426 i'HILUSOPHICAL TRANSACTIONS. [aNNO IjOj. 



As far then as we can guess of the parallax at the fixed stars from the prin- 

 ciples above laid down, we may reasonably expect that it should be exceedingly 

 small, even in those of the first magnitude; yet, besides the probability that 

 some of them may be either less, or less luminous than the sun, it is not so 



all as to leave us altogether without hopes, that we may some time or other 

 be able to discover it in some of them ; for it appears not impracticable to con- 

 struct instruments, capable of distinguishing even to the 20th part of a second, 

 provided the air will admit of that degree of exactness ; but such instruments 

 must be on a plan a good deal different from those hitherto made use of, as they 

 would otherwise be, not only vastly too expensive, but also much too large and 

 unwieldy to be of any use. 



But whatever room there may be to hope, that we may some time or other 

 be able to discover the parallax of a few among the fixed stars, yet at tlie same 

 time it seems probable that we shall never be able to discover any sensible mag- 

 nitude in their apparent diameters, which in Sirius himself, if he is not of less 

 native brightness than the sun, must be considerably less, whatever be his pa- 

 rallax, than the 1 00th, probably than the 200th part of a second; so that it 



ness with that of our common firesj but the sun's light exceeds tiie light of our brightest fires in so 

 very great a proportion, viz. of some thousands to one, that we want some middle terms to be able 

 to form any analogy, which might serve to carry us further. We find however in general, that those 

 fires which pioduce the whitest light, are much the brightest, and that the sun, which produces a 

 whiter light than any fires we commonly make, vastly exceeds them all in brightness; it is not there- 

 fore improbable, from this general analogy, that those stars, which exceed the sun in the whiteness 

 of their light, may also exceed him in their native brightness; now this is the case with regard to 

 many of them; and, on the contrary, there are some that are of a redder colour. 



If however it should hereafter be found, that any of the stars have others revolving about them, 

 for no satellites shining by a borrowed light could possibly be visible, we should then have the means 

 of discovering the proportion between the light of the sun, and the light of those stars, relatively 

 to their respective quantities of matter; for in this case, the times of the revolutions, and the 

 greatest apparent elongations of those stars, that revolved about the others as satellites, being known, 

 the relation between the apparent diameters and the densities of the central stars would be given, 

 whatever was their distance from us; and the actual quantity of matter which they contained would 

 be known whenever their distance was known, being greater or less in the proportion of the cube of 

 that distance. Hence, supposing them to be of the same density with the sun, the proportion of 

 the brightness of their surfaces, compared with that of the sun, would be known, from the compa- 

 rison of the whole of the light which we receive from them, with that which we receive from the 

 sun; but, if they should happen to be either of greater or less density than the sun, the whole of 

 their light not being affected by these suppositions, their surfaces would indeed be more or less lumi- 

 nous, accordingly as they were, on this account, lessor greater; but the quantity of light corres- 

 ponding to the same quantity of matter, would still remain the same. 



The apparent distances, at which satellites would revolve about any stars, would be equal to the 

 semiannual parallaxes of those stars, seen from planes revolving about the sun, in the same periodical 

 times with themselves, supposing the parallaxes to be such as they would be, if the stars were of the 

 same size and density with the sun. — Orig. 



