110 CONSTRUCTION OF THE MICROSCOPE. 



secondly, in the examination of objects under polarized light, a 

 class of appearances being produced by the rotation of the object 

 between the prisms, which is not developed by the rotation of 

 either of the prisms themselves. Below the stage, and in front 

 of the stem that carries the mirror, is a dovetail sliding bar, 

 which is moved up and down by the milled head shown at its 

 side ; this sliding bar carries what is termed by Mr. Ross the 

 " secondary stage" (omitted in the figure for the sake of simpli- 

 city), which consists of a cylindrical tube for the reception of the 

 achromatic condenser, the polarizing prism, and other fittings ; 

 to this secondary stage, also, a rotatory motion is communicated 

 by the turning of a milled head ; and a traversing movement of 

 limited extent is likewise given to it by means of two screws, 

 one on the front and the other on the left hand side of the frame 

 which carries it, in order that its axis may be brought into perfect 

 coincidence with the axis of the " body." The special advantages 

 of this instrument consist in its perfect steadiness, in the admirable 

 finish of its workmanship, and in the variety of movements 

 which may be given both to the object and to the fittings of the 

 secondary stage. Its disadvantages consist in the want of porta- 

 bility, necessarily arising from the * substantial mode of its con- 

 struction ; and in the multiplicity of its movable parts, which 

 presents to the beginner an aspect of great complexity. This 

 complexity, however, is much more apparent than real ; for each 

 of these parts has an independent action of its own, the nature 

 of which is very soon learned ; and the various milled heads are 

 so disposed, that the hand readily (and at last almost instinc- 

 tively) finds its way from one to the other, so as to make any re- 

 quired adjustment, whilst the eye is steadily directed to the ob- 

 ject. To the practised observer, therefore, this multiplication of 

 adjustments is a real saving of time and labor, enabling him to 

 do perfectly and readily what might otherwise require much 

 trouble, besides affording him certain capabilities which he would 

 not otherwise possess at all. 



38. Powell and Lealands Compound Microscope. This instru- 

 ment, represented in Fig. 28, is far lighter than the preceding in 

 its general "build," without being at all deficient in steadi- 

 ness ; it has not, however, some of those improvements for 

 which Mr. Ross's plan of construction is especially adapted. 

 The three-legged stand gives a firm support to the trunnions 

 that carry the tube to which the stage is attached, and from 

 which a triangular stem is raised, by the rack-and-pinion 

 movement set in action by the double milled head, whereby the 

 " coarse" adjustment of the focus is obtained. The triangular 

 stem carries at its summit the transverse arm, which contains 

 (as in Mr. Ross's Microscope) the lever action of the "fine" 

 adjustment ; and this is acted on by the milled head at the back 

 of the arm, whence also pass two oblique stays, which, being 

 attached to the upper part of the body, assist in preventing its 



