TERMS EMPLOYED. 83 



The mechanical parts of a microscope I shall illustrate 

 by a reference to fig. II, which exhibits the simplest 

 construction of a microscope with which I am acquainted. 

 It will be seen in the sequel to be the same as my achro- 

 matic engiscope, described in Chapter VI., but without 

 its apparatus and stand, which can be added at any time 

 by its possessor, and thus avoid the disadvantage usual 

 in the purchase of a cheap microscope, viz. that when 

 a better is required, the cheap one is too slight and 

 defective in its construction to form the basis of a supe- 

 rior instrument, and is, therefore, of necessity thrown 

 aside as useless. 



The Vertical Microscope, fig. 4, consists of a double 

 arm, n, a ; the former to receive the screwed end of the 

 compound body, d, fig. 8 (when employed as an engi- 

 scope), and the latter, a, to receive the single, doublet, 

 or triplet magnifiers, when used only as a microscope. 

 b is a triangular bar, with a rack cut on its posterior 

 edge, into which a pinion works, which is actuated by 

 turning the large milled head, s, which raises or depresses 

 it and the arm, o, n, and thus the adjustment of the 

 focus is effected, j is the stage on which the objects to 

 be viewed are placed, and these latter are kept down by 

 the forked piece, QV finger-springs, t. The mirror-socket, 

 m } slides upon the stem, c, and carries the mirror, o, 

 with its frame, p. Above the arm, a, n, is a milled head, 

 ff, which serves to tighten it in any position in which it 

 may be placed : e is the circular foot or stand. When 

 intended as a travelling instrument, for lightness and 



