666 
PHARMACEUTICAL BOTANY 
The function of an objective is to take in the divergent light 
from the object and change it into convergent rays that meet in 
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cy ular 
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Cx 
LE LLL EE 
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Fic. 494.-——Sec- 
tional view showing 
component elements 
of an apochromatic, 
high power objective. 
the upper end of the microscope tube in the 
front focal plane of the ocular forming there a 
magnified image of the object. ‘This image is 
further magnified by the lens of the human eye. 
OsjecTivEs are either dry lenses or zmmerston 
lenses. If an air space be present between the 
tip of the objective and the object, the lens is 
called a dry one; if a liquid is present between 
the tip of the objective and the object, the lens 
is called an immersion lens. If this liquid be oil, 
the objective is called an ozl immersion objective; 
if water, a water immersion objective. 
Dry objectives are of low, medium and high 
powers. Dry medium and dry high power 
objectives are the best types to use on objects in air. 
When examining objects in water, glycerin or 
balsam with the dry objective, the cover glass 
should be slightly under 0.17 mm. in thickness 
for best results. 
The important properties of the microscope 
objective are: (1) Numerical aperture (N.A.) which 
measures the amount of light received from the 
object and represents the index of resolving 
power. (2) Resolving power is that by which two 
small elements in the structure of an object and 
which are only a short distance Aper are dis- 
tinctly separated. 
The higher the N.A. the greater the resolving 
power of the objective and the finer the detail it 
can reveal. 
N.A. equal W times sin u 
wherein 
n = the lowest refractive index that appears 
between the objective and the front of 
the objective. 
Fic. . 495.—A 
new type of ob- 
jective in a correc- 
tion mount. Turn- 
ing the  correc- 
tion collar varies the 
distance between 
the two upper lenses 
and the two lower 
lenses fixed in the 
body of the mount. 
= half the angular aperture of the objective. 
