18 THE AMERICAN MONTHLY [Jan. 



As a manufacturer of lenses of some years standing, 

 as well as a worker with a microscope, I have had ample 

 opportunities to carry out many experiments, which are 

 almost impossible for the ordinary microscopist to imag- 

 ine, and I have also had free access to almost any lens of 

 other makers which I desired to examine. A microscope 

 objective then should have larger aperture, but that ap- 

 erture is worse than useless unless it is properly corrected. 



Some time since I carried out a series of experiments 

 with the aperture of lenses relating to corrections. I 

 constructed a lens of 1.30 N. A., and at the back fitted 

 an iris diaphragm, which could cut out all the marginal 

 rays. Trying it on a test diatom I found no difference 

 appeared when I cut the aperture down to 1.1 N. A. 

 Below that the minute markings disappeared. The ob- 

 jective showed the markings just as well with the aper- 

 ture of 1.1 N. A., as with 1.30 N. A., so of course it 

 proved that the marginal rays were not sufficiently cor- 

 rected. I may say that this could only be observed 

 when using a condenser with an aplanatic cone. When 

 used with an Abbe condenser, with its enormous aberra- 

 tion, these facts were indistinguishable. Correcting the 

 lens yet farther, and using a perfectly achromatized con- 

 denser, the image was remarkable. 



The fact was soon impressed on me that for very fine 

 resolution aperture is useless unless it is corrected in all 

 its zones. Otherwise it had better be cut away. For as 

 cones are enlarged, faults of objectives are revealed, and 

 as the objective is more perfectly corrected faults in the 

 cone stand out more clearly. Consequently for high res- 

 olution the solid cone will no longer suffice. 



With increased and perfectly corrected aperture, the 

 flatness of field should be as important to the maker. 

 But this is very difficult to correct, and. it has always 

 been taken for granted that definition and flatness were 

 incompatible. A glance at my objectives will be a proof 

 to the contrary. — Journal N. Y. Mic. Society. 



