WILLIAM HERSCHEL : HIS TELESCOPES 



AND WORK. 



Bv \V. F. DENNING. F.K..\.S. 



William Herschel completed his first telescope in March. 

 1774, and made his last observation in June, 1S21. What a 

 period of activity those forty-seven years included! His 

 sister Carolina assisted him in his observations, while his 

 brother Alexander helped hiui in constructing telescopes. 



When Herschel came upon the scene and recognised the 

 requirements of practical astronomy the heavens had not been 

 explored — Nebulae and double stars existed in myriads but 

 only in a few special instances had they been suitably 

 recorded. He resolved to search the firmament, to reap the 

 harvest of wonders it presented, and to properly arrange and 

 classify them for the advantage of ages to come. So he 

 swept the sky year after year with a skill unmatched, an 

 energy indomitable, and a success beyond anticipation. He 

 was a star indeed risen amid the dawn of a new astronomy. 

 He sounded the depths of space and brought to light great 

 numbers of interesting objects never previously discerned by 

 human eyes. When his work was done the hea\ens had 

 given up many of its secrets. 



He had advanced our knowledge, in a marvellously com- 

 prehensive manner, of the great expanse around ns, and 

 posterity w'ill honour his name as that of a great pioneer in 

 the field of methodical ob.servation. 



Yet he was not backed up by any national institution, 

 endowment, or observatory. He was a comparatively poor 

 man, but with a genius within him which conceived a noble 

 work, and mechanical abilities which enabled him to fashion 

 with his own hands the telescopes he required. True his 

 Sovereign encouraged and pecuniarily assisted him after he had 

 gained renown. But without any help from George III. his 

 great career was assured — Herscliel would have been Herschel 

 still I 



The quality of Herschel's telescopes has been sometimes 

 discussed, and the subject is interesting, though differences of 

 opinion must necessarily exist. We can hardly think that 

 telescopes made more than a century ago could equal the best 

 appliances of our own day. We must have learnt something, 

 and approached a little nearer perfection during the last 

 hundred years. Old mirrors carefully tested alongside 

 Calver's and ^^"ith's best work might be expected to suffer in 

 the comparison : at any rate that seems to be the reasonable 

 inference. 



But we may depend upon it that Herschel's mirrors were 

 as good as they could be made in his day. and that they 

 were very excellent in certain cases is sufficiently evident 

 from his own allusions, and from the high powers he 

 occasionally utilized so successfully. 



It may in some degree elucidate the question if a few 

 quotations are made from Herschel's writings. .And in gi\ing 

 this evidence relatively to his telescopes I should like to 

 mention that my intention is merely to mention facts on both 

 sides of the case, and give no expression of opinion. .'Xt this 

 distance of time it would be in bad taste, and certainly unjust, 

 to disparage the instruments with which such splendid results 

 were achieved. No doubt Herschel's powers were such that 

 he could have made discoveries with relatively inferior 

 instruments, but we have his word for it tliat liis telescopes 

 were of far different character from that. 



On April 12th, 1805, he speaks of viewing "Saturn with 

 a power of five hundred and seventy on a seven-feet mirror 



of six and three-tenths inches aperture and extraordinary 

 distinctness." 



Referring to the seventh satellite of Saturn, he says he 

 " saw it very well in the twenty-feet reflector, to which the 

 extjuisite figure of the speculum not a little contributes." — 

 .•\ugust 2Sth, 1789. 



On October 24th, 17Q1. with a " seven-feet reflector, having 

 a new machine-polished, most excellent speculum, I see that 

 the division in the ring of Saturn and the open space between 

 the ring and body are ecjually dark." 



.\n impression has prevailed that the forty-feet telescope 

 rather disappointed expectation, and that its defining powers 

 were certainly not on a par with its light-grasping capacity ; 

 at any rate Herschel generally used the twenty-feet and seven- 

 feet instruments. Burnham's opinion is that some of the 

 instruments utilized by the old observers of double stars could 

 not compare favourably with modern refractors, and particu- 

 larly with telescopes made by the Clarks. " Even when the 

 earlier observers had powerful instruments in point of light- 

 gathering power, as in the case of the Herschels, there can be 

 no doubt that they were far inferior in definition." 



In the Phil. Trans, for 1795 he gives ns an idea of the 

 number of instruments made, and says : — " When I resided at 

 Bath I had long been acquainted with the theory of optics and 

 mechanics, and wanted only that experience which is so 

 necessary in the practical part of these sciences. This I 

 acquired by degrees at that place . . . My way of doing 

 these instruments at that time, when the direct method 

 of giving the figure of any of the conic sections to specula was 

 still unknown to me, was to have many mirrors of each sort 

 cast and to finish them all as well as I could ; then to select by 

 trial the best of them, which I preserved ; the rest I put by to be 

 repolished. In this manner I made not less than two hundred 

 seven-feet, one hundred and fifty ten-feet, and about eighty 

 twenty-feet mirrors, not to mention those of the Gregorian 

 form." 



High magnifying powers involve a severe test of the perform- 

 ance of telescopes. In proof of the quality of his mirrors we 

 may quote him as saying " In beautiful nights when the 

 outsides of our telescopes are dripping with moisture dis- 

 charged from the atmosphere there are now and then 

 favourable hoars in which it is hardly possible to put a limit 

 to magnifying power." 



But these superlative hours were all too few-, alas, for he 

 mentions he " had recourse to his journals to find how many 

 favourable hours we may annually hope for in this climate. 

 It is to be noticed that the nights nuist be very clear, the 

 moon absent, no twilight, no haziness, no violent wind, and no 

 sudden change of temperature, and it appears that a year 

 which will afford ninety or at most one hundred hours is to 

 be called a very productive one." 



Herschel's favourite working instrument seems to have 

 been a seven-feet of six and three-tenths inches aperture. 

 Speaking of observations of Saturn he mentions that "all that 

 magnifying can do may be done as well with the seven-feet as 

 with any larger instrument." 



The great forty-feet telescope was not used very frequently 

 by Herschel, as its manipulation occupied valuable time and 

 required assistance. It has been stated that this large 

 instrument was discarded in consequence of its bad perform- 

 ance and cumbersomeness, but this is scarcely justified. 



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