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664 PHARMACEUTICAL BOTANY 
lower one being the larger and known as the collective or field lens 
because it increases the field of vision. ‘The upper or smaller 
lens is called the eye lens. It magnifies the image formed by the 
objective. Midway between the field and eye lens is a perforated 
diaphragm, the object of which is to cut out edge rays from the 
image. It determines the size of the field of view. 
Fic. 492.—Illustrating the parts of a compound microscope. 
According to the system adopted by the maker, oculars are 
designated by numbers, as 1, 2, 3, 4, etc., or by figures which 
represent focal lengths, and by magnification numbers, as 7X, 
10, ete. 
9. The objectives, which screw into the bottom of the body. 
tube or nose piece. They consist of a system of two, three or 
more lenses, some of which are simple, others compounded of a 
