156 MICROSCOPIC ILLUSTRATIONS. 



ment gives us the power of examining small parts of 

 large bodies without detaching them from their wholes. 



Thus we can examine the contents of a vase of polypi, 

 or aquatic insects, or a small part of a large specimen of 

 a mineral, or a nosegay of flowers, with the insects 

 which inhabit them ; or with an erecting eye-piece we 

 may apply the instrument to a turning-lathe or some 

 piece of delicate machinery, and work upon it much 

 more comfortably than by the help of single lenses. But 

 for these purposes the space occupied by the stage must be 

 left free and open; and to obtain this accommodation the 

 bar of the microscope must swing round on its stand in 

 any direction, on a ball and socket joint, which con- 

 joined with a rotatory motion of the arm on the top of 

 the said bar, will almost always enable us to gain the po- 

 sition required. An instrument which does not possess 

 all these properties will only do half the work it ought to 

 perform. It must be evident, also, that this construc- 

 tion gives the utmost facility for introducing a lamp or 

 candle, either close behind the stage for transparent 

 bodies, or before it for opaque ones ; and this arrange- 

 ment is a point of the last moment for demonstrating 

 and verifying a variety of objects, particularly proof 

 opaque ones, as I shall shew in due season, when I come 

 to treat on them and on the subject of verification. No 

 microscope which is from any cause lame or impotent, 

 either in its optical or mechanical construction, can be 

 fit for such purposes. 



