1898.] MICROSCOPICAL JOURNAL. 121 



others employed by highly skilled microscopists ; but 

 not only are these too seldom employed by even the 

 best makers before the lens is sent forth, but we have 

 seen that even their necessity is disputed, and the im- 

 portance of spherical aberration itself actually chal- 

 lenged, by adherents of the "spectrum" theory as here- 

 tofore understood. 



When, however, we do employ adequate tests, and at 

 the same time make careful comparisons between one ob- 

 jective and another, we find that the perfect correction 

 of spherical aberrations h ]u.st all-imp(yrtant in determin- 

 ing how far we can go in using with that lens the hetero- 

 geneous illuminating cone which is so important for de- 

 picting true contours in our image, still preserving suf- 

 ficient resolution of minute structure, (We are here pos- 

 tulating sufficient opacity in the details, to dispense with 

 much of the aid we have seen to be often necessary in 

 hyaline subjects.) High-claKs moderate powers now 

 easily utilize their full aperture, with ground-glass illu- 

 mination. With high powers, the amount of this, or of 

 aplanatic cone possible, is in almost direct proportion to 

 the perfection of spherical correction. Few lenses over 

 0'60N.A. will, however, even yet hear more than three- 

 fourths of their aperture as direct light; many very good 

 ones only two-thirds. And objectives differ strangely. 

 In Zeiss's apochromatic series, the half-inch of ()-65 N.A. 

 and the i immersion of 1-40 t*tand out from the rest : 

 some rare specimens of the former will bear their full 

 cone, and occasionally an i of 140 has been used in pho- 

 tography with a cone of 1-10. Very recently there was 

 sent me for examination by Messrs Swift, ?. new English 

 1-12 apochromatic of 140, which was remarkably well 

 corrected spherically. A rough but very fair idea of the 

 spherical correction may be obtained almost immediately 

 by focussing a Podura test-scale with small cone and 

 then ascertaining how far the iris can be opened without 



