_—— a — 
—". 
Cnap. III, § 2.] 
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- 
ian. 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 niount 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 inuse. 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. 
ASTRONOMY.—SIR WILLIAM HERSCHEL. 
839 
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 
itis 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 nebule, 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 
mislead rather than otherwise in comparing the effi- / 
cient means of different astronomers. The magnify- 
ing power used by Galileo did not exceed thirty-two 
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 Pulkowa and 
by (179.) The usefulness of telescopes depends on two dis- of Cambridge (in America), do not exceed 1822 and 
: a ee tinct qualities—magnifying power and illuminating 2004 times respectively. 
iemtonting power. The former may be gained indefinitely by | The motion of the earth and the troubled state of  (181,) 
power of 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 on 
several accounts, deserves farther enquiry.—Ltudes d’ Astron, Stellaire, p. 85. 
