Parallax of the Fixed Stars . 89 
our prefent opticians make infruments that enlarge the difk of 
the fun above forty thoufand times ? What would even cassini 
fay, if he were to view the firft far of Aries, which appeared to 
him as fplit in two, through a telefcope that will fhew y Co - 
ronae borealis and h Draconis to be double f ars ? 
But to proceed, I fhall now prove that this method, if f ars 
properly ftuated (fuch as I have found) are taken, is free from 
all the errors occafioned by refraction, nutation, preceflion of 
the equinoxes, changes of the obliquity of the ecliptic, and 
aberration of light ; and that the annual parallax, if it even 
fhould not exceed the tenth part of a fecond, may fill become 
vifible, and be afcertained at leaf to a much greater degree of 
approximation than it ever has been done. 
It will alfo appear, from the great number of obfervations I 
have already made upon feveral double f ars, efpecially e Bootis, 
that we can now with much greater certainty affirm the annual 
parallax to be exceedingly fmall indeed ; and that there is a 
great probability of fucceeding fill farther in this laborious 
but delightful refearch, fo as to be able at laf to fay, not only 
how much the annual parallax is not , but how much it really 
is. 
Let there be two f ars at a dif ance from each other, not ex- 
ceeding five feconds ; fuppofe them to be obferved at an altitude 
of 20° and let them be fo fituated with refpeCt to each other, 
that one of them may be 20°, and the other 20° and 5" high : 
then the whole effect of mean refraction at that altitude, by Dr. 
maskelyne’s excellent tables, will be z' 35",$ for 20% and 
z' 35 // ,4888 for 20° 3". The difference i'so'^oiii. Now, in 
the firff place, we have nothing to do with the refraCtion itfelf, 
fince the real altitude of the f ars is not in queff ion. In the 
next place, we alfo have no concern with the difference of refrac- 
Vol. LXXII. N tion 
