4?6 PHILOSOPHICAL TRANSACTIONS. [ANNO 1784. 



quantity of matter contained in several of the fixed stars; yet, if they have no 

 luminous satellites revolving about them, we shall still be at a loss to form any 

 probable judgment of their distance, unless we had some analogy to go on for 

 their specific brightness, or had some other means of discovering it; there is 

 however a case that may possibly occur, which may tend to throw some light on 

 this matter. 



37. I have shown in my Inquiry into the probable Parallax, &c. of the Fixed 

 Stars, in the Philos. Trans. 1767, the extremely great probability there is, that 

 many of the fixed stars are collected together into groups; and that the Pleiades 

 in particular constitute one of these groups. Now of the stars which we there 

 see collected together, it is highly probable, as I have observed in that paper, 

 that there is not one in a hundred which does not belong to the group itself; and 

 by far the greatest part therefore, according to the same idea, must lie within a 

 sphere, a great circle of which is of the same size with a circle, which appears 

 to us to include the whole group. If we suppose therefore this circle to be about 

 1° in diameter, and consequently only about a 30th part of the distance at which 

 it is seen, we may conclude, with the highest degree of probability, that by far 

 the greatest part of these stars do not differ in their distances from the sun by 

 more than about one part in 30, and thence deduce a sort of scale of the propor- 

 tion of the light produced by different stars of the same group or system in the 

 Pleiades at least ; and, by a somewhat probable analogy, we may do the same in 

 other systems likewise. But having yet no means of knowing their real distance, 

 or specific brightness, when compared either with the sun or with each other, 

 we shall still want something more to form a further judgment from. 



38. If however it should be found, that among the Pleiades, or any other like 

 system, there are some stars that are double, triple, &c. of which one is a larger 

 central body, with one or more satellites revolving about it, and the central body 

 should likewise be found to diminish the velocity of its light; and more especially, 

 if there should be several such instances met with in the same system; we should 

 then begin to have a kind of measure both of the distance of such a system of 

 stars from the earth, and of their mutual distances from each other. And if 

 several instances of this kind should occur in different groups or systems of stars, 

 we might also perhaps begin to form some probable conjectures concerning the 

 specific density and brightness of the stars themselves, especially if there should 

 be found any general analogy between the quantity of the diminution of the light 

 and the distance of the system deduced from it; as, for instance, if those stars, 

 which had the greatest effect in diminishing the velocity of light, should in ge- 

 neral give a greater distance to the system, when supposed to be of the same 

 density with the sun, we might then naturally thence conclude, that they are 

 less in bulk, and of greater specific density, than those stars which diminish the 



