36 



OPTICAL PRINCIPLES OF THE MICROSCOPE. 



Fig. 23. 



the observer. This is accomplished by the use of two Prisms, one of 

 them (Fig. 22, a) placed over the cone of rays proceeding upwards 

 from the Objective, and the other (b) at the base of the secondary 

 or additional body, which is here placed on the right (Fig. 23). 

 The Prism a has its upper and lower surfaces parallel ; one of its 

 lateral faces inclines at an angle of 45°, whilst the other is vertical. 

 When this is placed in the position 1, so that its inclined surface 

 lies over the left half (I) of the cone of rays, these rays, entering 

 the prism perpendicularly (or nearly so) to its inferior plane sur- 

 face, undergo total reflexion at its oblique face, and being thus 



turned into the horizontal direction, 

 emerge through the vertical surface 

 at right angles to it. They then enter 

 the vertical face of the other Prism B ; 

 and after suffering reflexion within 

 it, are transmitted upwards into the 

 right -hand body, r\ passing out of 

 the prism perpendicularly to the plane 

 of emersion, which has such an inclina- 

 tion that the right-hand or secondary 

 body (r, Fig. 23) may diverge from 

 the left or principal body at a suitable 

 angle. On the other hand, the right 

 half (r) of the cone of rays passes 

 upwards, without essential interrup- 

 tion, through the two parallel surfaces 

 of the prism A, into the left-hand body 

 (V), and is thus crossed by the other 

 in the interior of the prism. But if 

 the Prism A be pushed over towards 

 the right (by pressing the button a, 

 Fig. 23), so as to leave the left half 

 of the Objective uncovered (as in Mr. 

 Wenham's arrangement), that half 

 (I) of the cone of rays will go on 

 without any interruption into the 

 Ze/£-hand body (Z'), whilst the right half (r r') will be reflected 

 by the oblique face of the prism into the horizontal direction, 

 will emerge at its vertical face, and being received by the 

 second prism, B, will be directed by it into the right-hainA. 

 body (?"'). The adjustment for the distance between the axes 

 of the Eyes is made by turning the milled-head 5, Fig. 23, 

 which, by means of a screw-movement, acts upon a movable 

 chariot that carries the prism B and the secondary body r, the 

 base of which is implanted upon it. — Now in the^?^ position, the 

 two halves of the cone of rays being made to cross into the opposite 

 bodies, true Stereoscopic relief is given to the image formed by their 

 recombination, just as in the arrangements previously described. 



Nachet's Stereo-pseudosco 

 pic Microscope. 



