392 LECTURE XLI. 



however, suppose that each of Dr. HerschePs 2500 nebulae can be at all 

 comparable in magnitude to this supposed nebula, since many of them are 

 almost as much resolved by the telescope into single stars as the milky way 

 itself ; which would be utterly impossible, if the stars which they con- 

 tain were equally numerous with those of the nebula to which the milky 

 way belongs. Supposing all the stars of this nebula to be as remote rom 

 each other as the nearest of them are from the sun, it may be calculated 

 that the most distant are about 500 times as far from us as the nearest, and 

 that light, which is probably 15 or 20 years in travelling to us from Sirius, 

 would be nearly twenty thousand in passing through the whole diameter of 

 the milky way. A nebula of the same size as this, appearing like a diffused 

 light of a degree in diameter, must be at such a distance, that its light would 

 require a million years to reach us. (Plate XXXI. Fig. 464.) 



The stars are not, properly speaking, absolutely fixed with respect to 

 each other, for several of them have particular motions, which have been 

 discovered by a comparison of accurate observations, made at very distant 

 times. Arcturus, for instance, has a progressive motion, amounting to 

 more than two seconds annually. * Dr. Maskelyne found, that out of 36 

 stars, of which he ascertained the places with great precision, 35 had a 

 proper motion. Mr. Michellt and Dr. Herschel : have conjectured, that 

 some of the stars revolve round others which are apparently situated very 

 near them ; and perhaps even all the stars may in reality change their 

 places more or less, although their relative situations, and the directions of 

 their paths may often render their motions imperceptible to us. 



Respecting all these arrangements of stars into different systems, Dr. 

 Herschel has lately entered into a very extensive field of observation and 

 speculation, and has divided them into a number of classes, to each of 

 which he has assigned a distinct character. Some he supposes, like our 

 sun, to be insulated stars, beyond the reach of any sensible action of the 

 gravitation of others ; and around these alone he conceives that planets 

 and comets revolve. Double stars, in general, he imagines to be much 

 nearer to each other, so as to be materially affected by their mutual gravi- 

 tation, and only to preserve their distance by means of the centrifugal 

 force derived from a revolution round their common centre of inertia ; an 

 opinion which, he thinks, is strongly supported by his own observations of 

 some changes in the positions of double stars. Others again he supposes 

 to be united in triple, quadruple, and still more compound systems. A 

 fourth class consists of nebulae like the milky way, the clusters of stars 

 being rounded, and appearing brightest in the middle. Groups of stars 

 Dr. Herschel distinguishes from these by a want of apparent condensation 



* Halley, Ph. Tr. No. 355. Cassini, Mem. de 1'Acad. 1738, p. 231. Monnier, 

 ibid. 1767, p. 417, proves that the latitude of Arc. varies at the rate of two seconds 

 annually; and that the longitude decreases at the rate of 60 seconds in 100 years. 

 See also Mem. de 1'Acad. 1769, p. 21. La Caille, Fundamenta Astron. pp.169, 

 187. Hornsby, Ph. Tr. 1773, Ixiii. 93, is of opinion that his deductions prove that 

 the obliquity of the ecliptic has become less. 



t Ph. Tr. 1767, Ivii. 234 ; 1784, p. 35. 



I Ibid. 1783, Ixxiii. 247. 



On the Construction of the Heavens, Ph. Tr. 1785, Lxxv. 213. 



