(289.) 
Continua- 
tion of the 
history of 
Sidereal 
Astrono- 
my. 
(291.) 
His re- 
searches on 
double 
stars. 
Cuar. IH, § 6.] | ASTRONOMY.—M. STRUVE—SIR JOHN HERSCHEL. 861 
§ 6. Sidereal Astronomy since 1820.—M. StrRuvE—Double Stars. Observatories of Dorpat and 
Pulkowa. Sir JoHN HerscHeL—Orbits of Double Stars. Magnitudes of Stars. Variable 
Stars. EArt or Rosse—His Telescopes. Nebule. HENDERSON and BesspL—Parallaz of 
Stars. 
As it is absolutely necessary to bring this chapter 
to a speedy close, and as I have already anticipated, 
in the account of Sir William Herschel’s discoveries, 
part of what relates to the recent history of the As- 
tronomy of the Fixed Stars, particularly the “ Con- 
stitution of the Heavens,’ and the motion of our 
system in space, I shall condense within brief com- 
pass a few leading facts connected, in the first place, 
with the Orbits of Double Stars, the Brightness of 
Stars, and the constitution of Nebule, and these sub- 
jects I shall connect with the names of the elder M. 
Struve, Sir John Herschel,and Lord Rosse; the second 
topic shall be the Parallax and distance of the fixed 
stars, as ascertained more particularly by the late 
professors Henderson and Bessel. 
Frieprich Georg Wituetm Srrvve has been the 
most assiduous observer of double stars since the time 
of Sir William Herschel. No discovery in this de- 
partment can for a moment compete with the great 
one of the orbital revolution of one star round an- 
other. But M. Struve, by devoting his chief energies 
during the most active years of his life, since 1813, 
to the assiduous continuation of Herschel’s observa- 
tions, has added immensely to our knowledge of these 
systems, and has earned the reputation of one of the 
most skilful of modern practical astronomers. His 
most elaborate observations were made at the Rus- 
sian Observatory at Dorpat, with a noble refractor by 
Fraunhofer, nearly 10 inches aperture, and 13} feet 
in focal length. He has published three works on the 
subject of double stars, one in 1824, one in 1837, 
and one in 1852, besides minor papers. The second 
of these works contains the particulars of about 3000 
double stars, deduced from a survey of the heavens, 
in which at least 120,000 stars were examined. 
M. Struve’s papers are distinguished by the ela- 
‘boration of the reductions, and of the statistical re- 
sults deduced from them. In his last publication he 
has made an interesting estimate of the number of 
true double stars in the heayens, which, it appears, 
is much greater in proportion to the whole number 
than is usually believed. But it is first necessary to 
‘distinguish those which are physically double from 
those which are merely apparently or optically so. 
The criteria on which he principally depends are— 
(1.) the fact of observed orbital revolution ; but as 
‘this is established in comparatively few instances, he 
very reasonably admits (2.) a common proper motion 
of the two components as a proof of their connection. 
He thus finds the evidence for physical duplicity to 
be much stronger for the closer double stars, and also 
for brighter or nearer stars, as compared with those of 
less magnitude. On the whole he concludes, that of 
os 
1973 double stars, 1702, or six-sevenths of the whole, 
are physically connected, and that, amongst at least 
the brighter stars, the number of compound systems 
is to that of isolated stars in a ratio not less than 1 
to 3, perhaps even 1 to 2. 
M. Struve has also determined with extreme care _(292.) 
the places and proper motions of the double stars hei sone 
and he has formed a decided opinion, that the proper tovions 
motions diminish on the whole regularly with the order 
of magnitude,—thus confirming the criterion of in- 
creased distance from diminished brilliancy, by that 
of the apparent displacement of the stars by the mo- 
tion of our system. 
Besides these important works on Sidereal Astro-  (293.) 
nomy, M. Struve is well known as the head and di- pee the 
rector of perhaps the best organized observatory in tory of 
the world, that of Pulkowa near St Petersburg, of Puikowa. 
which he has published a very interesting descrip- 
tion. Besides other noble instruments, it contains 
the finest refractor in Europe, that by Merz, 15 inches 
in diameter, and 22 feet focal length. The observa- 
tions with this noble telescope are chiefly made by his 
son M, Otto Struve, the author of many good papers. 
We have seen in a former section (252), that we owe 
in great part to M. Struve the conduct of the most 
extensive trigonometrical operation ever undertaken. 
Sm Joun Freperick Wittiam Herscuet, son of 294.) 
Sir William Herschel, whilst conversant with almost Sir John 
every branch of science, has devoted himself with re- re pe oats 
markable success to the cultivation of Sidereal As~ ical ca- 
tronomy. ‘ Bearing a name honoured and revered reer. 
by all, his career at Cambridge reflected upon it fresh 
lustre ; the variety and extent of his acquirements 
gave him a reputation amongst his college contem- 
poraries, afterwards fully confirmed by the not more 
impartial voice of mankind at large.” He was senior 
wrangler in 1813. “Since that time he has been 
indefatigable as an author :—first, in systematizing 
the higher mathematics, and in forwarding their study 
in his own university ;—afterwards by treatises con- 
tributed to the Encyclopedia Metropolitana on Sound, 
Light, and Physical Astronomy, which still rank 
among the clearest, completest, and most philoso- 
phical in our language. About the same time he 
wrote experimental essays on different branches of 
Chemistry, Magnetism, and Optics, and commenced 
his purely astronomical investigations, chiefly on 
nebulz and double stars, partly in conjunction with 
Sir James South, of which the details are given in 
different volumes of the Astronomical, and of the 
Royal Society’s Transactions. These memoirs collec- 
tively include a complete revision of the objects of 
the same description catalogued and classified by Sir 
