12 HISTOLOGY OF MEDICINAL PLANTS 
centre of this tube there is a small diaphragm for holding the 
ocular micrometer. In the lower end of the tube a lens is fas- 
tened by means of a screw. This, the field lens, is the larger 
lens of the ocular. The upper, smaller lens is fastened in the 
Fic. 14. FIG. 15. 
Eye-Pieces. 
tube by a screw, but there is a projecting collar which rests, 
when in position, on the draw-tube. 
The longer the tube the lower the magnification. For 
instance, a two-inch ocular magnifies less than an inch and a 
half, a one-inch less than a three-fourths of an inch, etc. 
The greater the curvature of the 
lenses of the ocular the higher will be 
the magnification and the shorter the 
tube-length. 
FORMS OF COMPOUND 
MICROSCOPES 
The following descriptions refer to 
three different models of compound 
microscopes: one which is used chiefly 
as a pharmacognostic microscope, one 
as a research microscope stand, while 
the third type represents a research 
microscope stand of highest order, 
which is used at the same time for 
taking microphotographs. 
PHARMACOGNOSTIC MICROSCOPE 
The pharmacognostic microscope 
Fic. 16.—Pharmacognostic F : ‘ : 
Microscope (Fig. 16) is an instrument which 
