C M M E N T A T I A S T R N M I C A. 93 



■» sooner enable us to discover any apparent cliange 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 molion of the wbole solar System. " 

 , Transit ad eiplicaadam tlieoriam annuae parallaxeos stellarum duplicium , fundatam 

 bis duobus postulatis. 



« 1. Let the Stars be supposed , one with anotber, to be about the size of the sun. 

 « (See Mr. Mitchell's Inquiry into the probable Parallax and Magnitude of the Fixed 

 « Stars, Phil. Trans, vol. LVII. p. 234. 236. 237. 240. and Dr. Ha Hey on theNumber, 

 « Order, and Light ,^ of the Fixed Stars, Phil. Trans, vol. XXXI.) 



; « 2. Let the difference of their apparent magnitudes be owing fo their different di- 

 « stances , so that a star of Ihe second , third, or fourth maguitude is two , tliree , or 

 « four times es far off as one of the first. (The apparent magnitude ia here takea in: 

 « a stricter sense ihan is generally used; and by it is ralher meant the order inlo which 

 « the Stars ought to be distinguished than that into wl;ich they are eommonly 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 bave s 

 « and considering then that light diminishes in the inverse ratio of the Squares of tbe 

 «' distances, we ought to class the stars accordingly. An allowance ought also perhaps 

 « to be made for some lofs that may happen to the light of very remote stars in ils 

 « passage through immense tracts of space , most probably not quite destitute of some 

 « Tery subtle medium. This conjecture is suggested to us by the colour of t'ie very small 

 « telescopic stars, for I have generally found tbem red, or inclining to red; whicli 

 « seeriK to indicate, that the more feeble and i-efrangible rays of the other colours are 

 « either stopped by the way , or at least diverted from their course by accideatal de- 

 « flections. "} 



« Let OE be the whole diamefer of the earth's aimual orbit ; and. let a , 6 ,. c , be three 

 K Stars situated in the ecliptic, in such a manner that they may be seen all in one Ime 

 « Oabc when the earth is at O. Let the line Qabc be perpeodicular to OE , and draw 

 « FE parallel to cO. Then, if 0«, ab, bc , are equal to each other, a will be a star 

 « of the first magnitude , b of the second , and c of the third. Let us now suppose Ihe 

 « angle OaE, or parallax of the whole orbit of the earth,. to be 1" of a degree: then 

 « we hnve PEö!=OaE rr 1": and' because very small angles, having. the same sublcnse 

 « OE, maybe- taken to be in the inverse ratio of the lines Oa , Ob, Oc, etc. we shall have 

 <. 06E = 1", OoE = f", etc. Nbw, when the earth is rcmovcd to E , we shall have 

 « P£6=rE60=|", and PEa — PE4 '= aSb = l" ; «hat is the stars a, b, will appear 

 » to be l" dislant. We also have PE^- = EcO = f " , and PEa — PEc = aEc = }"; 

 k tl.ai is,, tiie slars a, c„ will appear to be | ' distant, when the earth is at E. Now 



JH. a «sin- 



