1912] on Sir William Herschel. 401 



densed, and more advanced to the maturity of its figure. An obvious 

 consequence that may be drawn from this consideration is that we 

 are enabled to judge of the relative age, maturity, or climax of a 

 sidereal system, from the disposition of its component parts ; and, 

 makinir the degrees of brightness in nebulae stand for the different 

 accumulation of stars in clusters, the same conclusions will extend 

 equally to them all. But we are not to conclude from what has been 

 said tiiat every spherical cluster is of an equal standing in regard to 

 absolute duration, since one that is composed of a thousand stars only 

 must certainly arrive to the perfection of its form sooner than another 

 which takes in a range of a million. Youth and age are comparative 

 expressions ; and an oak of a certain age may be called very young, 

 while a contemporary shrub is already on the verge of its decay. 

 The method of judging with some assurance of the condition of any 

 sidereal system may perhaps not improperly be drawn from the 

 standard laid down earlier ; so that, for instance, a cluster or nebula 

 which is very gradually more compressed and bright towards the 

 middle may be in the perfection of its growth, when another which 

 approaches to the condition pointed out by a more equal compression, 

 such as the nebula3 I have called Planetary seem to present us with, 

 may be looked upon as very aged, and drawing on towards a period 

 of change, or dissolution. This has been before surmised, when in a 

 former paper I considered the uncommon degree of compression that 

 must prevail in a nebula to give it a planetary aspect ; but the argu- 

 ment, which is now drawn from the powers that have collected the 

 formerly scattered stars to the form we find they have assumed, must 

 greatly corroborate that sentiment. 



" This method of viewing the heavens seems to throw them into 

 a new kind of light. They now are seen to resemble a luxuriant 

 garden, which contains the greatest variety of productions, in dif- 

 ferent flourishing beds ; and one advantage w^e may at least reap 

 from it is, that we can, as it were, extend the range of our experience 

 to an immense duration. For, to continue the simile I have borrowed 

 from the vegetable kingdom, is it hot almost the same thing, whether 

 we live successively to witness the germination, blooming, foliage, 

 fecundity, fading, withering, and corruption of a plant, or whether a 

 vast number of specimens, selected from every stage through whicli 

 the plant passes in the course of its existence, be brought at once to 

 our view ? " 



I now turn to another line of discovery of wdiich I cannot show 

 any pictures, but which, to me at any rate, is more interesting. 

 Until 1838 — that is to say, until sixteen years after Herschel's death — 

 no one had succeeded in determining the distance of a single fixed 

 star, but in that year Henderson and Bessel almost simultaneously 

 attained success in the cases of the two stars a Centauri and 61 Cygni. 

 The attempts at this measurement had already been numerous, and 

 Herschel amongst others had failed, but his failure was a glorious one. 



