8 



THE STRUCTURE OF 



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 grreat 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. In 

 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, 

 D, into the free end of which it 

 screws. The opposite end of the 

 arm is secured to the stem, B, 

 by a screw, on which it moves 



Fig. 3. Object^lass. ^« ^^ ^ P^^^^' ^J ^^^^ "^^^^^ 



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, U, 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- 



