on the Diameter of Optic Pencils , $oy 
Exp. 4. I reduced the aperture of the objedt-glafs to ,013;: 
hence the pencil was found to be the 724th part of an inch; 
and yet I faw with this conftrudtion very diftin&ly every object 
that was placed under the magnifier. 
Exp. 5, I made a fecond redu&ion of the aperture of the 
object- gl a fs, fo that now it was no more than ,0052; and' 
therefore the optic pencil lefs than the 1 8ooth part of an inch ; 
and yet I could very well count the briftles on the edge of tha- 
wing of a fly, and diftinguifh their length and thicknefs. 
Exp. 6. Changing the conftrudtion of the microfcope, I 
now reduced the pencils by an increafe of power. Solar focus 
of the eye-glafs ,52 ; diftance between the obje£t-glafs and' 
eye-glafs 7,6; aperture the fame as in the third experiments 
This gave me a pencil of the 336th part of an inch, with' 
which I faw very diftindtly. 
Exp. 7. Applying now the reduced aperture of the fourth 
experiment, I had a pencil of the 1139th part of an inch, with 
which I faw very well, 
Exp* 8. I changed the eye lens for another of ,171 focal* 
length; the objedhglafs and diftance between the two lenfes* 
remaining as in the two laft experiments ; aperture ,02. This* 
gave a pencil of the 2173d part of an inch, with which I 
could count, or rather fuccefiively fee, the briftles before-men- 
tioned very well; the field, on account of the great poweiv 
not taking in more than two large and afmall one at a time. 
Exp. 9. I was now convinced, that we may fee diftindtly 
with pencils incomparably lefs than the 40th or 50th f part of an 
inch; and indeed fo far from expedting anv abftruftion to 
diftindt vifion from the fmallnefs of the pencils, it appeared 
to me now as if their fize might in future be intirely left out 
of the account. With a view, however, of feeing what other 
caufe 
