TERMS EMPLOYED. 81 



though it has been objected to, from the liability of their 

 becoming loose by wear ; the reverse of which, however, 

 occurs in practice, for as the tubes become oxidized they 

 tighten. 



The eye-pieces usually consist of two lenses, as shown 

 at a and b, fig. 9. The first, or anterior lens, is called the 

 eye-glass ; the second, b, the field-glass. As I have made 

 some eye-pieces of quartz, it would be more appro- 

 priate to call them eye-lens, or field-lens. 



The diaphragm, stop, or field-bar, is shown at c, and 

 should be in the focus of the eye-lens, a. The lens 

 which screws into the body at d, is called the object- 

 glass; it may be composed of more than one lens: 

 they are termed collectively objectives or object -glasses , 

 if they consist of three pairs, which is the usual 

 form for the best deep achromatics, as shown at fig. 

 Fig. 10. 10, where o is the place for the object (that 

 being their combined acting focus) d is 



e < o the anterior or first pair of object-glasses 

 e, the middle pair and /, the third or posterior pair 

 of objectives. As the inner surfaces of each pair are 

 cemented together, to an ordinary observer they appear 

 as one lens, and consequently the objective appears to 

 be composed of only three lenses. 



Over the object-lenses is often fitted a concave silver 

 reflector, for throwing condensed light on opaque objects ; 

 this is termed the Leiberkuhn, silver cup, or speculum, and 

 is shown at Plate 4, fig. 17- 



The power of eye- pieces is determined by the focal 



