(205.) Calling the distance of a star of the first magni- illustrious son, little has been added to our know- ees 
Relative tude 1, and supposing that light takes three years to ledge of “ the constitution of the Heavens” since his Herechel ‘ 
en paraghag traverse it,!— death. We shall, therefore, hardly recur to the topic researches. 
Niiad diniesine ot ; Dist. Light travelsin in the latter part of this chapter. Some German 
of a star of the sixth mag- : ¢ * 
nitude (the limit of unassisted vision) astronomers of eminence have indeed tried to deduce 
WAL He sis cus bss cdiacdncgepeenvaccdsa viet 2S 36 years. from an assumed distribution of the stars in the Milky 
Limit of 20-feet telescope (used in gaug- Pr Way, the necessity of allowing that space is not per- 
| hinteot tcteek telcos ago, Sots, feetly transparent, and have even attempted to assign 
p Limit of Lord Rosse’s telescope (6 feet the law of extinction of light through the sidereal in- 
GUOMOLET) <..sssedeunaeeadis SKI +* peaubense 3,436 10,308 tervals; but having examined their reasoning with 
Yet none of these telescopes appear to touch the the best care I have been able to give it, I am 
: boundary of our nebula in the direction of its greatest bound to say that it appears to me very far short of 
: extension, for fresh optical power still yields fresh conclusive.’ 
harvests of stars. 
(206.) In a direction perpendicular to the plane of the IV. Of the Motion of our System in Space.—That 299.) 
All visible Milky Way, not only is the paucity of stars remark- the stars are not fixed, but have “ proper motions,” Proper mo- 
stars be- able, but we seem to look out, as Sir John Herschel not explicable by the gyrations of our earth round its oe € 
Bite = describes it, into a starless region external to the stra- centre of gravity (which produce precession and nuta- Halley. 
the Milky tum. Sir W. Herschel concluded, in 1817, that the tion), nor by its revolution round the sun (on which 
Way. depth of the stratum in its least dimension is suchas aberration and parallax depend), was a fact first de- 
Cuap. III., § 2.] 
(hypothetically uniform) between any two stars. But 
in his later memoirs, Herschel seems to have felt the 
insecurity of his conclusions, and that there is no 
reason, but the contrary, to believe in a uniform dis- 
tribution of the stars; that, at least in some direc- 
tions, the Milky Way is unfathomable, or its limits 
unattained even by the 40-feet telescope. (See his 
Memoir of 1817.) He therefore judiciously recurs 
to photometrical measures as the best criterion of the 
distance of the stars. The estimate of ‘the space- 
penetrating power” of his telescopes, already referred 
to (182), enabled him to infer the relative distances at 
which a star would become lost to vision aided by 
each telescope. 
to include all stars down to those three times more 
distant than the naked eye perceives. His son, 20 
years later, infers, from the investigation of the 
southern hemisphere, that the distribution of stars is 
comparatively independent of direction for all those 
which are brighter than the 8th magnitude, and that 
it is by the influence of the hosts of lesser brightness 
ASTRONOMY.—SIR WILLIAM HERSCHEL. 
845 
Milky Way takes effect. He is therefore disposed to 
conclude that “ our system is plunged in the sidereal 
stratum constituting the Galaxy, reckoning from the 
southern surface to a distance which corresponds to 
the light of a star of the 9th or 10th magnitude.’’? 
These great and arduous enquiries occupied Sir 
William Herschel during nearly the whole of his 
scientific career, extending to almost half a century. 
His first observations (Struve remarks) dated from 
1774, his last observations on this subject which have 
been published were of 1804, and his latest paper, 
still on the same topic, was in 1821, when he was 
almost on the verge of the grave. 
Excepting the continuation’ of his labours by his 
tected by Halley from a comparison of ancient and 
modern observations. Though small in amount, yet 
as it continually increases in the same direction, this 
Proper Motion becomes in the course of ages an im- 
portant quantity, In 2000 years, Arcturus, ~ Cas- 
siopeie and 61 Cygni have moved 24, 3}, and 6 times 
the moon’s diameter. 
(207.) 
(208.) 
In 1748, Bradley (in his paper on Nutation) stated 
the probable explanation of these apparent motions 
visible in the 20-feet reflector, that the very remark- (210.) 
able law of condensation towards the plane of the 
1 These startling results appear to be ily true, if the following assumptions be admitted :—(1.) That the stars have 
on an average the same brilliancy in all regions of space. (2.) That the light received from a star is inversely as the square of 
the distance ; or that the laws of radiation and of vis viva are rigorously true, and that there is no absorption of light in space. 
(3.) That the visibility of a star is exactly as the quantity of light received from it in the telescope. Probably this last assump- 
tion is the least sure. We know that magnifying power (independently of illumination) affects the result. 
mi 2 Cape Observations, p. 383. 
q 3 I ought perhaps to state that in this passage I have referred to the reasonings of M. Struve, contained in the latter part of his 
Etudes d’ Astronomie Stellaire. Waving taken some pains to follow the argument of that work, which professes to be “ based en- 
} tirely upon observation, without any arbitrary hypothesis,” I was reluctantly led to the conclusion that M. Struve’s assumptions, 
} if tacit, were not the less arbitrary and questionable. Indeed it appears self-evident that no geometrical certainty can be at- 
7 tained as to the relative distances of the stars composing the Milky Way, but from some fundamental hypothesis respecting their 
4 magnitudes and distribution. Iam persuaded that the popular writers and reviewers who have given additional publicity to 
the most striking and positive of M. Struve’s conclusions, have (very naturally) done so on the strength of the author’s well-de- 
served reputation as an observer, and without attempting to analyse his reasoning, which it must be owned is sometimes obscure. 
My objections to M. Struve’s argument were put in writing several years ago (1850), but not published except in my lectures. 
It was only whilst correcting the proofs of these sheets (1855) that I saw for the first time a memoir by Professor Encke in the 
Astr ische Nachrichten, vol. xxvi., No, 622 (published in 1848), maintaining the same view of the invalidity of M. Strave’s 
j reasoning, and questioning the hypotheses (of which M. Encke reckons jive) tacitly assumed by him. The Dutch Academy of Sciences 
(Haarlem) has proposed the points at issue between MM. Struve and Encke as the subject of a prize question. 
