VOL. LXXII.] PHILOSOPHICAL TRANSACTIONS. 20Q 



their surrounding planets there situated, which may perhaps occasion a gravita- 

 tion of our whole solar system towards it. If this surmise should have any foun- 

 dation, it will show itself in a series of some years; as from that motion will 

 arise another kind of hitherto unknown parallax,* the investigation of which 

 may account for some part of the motions already observed in some of the prin- 

 cipal stars; and for the purpose of determining the direction and quantity of 

 such a motion, accurate observations of the distance of stars that are near enough 

 to be measured with a micrometer, and a very high power of telescopes, may be 

 of considerable use, as they will undoubtedly give us the relative places of those 

 stars to a much greater degree of accuracy than they can be had by transit in- 

 struments or sectors, and thus much sooner enable us to discover any apparent 

 ^chano-e in their situation occasioned by this new kind of systematical parallax, if 

 I may be allowed to use that expression, for signifying the change arising from 

 the motion of the whole solar system. 



I shall now endeavour to deliver a theory of the annual parallax of double 

 stars, with the method of computing from it what is generally called the parallax 

 of the fixed stars, or of single stars of the first magnitude, such as are nearest 

 to us. It may be observed, that the principles on which I have founded the 

 following theory are of such a nature, that they cannot be strictly demonstrated, 

 in consequence of which they are only proposed as postulata, which have so great 

 a probability in their favour, that they will hardly be objected to by those who 

 are in the least acquainted with the doctrine of chances. 



General Postulata. — 1. Let the stars be supposed, one with another, to be 

 about the size of the sun.-f- 



2. Let the difference of their apparent magnitudes be owing to their different 

 distances, so that a star of the 2d, 3d, or 4th magnitude, is 2, 3, or 4 times as 

 far off as one of the first. t 



* See the note in the Rev. Mr. Mitchell's paper on the Parallax of the Fixed Stars. Phil. Trans, 

 vol. 57, p. 252.— Orig. 



+ See Mr. Mitchell's Inquiry into the probable Parallax and Magnitude of the Fixed Stars, Phil. 

 Trans, vol. 57 ; and Dr. Halley on the Number, Order, and Light, of the Fixed Stars, Phil. Trans, 

 vol. 31. — Orig. 



t The apparent magnitude is here taken in a stricter sense than is generally used; and by it is rather 

 meant the order into which the stars ought to be distinguished than that into which they are commonly 

 divided ; for as the order of the magnitudes is here to denote the different relative distances, we are 

 to examine carefully the degree of light each star is accurately found to have : a.id considering then 

 that light diminishes in the inverse ratio of the squares of the distances, we ought to class the stars 

 accordingly. An allowance ought also perhaps to be made for some loss that may happen to the light 

 of very remote stars in its passage through immense tracts of space, most probably not quite destitute 

 of some very subtle medium. This conjecture is suggested to us by the colour of the very small 

 telescopic stars, for 1 have generally found them red, or inclining to red ; which seems to indicate, 

 that the more feeble and refrangible rays of the other colours are either stopped by the way, or at 

 least diverted from their course by accidental deflections. — Orig. 



VOL XV. E E 



