THE STELLAR UNIVERSE. 



they existed in distinct cLsses. Such, classification must, there- 

 fore, be to a great extent arbitrary, the individuals placed at 

 the bottom of one class being just as well entitled to a place at the 

 top of the next. 



Astronomers, accordingly, in the classification of the visible 

 stars, in the order of their relative splendour, have encountered a 

 like difficulty. The ancient astronomers, by common consent, 

 distributed all the stars visible to the naked eye into six orders of 

 what they called magnitude. The most splendid stars were said 

 to be stars of the first magnitude : the next in the order of 

 splendour, of the second magnitude ; and so on to the sixth, which 

 included the stars barely perceptible with the naked eye. As may 

 be expected, from what has been stated, much difference between 

 astronomer and astronomer arose in settling this classification. It 

 necessarily occurred, that numerous stars had such brightness as 

 would equally entitle them to be placed at the foot of the stars of 

 the first magnitude, or at the head of those of the second magni- 

 tude. A still greater number raised a like question as to their 

 title to a place in the classes of the second and third magnitude, 

 and so on. Notwithstanding these vague and uncertain conditions, 

 the ancient classification has still maintained its place, and has 

 been accepted by modern astronomers as the least inconvenient in 

 principle, and, as will presently appear, they have even extended 

 the principle, defective as it is, to the far more numerous classes 

 of stars which the telescope has rendered visible. 



22. An expedient has occasionally been adopted by observers 

 aiming at more than usual precision to distinguish stars whose 

 brightness renders it doubtful to which of two succeeding magni- 

 tudes they ought to be assigned, consisting of a fraction annexed 

 to the number which designates the higher of the two orders. 

 Thus, for example, a star whose brightness appears to give it 

 equal titles to be placed at the foot of those of the second, or the 

 head of those of the third magnitude, is designated as a star of 

 the 2 1 magnitude. 



Modern observers have also extended the ancient classification 

 to seven orders of magnitude ; dividing the ancient stars of the 

 sixth magnitude into two, designated the sixth and the seventh 

 magnitudes ; so that, according to the classification received at 

 present, the most minute stars visible to the naked eye, under the 

 most favourable atmospheric conditions at midnight, when all 

 interference of solar light is removed, are classed as stars of the 

 seventh magnitude. 



We must here, however, observe, that we fall again into the 

 difficulties arising from arbitrary classification, since certain stars 

 are visible to some eyes, which, at the same time and place, are 

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