Ch. I] 



OBJECTIVES ink III i: MICROSCOPIC 



17 



Optical Parts of a Compound Microscope 

 § 18. Objective. — This is a lens, or combination of lenses, which, 

 under the proper conditions, produces an enlarged, inverted image 

 of some object (tig. 11, 20). 



Fig. 19. Adjustable Lens Holder with Joixts. 



Base. The heavy base supporting the lens holder. 



Coarse Adjustment The rack and pinion for focusing the lens. 



Joint, Joint The joints enabling one to put the lens in any desired position. 



Lens This is held in a spring fork or in a socket. 



Practically all microscopic objectives are composed of one or of 

 several combinations of lenses. The purpose of combining the 

 lenses is to produce an image as nearly as possible like the object 

 itself, by doing away with certain defects or aberrations inherent in 

 simple lenses (fig. 21). 



