MANIPULATION OF THE MICROSCOPE. 35 
ter IV. The resolving power by which fine structures are 
made visible varies directly with the numerical aperture, 
as has been already explained. This power is ordinarily 
tested by examining the wing-scales of certain Lepidop- 
tera or the shells of diatoms which possess very fine 
markings. For example, the shell of Pleurosigma shows 
three systems of striations when examined under a mag- 
nification of 250. diameters with a numerical aperture 
of over .80, while two of them disappear with lesser aper- 
ture. Penetration, or the power to see clearly different 
planes of the object at the same time, varies inversely 
with the numerical aperture and directly with the square 
of the equivalent focus of the lens. Illuminating power 
“varies with the square of the numerical aperture, and 
with the square of the equivalent focus. 
Two common faults should be looked for in a micro- 
scope, curvature of the field and imperfect correction of 
chromatic and spherical aberration. If a stage microme- 
ter, marked with lines at right angles, be examined, the 
image should resemble a, Fig. 23. If it has the appear- 
ee 
| 
a b c 
Fic. 23.—CURVATURE OF THE Firetp. (After Hager-Mez.) 
ance of 6 or ¢ the lenses are so ground that the magnifica- 
tion is greater or less at the periphery than at the center. 
