Parallax of the Fixed Stars. ioi 
i Bootis, cannot be diftinclly perceived to be fairly feparated. 
becaufe it is almoft as fine a miniature of i Bootis as that is of 
a Geminorum. If the obferver has been fuccefsful in all thefe, 
he may then, at the fame time, try h Draconis, though I 
queftion whether any power leis than 4 or 500 will (hew it to 
be double ; but the former I have all feen very well with 2 2~. 
To try the ftars of unequal magnitudes it will be expedient to 
take them in fome fuch order as the following : a Herculis, 
u Aurigae, $ Geminorum, k Cygni, e Perfei, and b Draconis ; 
from thefe the obferver may proceed to a moil beautiful object, 
e Bootis, which I have, clofely attended thefe two years as very- 
proper for the inveftigation of the parallax of. the fixed ftars. 
It appears,, from what has. been laid, that thefe double ftars 
are a moft excellent way of trying a telefcope ; and as the fore- 
going remarks have fuggefted the method oi feeing how far the 
power and diftindtnefs of our inftruments will reach, I fhall 
add the way of finding how much light we have. The ob- 
ferver may begin with the pole-ftar and a Lyras ; then go to the 
ftar fouth of s Aquilae, the treble ftar near k Aquilae, and laid 
of all to the ftar following 0 Aquilaa. Now, if his telefcope 
has not a great deal of good diftinct light, he will not be able 
to fee fome of the fmall ftars that accompany them. 
In the fecoixd clals of double ftars I have put all thofe that 
are proper for eftimations by the eye or very delicate meafure* > 
of the micrometer. . To compare the diftances with the appa- 
rent diameters the power of the telefcope fhould not be much 
lefs than 200,. as they , will otherwil'e be too clofe for the pur- 
pofe. The inftrument ought, moreover, to be as much as 
poflible free from rays that furround a ftar in common tele- 
fcopes, and Ihould give the apparent diameters of a double ftar 
perfe&ly round and well-defined, with a deep black divifion 
between . 
