MANIPUL/ITION 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- 



Fig. 23. — Curvature of the Field. (After Hager-Mez.) 



ance of & or c the lenses are so ground that the magnifica- 

 tion is greater or less at the periphery than at the center. 



