VOL, LVII.] PHILOSOPHICAL TRANSACTIONS. 433 



doubtful. Those stars which are found in clusters, and surrounded with many 

 others at a small distance from them, belong probably to other systems, and not 

 to ours. And those stars, which are surrounded with nebulae, are probably only 

 very great stars, which, on account of their superior magnitude, are singly 

 visible, while the others, which compose the remaining parts of the same system, 

 are so small as to escape our sight. And those nebulae, in which we can 

 discover either none, or only a few stars, even with the assistance of the best 

 telescopes, are probably systems that are still more distant than the rest. 



The Pleiades, as they appear to the naked eye, have been shown above to be 

 probably a system by themselves ; and if we examine them still further by means 

 of telescopes, we shall find that they are surrounded with so large a number 

 of smaller stars, as to increase the odds against the contrary opinion many 

 millions to one. Now supposing the Pleiades to be in reality a system of stars, 

 the probability is at least, Mr. M. supposes, a hundred to one that no one 

 among them, of those visible to the naked eye, belongs to the same system with 

 the sun; but that these are only such stars as are greater than the rest. The 

 exact quantity of this probability depends on the number of stars, visible to the 

 naked eye, belonging to this system; the proportion, that the space occupied by 

 the Pleiades bears to the whole heavens ; and lastly, how far the situation of any 

 one of the Pleiades falls in with the general analogy of the stars composing 

 this system, if any such general analogy should appear. 



As the nebulae, and smaller constellations, composed of a great number of 

 stars, within a small distance from one another, belong probably to other 

 systems; so those, which being placed at greater distances from each other 

 compose the larger constellations, and such as have few or no smaller stars near 

 them, when examined with telescopes, belong probably to our own system. 

 Most of the stars of the first and second magnitude have this criterion to 

 distinguish them, as belonging to the same system with the sun, besides several 

 other circumstances, such as their greater brightness; the proper motions * that 

 have been observed among some of them ; their being more numerous than we 

 might naturally expect, in proportion to the smaller stars, if they did not 

 compose a part of the same system with ourselves, &c. 



• The apparent change of sitiiation that has been observed among a few of the stars, is a strong 

 circumstance in favour of those stars being some of the nearest to us. This apparent change of 

 situation may be owing either to the real motion of the stars themselves, or to that of the sun, or 

 partly to the one, and partly to the other. As far as it is owing to the latter (which it is by no 

 means improbable may in some measure be the case) it may be considered as a kind of secular 

 parallax, which, if the annual parallax of a few of the stars should some time or other be 

 discovered, and the quantity and direction of the sun's motion should be discovered also, might 

 serve to inform us of the distances of many of them, which it would be utterly impossible to find 

 out by any other means.— Orig. 



VOL. XII. . 3 K 



