es a, 
(185.) 
on the Sun: 
Guar. IIT., § 2.] 
tions on the other planets and on the moon we here 
pass over for want of space. 
I have reserved the observations on the nature of 
the sun to this place, because everything leads us to 
assimilate the nature of the sun and of the fixed stars. 
The belief that the luminous disk of the sun is a photo- 
sphere or luminous atmosphere of great tenuity sur- 
rounding a globe of comparative density and dark- 
ness, was long anterior to Herschel, and in fact due 
’ to Dr Patrick Wilson of Glasgow, whose admirable 
followed 
Patrick 
Wilson. 
paper on this subject was published in the Philoso- 
phical Transactions for 1774, in which he explains 
the phenomena of the solar spots by apertures in the 
luminous atmosphere, discovering the dark nucleus 
below, and some shell or shells of intermediate 
brightness which form the penumbra. These con- 
clusions were most clearly and ably deduced from a 
careful observation of the changing aspect of the spots, 
as they move by the solar rotation from the centre 
to the edge of his disk. It is to be regretted that 
Herschel does not more pointedly refer to the dis- 
coveries of Wilson, which were more than twenty 
years antecedent to his first paper on the subject, 
and with which he could hardly fail to have been ac- 
quainted. A similar remark applies, in a less degree, 
to his papers generally, which rarely contain references 
to the observations and speculations of his predeces- 
sors. Herschel adopted Wilson’s hypothesis almost 
literally, and his long series of patient observations on 
_ the sun, made with high powers and at an eminent risk 
(186.) 
Herschel’s 
sidereal 
discoveries, 
to his eyesight, enabled him to classify the singularly 
varied appearances of that wonderful orb, and to 
draw some probable conclusions from the excessive 
rapidity and seeming tumult of the exterior portions 
of it. That the photosphere is strictly gaseous he 
rendered very probable, an inference confirmed by 
the direct observations of Arago as to the un- 
polarized character of its light. The singular dis- 
closure of faint red prominences extending far beyond 
the disk, and observed in the total eclipses of 1842 
and 1851, shows that there is still much which re- 
gards the mysterious nature of the sun within reach 
of direct observation ; and the same may be observed 
of the direct experiments lately made on the heat 
and light of different parts of the disk, which diminish 
to one-half between the centre and the edge, and 
appear to attain a maximum at the solar equator. 
A convenient, though not a strictly chronological 
arrangement of Sir W. Herschel’s more important 
sidereal discoveries and speculations may be made 
under the following heads :-— 
I. Of double Stars and their mutual connection. 
II. Of the Nature of Nebulz, and the so-called 
Nebular Hypothesis. 
ASTRONOMY.—SIR WILLIAM HERSCHEL. 
841 
III, Of the Grouping of the Stars generally in 
space, and the significance of the Milky Way. 
TY. Of the Motion of our System in space. 
I. On Double Stars. Discovery of Binary Sys- 
tems.—Double stars were noticed as objects of curio- 
sity even before the discovery of the telescope. The 
group of the Pleiades attracted attention from the 
earliest times. Amongst the earliest double stars 
carefully observed were  Urse Majoris (by Kirch, 
1700); @ Centauri in 1709; y Virginis and Castor 
by Bradley (1718 and 1719); Mayer made a con- 
siderable catalogue of double stars in 1756. But 
Lambert first announced in 1761 (in his Lettres Cos- 
mologiques) the probability of the mutual revolution 
of suns, in these remarkable words (speaking of clus- 
ters of stars), ‘It will perhaps be decided whether 
there are not fixed stars which make their revolutions 
in no long periods round their common centres of 
gravity.’ Mitchell, in 1767 and 1784, maintained 
the same views, but supported them by an applica- 
tion of the then young science of probability, hazard- 
ous in its principle, and unquestionably wrong in its 
numerical solution.? 
Sir W. Herschel commenced his observations on 
double stars with the hope of ascertaining the Annual 
Parallax in the manner previously indicated by 
Galileo and James Gregory ; but, as in many parallel 
instances, whilst he failed of his main result, he dis- 
covered unsought a phenomenon more unexpected 
and probably more interesting. With the micro- 
metrical means at his disposal, he entirely failed in 
detecting any semi-annual fluctuation of the inter- 
val between the members of the pair of stars, but 
he found (in some instances) progressive and con- 
tinually increasing changes both in the relative 
position and distance of the two. He com- 
menced his observations in 1779, but it was not 
until 1802 that he thought himself entitled to an- 
nounce with confidence his discovery of the cireula- 
tion of one sun round another, or rather of both 
round their common centre of gravity. Herschel’s 
first list of orbital stars (Phil. Trans. 1803, where 
this splendid discovery was first published*) includes 
the chief examples now known ; and they have all 
been confirmed. That of which the revolution is 
most rapid is  Hereulis, which has a period of 314 
years, and consequently has revolved twice round 
since it was first observed, whilst the slow planet 
Uranus has not yet returned to the position of its 
first discovery. Herschel does not appear to have 
received a medal or other public recognition of this 
signal success, 
Of the subsequent progress of this interesting in- 
quiry we shall speak in the latter part of this chapter. 
1 Flamsteed appears to have entertained in 1681 a somewhat similar opinion, as we find from a letter published (1855) in 
Sir David Brewster’s Life of Newton, vol. ii., p. 103. 
2 See art. (85) and note. 
3 It was in fact distinctly though incidentally announced in his paper of 1802, though the proofs were given the following 
year. See Phil. Trans. 1802. 
VOL, I. 
5o 
(187.) 
Double 
stars—or- 
bital mo- 
tion ; 
(188.) 
failed to 
discover 
parallax. 
(s9.) 
