> X 
on the Diameter of Optic Pencils ♦> j 0 ^ 
Exp. ii. Increafing the aperture of the objed-lens to ,0124,, 
I had a pencil of the 758th part of an inch, but could fee no 
better with it. 
Exp. 1 2. Proceeding in the track now pointed out to me, I ad- 
mitted an aperture of ,017, which gave a pencil of the 550th 
part$of an inch, but could fee not much better with it than 
before. 
Exp. 13. On a farther increafe of the aperture to ,0231,. 
and a pencil of the 406th part of an inch, I faw a little better ; 
but ftill had not diftindnefs enough even to fee the briftles 
before- mentioned at all. Hence we may conclude, that, in 
fuch conftrudions as the prefent one, the aperture of the objed- 
glafs muft bear a confiderable proportion to its focal length ; 
ftnce the 54th part (for ,0231 ; 1,25 :: 1 : 54) is here not 
nearly fufficient. 
Exp. 14. To the fame apparatus I applied a higher powery, 
by an exchange of the eye-glafs ; but the indiftindnefs re- 
mained as before. 
Exp. 1 5. Returning again to the former conftrudion, I ad- 
mitted an aperture of about ,037 ; and having now a pencil of 
nearly the 250th part of an inch, I could but juft perceive fome 
of the large briftles, which fhews that even the 34th part (for 
,037 : 1,25 :: 1 : 34) of the focal length is not a fufficient 
aperture for objed-lenfes that ad under fuch circumftances as 
the prefent. 
So far I have only related experiments that were made in the 
year 1778; and my opinion that the fmallnefs of the optic 
pencils could be no objedion to feeing well being thus fup- 
ported by evident fads, I hefitated not,. uv a Paper on the 
Parallax of the Fixed Stars (Phil. Tranf. vol. LXXII. p. 96.) 
to affirm, that we might fee diftindly with pencils much 
fm alien 
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