Parallax, of the Fixed Stars. 99 
tmfkiifhed as they are, that every perfon who is Inclined to en- 
gage in this purfuit may become a fellow-labourer. 
In fettling the diftances of double ftars I have occafionally 
ufed two different ways. Thofe that are extremely near each 
other mav be eftimated by the eye, in meafures of their own 
apparent diameters. For this purpofe their diftance fhould not 
much exceed tw r o diameters of the largeft, as the eye cannot fo 
Well make a good eft imation when the interval between them is 
greater. This method has often the preference to that of the 
micrometer : for inftance, when the diameter of a fmall ftar, 
perhaps not equal to half a fecond, is double the vacancy be- 
tween the two ftars. Here a micrometer ought to meafure 
tenths of feconds at leaft, otherwife we could not, with any 
degree of confidence, rely on its meafures; nay, even then, if 
the ftars are fituated in the fame parallel of declination and 
near the equator, their quick motion acrols the micrometer 
makes it extremely difficult to meafure them, and in that cafe 
an eftimation by the eye is preferable to any other meafure ; 
hut this requires not a little pra&ice, precaution, and time, 
and yet with proper care it will be found that this method is 
capable of great exadtnefs. Let two fmall circles be drawn 
either equal or unequal, at a diftance not exceeding twice the 
diameter of the largeft ; let thefe be fhewn to feveral perfons 
in the fame light and point of view. Then, if every one of 
them will feparately and carefully write down his eftimation of 
the interval between them, in the proportion of either of their 
diameters, it will be found upon a comparifon that there vrili 
feldom be fo much as a quarter of a diameter difference be- 
tween all the eftimations. If this agreement takes place with 
fo many different eyes, much more may we expedf it in the 
O 2 eftimations 
