CHAP. III., 2.] 



ASTRONOMY. SIR WILLIAM HERSCHEL. 



47 



(205.) 

 Relative 

 distances of 

 fixed stars. 



(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. 



Calling the distance of a star of the first magni- 

 tude 1, and supposing that light takes three years to 

 traverse it, 1 



DM, 



The distance of a star of the sixth mag- 

 nitude (the limit of unassisted vision) 



will be 12 



Limit of 20-feet telescope (used in gaug- 

 ing) 900 



Limit of 40-feet telescope 2,304 



Limit of Lord Rosse's telescope (6 feet 

 diameter) 3,436 



Yet none of these telescopes appear to touch the 

 boundary of our nebula in the direction of its greatest 

 extension, for fresh optical power still yields fresh 

 harvests of stars. 



(206 ) '- 11 a Direction perpendicular to the plane of the 

 .\11 visible Milky Way, not only is the paucity of stars remark- 

 stars be- able, but we seem to look out, as Sir John Herschel 

 describes it, into a starless region external to the stra- 

 tum. Sir W. Herschel concluded, in 1817, that the 

 depth of the stratum in its least dimension is such as 

 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 

 visible in the 20-feet reflector, that the very remark- 

 able law of condensation towards the plane of the 



Light travels in 



36 years. 



2,700 ... 

 6,912 ... 



10,308 ... 



ong to th 

 lystem of 

 he Milky 

 vVay. 



(207.) 



(208.) 





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." 2 



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 v continuation of his labours by his 

 illustrious son, little has been added to our know- 

 ledge of " the constitution of the Heavens" since his Herschei' 

 death. We shall, therefore, hardly recur to the topic researches. 

 in the latter part of this chapter. Some German 

 astronomers of eminence have indeed tried to deduce 

 from an assumed distribution of the stars in the Milky 

 Way, the necessity of allowing that space is not per- 

 fectly transparent, and have even attempted to assign 

 the law of extinction of light through the sidereal in- 

 tervals ; but having examined their reasoning with 

 the best care I have been able to give it, I am 

 bound to say that it appears to me very far short of 

 conclusive. 3 



IV. Of the Motion of our System in Space. That (209.) 

 the stars are not fixed, but have " proper motions," Proper mo- 

 not explicable by the gyrations of our earth round its tlons ot 

 centre of gravity (which produce precession and nuta- Haiiey. 

 tion), nor by its revolution round the sun (on which 

 aberration and parallax depend), was a fact first de- 

 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, /t* Cas- 

 siopeise and 6 1 Cygni have moved 2 J, 3 J, and 6 times 

 the moon's diameter. 



In 1748, Bradley (in his paper on Nutation) stated (210.) 

 the probable explanation of these apparent motions 



1 These startling results appear to be necessarily 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. 



2 Cape Observations, p. 383. 



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. Having 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- 

 tained as to the relative distances of the stars composing the Milky Way, but from some fundamental hypothesis respecting their 

 magnitudes and distribution. I am 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 

 Astronomische Nachrichten, vol. xxvi., No, 622 (published in 1848), maintaining the same view of the invalidity of M. Struve's 

 reasoning, and questioning the hypotheses (of which M. Encke reckons five) 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. 



