316 



THE MICROSCOPE AND ITS REVELATIONS. 



out in the ordinary small Continental model, is shown in Fig. 493; 

 whilst, on the other hand, Fig. 494 represents the largest and most 

 complete form of the instrument. In this last, the upper part of the 

 body, carrying the eye-piece and analyzing prism, can be raised or low- 

 ered by the pinion attached to the fixed arm that carries it. At M, im- 

 mediately beneath the eye-piece, is a small mirror, so placed as to illu- 

 minate the cross-wires when the field is dark. The analyzing prism is 



FIG. 494. 



Nachet's Large Mineralogical Microscope. 



inserted at A, in such a manner as to allow of being readily withdrawn 

 when its action is not required. The Stage, with its traversing object- 

 platform D, is made to rotate in the optic axis by the pinion E; which 

 can be thrown out of gear so as to enable the rotation to be made by 

 hand; and the object-platform which is graduated in both directions, is 

 fitted with a square against which the slide abuts, so that any particular 

 point in a section, whose place has been once noted by the scales, can be 

 readily found again. The Polarizing prism N, is mounted quite inde- 



