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SCIENCE 



[N. S. Vol. XXXVI. No. 917 



complete in this figure, must have been the longest 

 exposed to the action of these causes. This will 

 admit of various points of view. Suppose, for 

 instance, that 5,000 stars had been once in a cer- 

 tain scattered situation, and that other 5,000 equal 

 stars had been in the same situation, then that of 

 the two clusters which had been longest exposed to 

 the action of the modelling power, we suppose 

 would be most condensed, 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, ma- 

 turity or climax of a sidereal system, from the dis- 

 position of its component parts; and, making the 

 degrees of brightness in nebulae stand for the dif- 

 ferent 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 

 that 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 yoimg, 

 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 nebulae 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 con- 

 sidered the uncommon degree of compression that 

 must prevail in a nebula to give it a planetary 

 aspect; but the argument which is now drawn 

 from the powers that have collected the formerly 

 scattered stars to the form we find they have as- 

 sumed, 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 con- 

 tains the greatest variety of productions, in dif- 

 ferent flourishing beds; and one advantage we 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 bor- 



rowed from the vegetable kingdom, is it not almost 

 the same thing, whether we live successively to 

 witness the germination, blooming, foliage, fe- 

 cundity, fading, withering and corruption of a 

 plant, or whether a vast number of specimens, 

 selected from every stage through which the plant 

 passes in the course of its existence, be brought at 

 once to our view? 



I now turn to another line of discoveiy 

 of which I can not show any pictures, but 

 which, to me at any rate, is more interest- 

 ing. Until 1838 — that is to say, until six- 

 teen years after Herschel's death — no one 

 had succeeded in determining the distance 

 of a single fixed star, but in that year Hen- 

 derson 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, for he made incidentally a discovery 

 of another kind and of at least equal in- 

 terest. 



The earth moves around the sun at a 

 distance of 93 million miles, so that in six 

 months we shift our position by 186 mil- 

 lion miles. If, then, there are two stars of 

 which one is relatively near to and the 

 other far from the sun, but so situated as 

 to appear to us very close together, the near 

 one ought to shift its position relatively to 

 the distant one in the course of each six 

 months. The amount of this change of 

 position, called by astronomers annual par- 

 allax, should furnish the distance of the 

 nearer of the pair, provided that the other 

 is very far off. This idea is as old as the 

 time of Galileo, but no one had been able 

 to make successful use of it. 



As I have already said, the only general 

 test of the distance of a star is its bright- 

 ness, and therefore Herschel chose pairs of 

 stars of very different brilliancy. He 

 thought, at least at first, that it was mere 

 chance which brought the stars so near to 



