668 PHARMACEUTICAL BOTANY 
Two objectives of the same equivalent focal length (E.F.) and 
the same N.A. should show the same illuminated area in the 
back lens, when viewed without an eyepiece and illuminated 
with the widest cone of light they can take in. 
It also is evident that an objective cannot show its full effi- 
ciency if it is not used with a condenser of an N.A. large enough 
to fill the back of the objective with light. 
DeptH oF Focus.—Depth of focus (known also as depth of 
sharpness of penetration) depends on the N.A. and the magnifi- 
cation and is inversely proportional to both. The higher the 
N.A. and the higher the magnification, the less the depth of 
focus. An increase of the depth of focus, for instance, by insert- 
ing diaphragms above the back 
lens of the objective must neces- 
sarily decrease the effective diam- 
eter of the back lens and thus 
decrease the N.A., thereby lower- 
ing the efficiency of the objective. 
: | To comply with different re- 
Fic. 497.—Aplanatic condenser (Zeiss) quirements in this respect the 
N.A. 1.4. Focal length 10.5 mm. A 
makers offer two 4 mm. objec- 
tives, whose depths of focus, when used with the same magnifica- 
tion, are in the ratio of 3:4. 
It should be borne in mind that the finer the definition of an 
objective the more sensitive it is to incorrect focusing and to 
slight changes of the fine adjustment screw. This means less 
depth of focus. Objectives that show greater depth of focus 
than others of the same N.A. cannot be well corrected. _ 
Working distance of an objective is sometimes confused with the 
equivalent focus. It varies for different powers being dispropor- 
tionately less for high powers, and is measured from the front of 
the objective to the upper surface of the cover glass. For 
instance, the high dry objective (4 mm. ef. 0.65 N.A.) has a 
working distance of 0.6 mm. 
Some microscopes are fitted up with a nose-piece, capable of 
carrying two, three, or four objectives, which may be revolved 
into place at the lower end of the body tube. Others have a 
condenser which is employed to concentrate the light upon the 
