a new prifmatic Micrometer. 801 
to tliofe pencils, will be collected into one point upon the 
retina of the eye; and confequently, the two oppolite 
limbs of the two images of the Sun will feem to coin- 
cide, and the two images of the Sun to touch one ano- 
ther externally. But if the hate of the eye fhould alter, 
the place of the eye-glafs remaining the fame, the eye 
will be no longer difpofed to fee the image formed at the 
point a diftindtly, but to fee an object placed at ef, 
nearer to or farther from the objedt-glafs diftindtly ; and 
therefore an image will be formed on the retina exactly 
limilar to the fomewhat confufed image formed by the 
rays on a plane perpendicular to their courfe at ef. Con- 
/ 
fequently, as the two cones of folar rays, boa, cva, 
formed by the two femi-circles, are feparated or en- 
croach upon one another at this point of the axis by the 
diftance ef, the two images of the Sun will not feem to 
touch one another externally, but to feparate or to encroach 
upon one another by the interval ef. The error hereby 
introduced into the meafure of the Sun’s diameter will 
be the angle erf, fubtended by ef at r the middle point 
between o and v, which is to eaf or o av, the Sun’s ap- 
parent diameter, as ae to er, or even to ar, on account 
of the fmallnefs of ae with refpedt to ar. 
Thefe conliderations concerning the caufe of a prin- 
cipal error that has been found in the objedl-glafs micro- 
Vo l. LXVII. 5 H meter, 
