_—" 
— Oe EEE ——e7~ 
(299.) 
Lord Rosse 
—his re- 
flecting 
telescopes. 
(300.) 
Difficulties 
encoun- 
tered. 
Cuapr. IIL, § 6.] 
disappeared, The last two hundred years haye not 
presented any such astonishing phenomena as the 
new stars recorded in the sixteenth and seventeenth 
centuries ; but singular variations in the brightness 
of some of the most conspicuous stars, as « Orionis 
and » Argis, have been discovered by Sir John 
Herschel, Several stars of short and irregular 
periods of varying brightness were recorded towards 
the close of last century, But upon this very in- 
teresting subject I must content myself with refer- 
ring to the details given by Professor Argelander in 
the third volume of Humboldt’s Cosmos, 
Eant or Rossz. Latest observations on Nebule.— 
It is a remarkable circumstance, that as the reflect- 
ing telescope was a British invention, so the more 
important improvements and applications of it have 
been almost confined to the United Kingdom. It is 
also worthy of notice that the manufacture has pros- 
pered more in the hands of amateurs than of regular 
opticians, Sir William Herschel appeared at one 
time to have brought the invention to its highest per- 
fection, but the Earl of Rosse has made an important 
step farther; not only by constructing a larger tele- 
scope than had been made before, but by adapting 
machinery driven by steam power, to the grinding 
and polishing of the mirror; so that the largest 
speculum may be finished with nearly the same ac- 
curacy and expedition as the smallest, The chef 
d@euvre of Lord Rosse is a telescope of 6 feet aper- 
ture, and 53 or 54 feet of focal length. It was com~- 
pleted in the latter part of 1844, and erected at Par- 
sonstown in Ireland, 
Let me here record the important fact, that neither 
rank nor wealth could absolve Lord Rosse from those 
toils and disappointments which attend all new and 
original efforts. Thereisnoroyalroadto such triumphs. 
The Irish nobleman owes his success entirely to his 
unwearying perseverance and mechanical skill. Even 
his assistants were countrymen instructed by him- 
self in his own workshops, where the very steam-en- 
gine which drives the polisher was fabricated. His 
labours to improve the telescope date from 1828 
(when he was Lord Oxmantown), or even earlier, and 
they appear to have been unremitting until 1844; in- 
deed I might say until the present time. Com- 
mencing with a variety of ingenious attempts to cor- 
rect spherical aberration, and to overcome the ex- 
treme difficulty of procuring large castings of so ex- 
cessively brittle a material as speculum-metal, they 
terminated in the rejection of all minor helps and 
expedients, and in the fortunate completion of im- 
mense mirrors at a single casting, and of correctly 
parabolic figure when ground and polished. The 
speculum of his largest telescope weighs four tons. 
It was polished in six hours, and its surface is con- 
ASTRONOMY.—LORD ROSSE—-HENDERSON AND BESSEL, 
863 
siderably more than twice as large as that of Sir 
William Herschel’s forty-feet instrument, 
We cannot enter into the details of the methods, 
(301.) 
which evince no small mechanical skill and scientific Methods of 
ingenuity, together with a perseverance admirable in 
itself I will only mention how the upper and 
lower surfaces of the casting were made to coo] nearly 
equally fast. To effect this the lower surface of the 
mould (which naturally retains the heat more than 
the upper) was made of iron, a good conductor, whilst 
the upper surface was made of sand. This effected 
the purpose; but it being found that air bubbles 
entangled in the fluid metal could not escape beneath, 
and injured the casting, the iron bed was constructed 
of hoops set on edge and closely packed, the crevices 
allowing the escape of air, whilst the cooling pro- 
ceeded as before; and this ingenious contrivance was 
perfectly successful, 
construc- 
tion. 
Many difficulties in detail have been found in the 302.) 
mounting and use of so gigantic a mass, particularly 
on account of the distortion of the mirror by flexure. 
Applica- 
tion to Ne- 
bule, and 
But these have gradually been surmounted by Lord its results. 
Rosse. His published observations (Philosophical 
Transactions, 1850) relate almost entirely to objects 
of the class of nebulw; and as I cannot enter into 
details, I may state the general results in two or three 
sentences, (1.) As might haye been expected, many 
nebulz which resisted the power of former telescopes 
(for, except in rare instances, nothing greater than 
eighteen-inch apertures have been directed to themi) 
have been “resolved” into stars by the six-feet specu- 
lum, (2.) The aspect of a great number of nebula 
described by the two Herschels is materially modified 
by the power of the telescope to embrace the fainter 
prolongations of these singular objects, In general, 
the symmetrical forms are yery much cut up and 
confused, and in many cases yanish altogether. 
(3.) Instead of these, a certain species of symmetry, 
of a vague yet very remarkable description, has been 
detected by Lord Rosse, probably for the first time. 
It is a spiral arrangement of the nebulous coils 
round a centre, resembling somewhat the spiral 
emanations of revolving fireworks. The well-known 
nebula No, 51 of Messier’s Catalogue shows this 
in a remarkable manner, 
Some obseryations have been made upon the moon, 
It is much to be desired that these were continued, 
and that the planets could also be observed; but I 
believe that the climate of Parsonstown affords but 
few nights favourable to observation. 
Henperson anp Bessex. Parallax and distance 
of the Fived Stars,—Tuomas Henperson, at -one 
(303.) 
(304.) 
Henderson 
—his birth 
time government astronomerat the Cape of Good ana charac- 
Hope, and subsequently professor of practical as-ter. 
tronomy in the university of Edinburgh, and his 
2 For a comparison of Lord Rosse’s and Messrs Naysmith and Lassell’s (subsequent) methods of mechanical grinding and 
polishing, see Astron. Soc. Notices, vol. ix, p.110. In Lord Rosse’s apparatus every stroke of the polisher is almost a straight 
line ; in Mr Lassell’s it never is. 
