CHAP. III., 2.] 



ASTRONOMY. SIR WILLIAM HERSCHEL. 



41 



(177.) 



(178.) 

 Herschel as 

 an optician. 



(179.) 

 Magnify- 

 ing and il- 

 luminating 

 power of 

 telescopes. 



observations on the physical appearance and con- 

 stitution of the sun and planets ; but his great glory 

 was in sidereal astronomy, of which he laid almost 

 the foundations, by means of discoveries which he 

 fortunately lived long enough to see confirmed and 

 enlarged beyond his reasonable hopes. 



In this brief sketch of Herschel's achievements 

 (which in his own department may be said to embrace 

 those of his age, for he had hardly an imitator, and cer- 

 tainly no rival) we shall, in the first place, condense 

 into small compass what is to be said on the two 

 former heads his merit as an optician and as a 

 planetary observer. 



To construct good telescopes is itself no easy art, 

 especially if they are to be carried to a size and per- 

 fection previously unattained. To ascertain the best 

 composition, whether of glass or metal, to melt and 

 to cast it in the right way, is one branch of the art 

 which may be called generally the chemical part. To 

 fashion the lens or mirror correctly by grinding, and 

 to fit it for optical use by giving it an exquisite polish, 

 is a second step requiring a peculiar mechanical skill 

 and perseverance. To mount the telescope effec- 

 tively is another and entirely different problem in 

 mechanics ; and all these the amateur astronomer 

 must be prepared to accomplish with his own hands, 

 unless he command the services of practical opticians 

 to an extent rarely to be bought. Amateurs have 

 indeed seldom succeeded except with reflecting in- 

 struments, and Herschel acted very wisely in devot- 

 ing himself to their improvement at a time when the 

 success of Dollond with achromatic telescopes had 

 rendered the Newtonian form unfashionable. The 

 real secret of Herschel's success was his astonishing 

 perseverance ; his determination was to obtain tele- 

 scopes of twenty feet focal length or more, and of a 

 perfection equal or superior to that of the small ones 

 then in use. He himself relates, that whilst at Bath 

 he had constructed 200 specula of seven feet focus, 

 150 of ten feet, and about 80 of twenty feet ; a proof 

 of extraordinary resolution in a man of limited means 

 and engaged in a laborious profession. By making 

 so great a number he could select those having 

 the most perfect figure, especially before he had con- 

 trived a method of obtaining mechanically a para- 

 bolic form, almost with certainty. This method was 

 not divulged. He was justified in keeping his secret, 

 whilst he made a handsome income from the manu- 

 facture of telescopes for sale. Lord Rosse and Mr 

 Lassell have had the merit of publishing their 

 methods, which by their results would appear to be 

 at least equal to Herschel's. 



The usefulness of telescopes depends on two dis- 

 tinct qualities magnifying power and illuminating 

 power. The former may be gained indefinitely by 



diminishing the focal distance of the eye-glass ; the 

 latter can be had only by increasing the diameter of 

 the object-glass or speculum, so as to collect the rays 

 which fall upon a circle of large diameter from every 

 point of the object examined. But in order that 

 the whole light may be effective, the magnifying 

 power of the eye-glass must be sufficient to condense 

 the emergent pencil within the diameter of the pupil 

 of the eye. The proportion in which these two 

 qualifications are requisite to obtain the best results 

 in astronomy is a matter involving nice questions 

 both of theory and practice. Sir William Herschel 

 did more than any one who preceded or fol- 

 lowed him to solve them. On the whole, the illu- 

 minating power is perhaps the most important, whilst 

 it is the most difficult of attainment ; but in truth 

 each class of telescopic objects have their own rule 

 in this respect. Brilliant objects, such as Venus, and 

 moderately bright stars, do not require large aper- 

 tures ; very feeble objects, as the remoter satellites 

 and the nebulse, require indispensably great illumina- 

 tion. Five satellites of Saturn were seen by Sir W. 

 Herschel in his great telescope by the naked eye alone, 

 that is, without the advantage of the magnifying 

 power of a lens. On the other hand, Sir John Her- 

 schel tells us that the satellites of Uranus cannot be 

 clearly made out without a magnifying power of at 

 least 300, whatever may be the aperture of the tele- 

 scope}- 



The mention of magnifying powers is calculated to (180.) 

 mislead rather than otherwise in comparing the effi- Magmfy- 

 cient means of different astronomers. The magnify- ^tiPi 

 ing power used by Galileo did not exceed thirty-two Astronomy, 

 times the diameter. Huygens, who used aerial tele- 

 scopes of immense length, pushed it to 163 times. 

 Auzout is said to have given a power of 500 to one 

 of these gigantic instruments of 300 feet in length ; 

 but it is easily understood that such achievements 

 were little more than nominal, the mechanical diffi- 

 culty of managing these instruments being excessive. 

 Short professed to carry the magnifying powers of 

 his Gregorian reflectors to 1200, but still they yielded 

 only trifling fruits to astronomy. It required that 

 such instruments should be made and used fami- 

 liarly, not as objects of luxury of which but one or two 

 were ever brought into actual use. Herschel applied 

 even to his seven-feet Newtonian (his favourite and 

 smallest working size, having 6-3 inches aperture) 

 powers exceeding 2000. To his largest instrument 

 (thirty-nine feet focus) was occasionally applied a 

 power of 6500. The highest powers usefully em- 

 ployed with the gigantic achromatics of Pulkowaand 

 of Cambridge (in America), do not exceed 1822 and 

 2004 times respectively. 



The motion of the earth and the troubled state of (181.) 

 Its limits. 



1 Struve found that with a small telescope, magnifying three times, and having the same aperture with the pupil of the eye 

 (0-2 inch), he could count nearly twice as many stars (accurately 1-83 times) as with the naked eye. This result, interesting oa 

 several accounts, deserves farther enquiry. Etudes d'Aatron. Stellaire, p. 85 . 



