8 
angular aperture might possibly bring out some new points ; 
and provided particular rings or annular areas of the objec- 
tive surfaces could be instantaneously compared, a further 
application might be made in the direction of testing the 
comparative aberrations of special areas. For the whole 
question of aberration hinges upon the unequal effects 
of these rings of surface. Such an instrument has been 
adapted by the writer, which gives apertures from one 
thirtieth to half an inch, twelve in number, and has three 
central stops for cutting off the central pencils. The aberram- 
eter being applied to the nose of the microscrope, into this 
the objective to be critically examined is inserted; various 
areas are successively cut off and the effects noted. ‘The 
unbalanced state of the aberrations in general, with proper 
tests, become at once apparent, and no adjustment of focus or 
*“ screw collar” gets rid of the defects. 
Experiment 8.—Objective’ one inch focal length, quality 
“good.” The definition of the central pencil is obscure. As 
aperture increases, so does the definition improve up to 
No. 9 (full aperture). Strie of azure blue, finest scales 
visible, best with outer ring of pencil; invisible with central 
rays, aperture No. 6 (600 diameters). 
Result.—Outer and inner rays are not in aplanatic co- 
operation. Aberration uncorrected. 
Experiment 9.—Dots on “battledore scales” intensely 
black with No. 7 aperture ; the black annuli are developed. 
Experiment 10.—Same object. Fine 4, full aperture, 
power 400. JBattledore bulbs translucent and ill-defined. 
Aperture reduced to No.1. The surface starts out in full 
relievo, shaded black and sharply marked. Direct light. 
All the perspective of an inch objective is obtained; the 
focal plate gains extraordinary depth; the surface resembles 
rather an intaglo than a flat-drawn picture; reduced to a 
flat surface, as usual, with one-fourth objective and full 
aperture. : 
From repeated experiments of this kind it appears that the 
concentric areas or rings of the objective surfaces seldom 
exhibit equal defining powers, that an even quality in the 
corrections is infrequent, and that careful focussing does not 
correct the definition. The contrivances of the aberrameter 
readily detect the aberrating annulus, if it is provided with 
a sufficient number of graduated stops to obliterate either 
the centrical or excentrical pencils; and it affords a new 
means of examining an object, so as to bring out its details 
in strong relief. No. 2 aperture evolves relievo details even 
