USE OF THE MICROSCOPE 29 
when viewed without an eyepiece and illuminated with the widest 
cone of light they can take in. 
“The foregoing explanation shows the importance of the N.A. to 
the efficiency of an objective. 
“Tt is also evident that an objective cannot show its full efficiency 
if it is not used with a condenser of a N.A. large enough to fill the back 
of the objective with light.”’ 
The depth of sharpness of an objective is in inverse ratio to the 
N.A. great penetration goes along with a low N.A. and low penetration 
but great resolving power with a high N.A. 
Iliuminating Power. The brilliancy of the objective increases 
with the square of the numerical aperture of the objective. An objec- 
tive of 0.40 N.A. will give an image four times as brilliant as one of 
0.20 N.A., provided the magnification is the same and the full cone of 
the illumination is used in both cases. 
Magnifying Power. The magnifying power of an objective is in 
inverse ratio to its focal distance. An objective of 2 mm. focal distance 
will give, with the same ocular, a magnification eight times greater 
than one of 16 mm. focal distance. Numerical aperture and magnify- 
ing power are of little advantage if the definition is not good. 
Chromatic Aberration. This is due to the fact that a ray of white 
light passing from one medium to another of different refractive index 
at any angle other than 90° to the surface between them is refracted 
and dispersed into its component colors. 
Spherical Aberration. This is due to the fact that a spherical 
surface cannot bring a beam of light which passes through its vertex 
to the same focus as that of a beam of light passing through any other 
zone. 
Both aberrations are corrected by the use of different kinds of glass 
(crown and flint) combined as double and triple lenses in the objective. 
Neither can be corrected absolutely for all colors in an achromatic 
objective. Apochromatic objectives approach the ideally corrected 
objective almost to perfection. 
An objective can be tested for chromatic correction by using a 
narrow cone of oblique light and a coarse grating. Abbe’s test plate is 
best. Diatoms are good. No stained object should be used. 
If the spherical correction is perfect (see next paragraph) and one 
side of a line passing through the center of the field shows a clear, 
narrow greenish yellow border, while the other side is fringed with a 
violet red (secondary colors) the objective is chromatically corrected. 
The colors shown in the higher power objectives are of a more primary 
