VOL. LXXVI.] PHILOSOPHICAL TRANSACTIONS. 1 67 



this construction I saw very distinctly every object I placed under the micro- 

 scope. 



Exper. 4. I reduced the aperture of the object-glass to .013; hence the pencil 

 was found to be the 724th part of an inch; and yet I saw with this construction 

 very distinctly every object that was placed under the magnifier. 



Exper. 5. I made a id reduction of the aperture of the object-glass, so that 

 now it was no more than .0052; and therefore the optic pencil less than the 

 1800th part of an inch; and yet I could very well count the bristles on the edge 

 of the wing of a fly, and distinguish their length and thickness. 



Exper. 6. Changing the construction of the microscope, I now reduced the 

 pencils by an increase of power. Solar focus of the eye-glass .32; distance be- 

 tween the object-glass and eye-glass 7.Q\ aperture the same as in the 3d experi- 

 ment. This gave me a pencil of the 336th part of an inch, with which I saw 

 very distinctly. 



Exper. 7. Applying now the reduced aperture of the 4th experiment, I had 

 a pencil of the 1 ISQth part of an inch, with which I saw very well. 



Exper. 8. I changed the eye lens for another of .171 focal length; the object- 

 glass and distance between the two lenses remaining as in the two last experi- 

 ments; aperture ,02. This gave a pencil of the 2173d part of an inch, with 

 which I could count, or rather successively see, the bristles before-mentioned 

 very well ; the field, on account of the great power, not taking in more than 2 

 large and a small one at a time. 



Exper. 9. I was now convinced, that we may see distinctly with pencils in- 

 comparably less than the 40th or 50th part of an inch ; and indeed so far from 

 expecting any obstruction to distinct vision from the smallness of the pencils, it 

 appeared now as if their size might in future be entirely left out of the account. 

 With a view however of seeing what other cause might bring on that indistinct- 

 ness which had been ascribed to the smallness of the optic pencils, I continued 

 these experiments with a variation in the apparatus, and used now an object lens 

 of a different focal length; the aperture and other particulars being as in the 

 4th experiment. By this construction, which gave me a pencil of the 724th 

 part of an inch, I could see objects very well ; but though they appeared dis- 

 tinctly, they were not so sharp on the edges aS one would wish to see them. 

 This being compared with the 4th experiment, it appeared that, with equal pen- 

 cils, unequal degrees of distinctness may take place; and a pretty striking cir- 

 cumstance, which served to lead, me in the following experiments, was, that the 

 smallest power gave me the least distinct image; notwithstanding, from former 

 trials, the goodness of the lenses I employed could not be doubted. 



Exper. 10. On an examination of circumstances it occurred to me, as indeed 

 I had already before surmised, that a certain proportion of aperture might be 



