Parallax of the Fixed Stars* 87 
are very far from being perfectly known. Secondly, the 
change of pofition of the earth’s axis arifing from nutation, 
preceffion of the equinoxes, and other caufes, is fo far from 
being completely fettled, that it would not be very eafy to fay 
what it exactly is at any given time. In the third place, the 
aberration of light, though be ft known of all, may alfo be 
liable to fome fmall errors, fince the oblervations from which it 
was deduced laboured under all the foregoing difficulties. I do 
not mean to fay, that our theories of all thefe caufes of error 
are defective ; on the contrary, I grant that we are for moft 
agronomical purpofes fufficiently furniffied with excellent tables 
to correct our obfervations from the above mentioned errors. 
But when we are upon lb delicate a point as the parallax of the 
ftars ; when we are inveftigating angles that may, perhaps, not 
amount to a fingle fecond, we muft endeavour to keep clear of 
every poffibility of being involved in uncertainties ; even the 
hundredth part of a fecond becomes a quantity to be taken 
into confideration. 
I lhall now deliver the method I have taken, and fhew that 
it is free from every error to which the former is liable, and is 
ffill capable of every improvement the telefcope and mecha- 
nifm of micrometers can furnifh. 
Let OE (fig. 1 .) be two oppofite points of the annual orbit,, 
taken in the fame plane with two ffars h, of unequal magni- 
tudes. Let the angle a Ob be obferved when the earth is at 
O : and let the angle aF,h be alfo obferved when the earth is 
at E. From the difference of thefe angles, if any Ihould be 
found* we may calculate the parallax of the flars, according to 
a theory that will be delivered hereafter. Thefe two ftars, for 
reafons that will foon appear, ought to be as near each other as 
poffible,, 
