Parallax of the Fixed Stars . 105 
or fourth magnitude is two, three, or four times as far off as 
-one of the firft *. 
In fig. y. let OE be the whole diameter of the earth’s an- 
nual orbit ; and let a , b , c, be three liars htuated in the eclip- 
tic, in fuch a manner that they may be feen all in one line O abc^ 
when the earth is at O. Let the line Qabc be perpendicular 
to C)E, and draw PE parallel to eO. Then, if Oa , ah, be, 
are equal to each other, a will be a ftar of the firffc magnitude, 
b of the fecond, and c of the third. Let us now fuppofe the 
angle CLE, or parallax of the whole orbit of the earth, to be 
1" of a degree : then we have PE a — 0 ^?E= i r . : and, becaufe 
very fmall angles, having the fame fubtenfe OE, may be taken 
to be in the inverfe ratio of the lines Oa , Ob, Oc, &c. we (hall 
have ObE=p\ OcE = L, &c. +. Now, when the earth is 
removed 
* The apparent magnitude is here taken in a ftridier fenfe than is gene, •■illy 
ufed ; and by it is rather meant the order into which the ftars ought to be diiiin- 
guifhed than that into which they arc commonly divided : for as the order of the 
magnitudes is here to denote the different relative ditlances, we are to examine 
carefully the degree of light each ftar is accurately found to have : and conlidering 
then that light diminifhes in the inverfe ratio of the fquares of the diilances, we 
ought to clafs the flars accordingly. An allowance ought alfo perhaps to be made 
for fome lofs that may happen to the light of very remote liars in its paffage 
through immenfe tradls of fpace, moll probably not quite deftitute of fome very 
fubtle medium. This conjecture is fuggelled to us by the colour of the very 
fmall telefcopic ftars, for I have generally found them red, or inclining to red ; 
which feems to indicate, that the more feeble and refrangible rays of the other 
colours are either flopped by the way, or at leaft diverted from their courfe by 
accidental deflections. 
f This proves what I have before remarked on the parallax of y Draconis ; for 
that liar, (admitting it to he a ftar of between the fecond and third magnitude, 
which ought to be afeertained by experiments, as mentioned in the note above) 
by the poftulata, will have its place aftigned fomewhere between b and c , 
and therefore its parallax will be between | and | of the parallax of a ftar of the 
firft magnitude. And if Dr. bradley thought that he fliould have perceived a 
VoL. LXXIL P parallax 
