226 Dr. North on ihe Angle of Aperture in Microscopes. 
the perfection of the human eye than this power of looking at 
points in the plane of the-exact focal distance, and yet at the 
same time receiving impressions appropriately indistinct from 
points within or beyond the focus. It is the same, thongh in less 
pefection, with a well corrected object glass of large angle; in 
proportion to the length of its focus, do points beyond the focal 
distance form suitable images. But upon this requisite more will 
e said presently. The power of exhibiting minute variations 
of light and shade thus being essential in reference to solid form 
and also being that chiefly by which lines are separated on @ 
plane surface, becomes obviously the most valuable quality of an 
objective. 
No object glass can be wholly freed from the two kinds of ab- 
erration. Of these the spheroidal is the most difficult to correct, 
particularly when the residuum of error is to be magnified by an 
eye-piece. ; 
Mr. Pritchard should therefore have explained that when an 
objective has a small aperture, and is at same time not well 
corrected, the more light is sent through it by increasing the 
illumination, the greater is the amount of uncorrected aberration, 
which has its confusing effect subsequently magnified with the 
rest of the image. But if additional brightness of the image and 
greater amount of light entering the eye is gained by enlargig 
the angular aperture, the lens must be corrected for this aperture 
or no definition is gained. The benefit from large aperture, then 
is as before, simply that of an increase of light. If the spherical 
and chromatic aberrations could be perfectly corrected, a stronger 
illumination would assist an objective of small aperture, precisely 
as it does the vaked eye, up to a surprising degree of intensity; 
while injurious extremes would be farther removed by suce 
in discrimination, and the pleasure of the exhibition. 
But images are formed more by the marginal than the central 
rays, as these are the most numerous; yet as spherical aberration 
increases in proportion as rays are further from the centel, the 
ity of the most perfect object glasses being as much superior for 
a stronger as for a weaker illumination. Dr. Goring repo 
that only the coarsest tests could be resolved by sun-light; 
present some rely upon it solely for those of extreme difficulty. 
* A striking illustration of the effect of large apertures, and of that degree of 
correction which is indispensable, was afforded by the surprise which some DUCT” 
* 
Lia 
