8 THE STRUCTURE OP 



much of the utility of a Microscope depends on 

 good object-glasses. Where they are faulty, the 

 image they form is also faulty ; and when these 

 faults in the first image are multiplied by the 

 power of the eye-piece, they become like the faults 

 of our friends when viewed through a similar 

 medium of great magnitude. 



A good object-glass may be known by its giving 

 a clear and well-defined view of any object we may 

 wish to examine ; while a bad lens may be equally 

 well known by the absence of these qualities. Tn 

 short, a badly constructed objective is more apt to 

 mislead than to guide the student, by the fictitious 

 appearances it creates appearances that may be 

 erroneously taken for realities, which have no exist- 

 ence in the object itself. The object-glasses of our 

 best opticians consist of several 

 lenses arranged in pairs, set in a 

 small brass tube. A screw at 

 one end serves to attach them 

 to the lower extremity of the 

 compound body, A. (Fig. 3.) 

 The body of the Microscope is 

 supported by a stout metal arm, 

 Z), into the free end of which it 

 screws. The opposite end of the 

 arm is secured to the stem, E, 

 by a screw, on which it moves 



Ffg.8. Object-GlasB. as on a P ivot ' B 7 this meanS 

 the tube of the Microscope can 



be turned away from the stage an arrangement 

 that gives this form of Microscope an advantage 

 over those that are not so constructed. To the 

 stem, E, which works up and down a hollow pillar 

 by rack-work and pinion, is attached the stage, G. 

 This, in its simplest form, consists of a thin flat 

 plate of brass, for holding objects undergoing ex- 



