oj Double Stars. j.59 
be extremely detective. Nor could his micrometer give the 
declination much better unlefs the telefcope had bore a power 
ot at leaft 4 or 500. When the angle of pofition is but fmall, 
fuch as 3, 4, 5, or 6 degrees, and the diftan.ee of the ftars not 
above a few feconds, it is evident, that a micrometer muft be able 
to meafure tenths of a fecond at leaft to give even a tolerable 
exactnefs of pofition* On the contrary, the pofition being 
meafured with fuch a micrometer as 1 have conftruCted for the 
purpofe, we may from thence deduce the declination, with 
great confidence,, true to a. quarter of a tenth of a fecond for 
every fecond of the diftance of the ftars. 
Mr., mayer’s account of es Geminorum, for inftance, gives 
a difference of o ",7 of time in JR, of 3", 8 in declination, 
and of 1 to 6 in magnitude or degree of light of the ftars.. 
Thele quantities reduced to my notation, and compared with 
my mealures- of the fame ftar, give 
tance9",635 from center to center 5”, 156 diameters included. 
Pofit’on 23 0 14' n. preceding 
y 
sj 
Magnitude extremely unequal 
c , 32 0 47' n. preceding. 
t— 1 ! 
A little unequal. 
To account for this difference I aferibe Mr. mayer’s error in 
diftance to his method of meafuring by time. The error of 
pofition follows always from an obiervation of the declination 
taken with the common micrometer, when it is deduced from 
an erroneous JR . In my meafures the diftance and pofition 
are independent of each other, which I look upon as no fmall 
advantage of my crofs-hair micrometer. The error in the magni- 
tudes of the ftars I aferibe to the want of power in Mr. 
mayer’s telefcope, which did not feparate the ftars far enough 
for him to judge accurately of their fize, otherwife he would 
foon have found,, that inftead of five there is hardly fo much as 
one 
