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 



Fig. 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 



Fig. i6.-Pharmacognostic . The pharmacognosy microscope 



Microscope (Fig. 16) is an instrument which 



