites, respecting 

 which the improvements he made in the construction of 

 telescopes enabled him to advance knowledge so greatly. 

 He was not particularly occupied with the inferior planets, 

 but he determined the time of axial rotation of Mars with 

 greater precision than before, and also the position of his 

 axis. The times of the rotation of the satellites of Jupi- 

 ter were found from obser\'ations on their changeable 

 brightness, and Herschel also remarked the as yet im- 

 perfectly explained phenomena attending the transits of 

 the satellites across the disk of the planet. Saturn, as 

 Holden remarks, was the object of his constant attention : 

 in addition to the discovery of the interior satellites 

 Enceladiis and Mimas, he left upon record an extensive 

 series of observations of the seven attendants upon 

 Saturn at that time known, and determined the time of 

 rotation of the outer satellite Japetus upon its axis, by 

 similar observations to those made upon the satellites of 

 Jupiter. He ascertained the time of axial rotation of 

 Saturn, and was the first who had succeeded in effecting 

 this in a reliable manner. He also remarked the curious 

 square-shouldered appearance which the globe of the 

 planet has been suspected to present, and of which we 

 still occasionally hear, though it was long ago proved by 

 Bessel to be an illusion. It is remarkable that notwith- 

 standing Herschel's frequent scrutiny of the planet, with 

 all his experience of observation and the advantages of 

 optical means surpassing by far those of his contempo- 

 raries, he does not appear to have at any time suspected 

 the existence of the interior obscure ring. He proved 

 beyond doubt that Uranus was attended by two satellites, 

 and believed he had observed four others, and for a long 

 time on his authority the planet was credited with six 

 attendants. 



In 1795 Herschel communicated to the Royal Society 

 a memoir upon the nature and construction of the sun 

 and fixed stars. As to the former he adopted a modified 

 view of the theory which had been advanced by his friend 

 Wilson of Glasgow ; he regarded the sun as consisting of 

 three essentially different parts : a solid and non-luminous 

 nucleus, cool and perhaps capable of habitation, above it 

 the atmosphere proper, and still higher the clouds or 

 bodies which cause the sun's intense brilliancy. In this 

 paper occurs a remark which, as Prof. Holden observes, 

 has often been brought to bear, in consideration of the 

 causes which maintain the solar light and heat. " Per- 

 haps," he says, "the many telescopic comets may restore 

 to the sun what is lost by the emission of light." We 

 know that however credible in his day points in his 

 theory have given way under our greatly advanced 

 knowledge. 



One of the discoveries, or perhaps we should rather say 



