192 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE 



are not completely parallelised ; they may be regarded as wedges. 

 With that fact before us we can see how images approximate and 

 retire when the mirror is revolved. Let the surfaces A and B, 

 li-. I .">."). hnve ;ni inclination of 1 ; then, viewing a small object at E 

 (close to the eye), one image appears towards 1 i.e. at right angles 

 to A and another in the direction E 2, H from E 1, which, after 



being refracted to 1 in the 

 glass, is reflected at right 

 angles from surface B. 



If this mirror is re- 

 volved in the plane of A, 

 of course No. 1 image will 

 remain still, and No. 1 and 

 subsequent images will re- 

 volve with the mirror 

 round No. 1 . 



If we exaggerate the 

 wedge shape of our mirror, 

 we can see that at a par- 

 ticular angle these images 

 can be made to superim- 

 pose. In fig. 156 let the 

 signs be as before, and the 

 images whose rays pass re- 

 spectively from to 1 and 

 1 l will be reflected to E as 

 one image. The images 

 vary in size owing to the 

 various distances. No. 2 

 is the brightest except at 

 great obliquity. 



In practice we find that 

 these images may be obvi- 

 ated by rotating the mirror 

 in its cell until a certain 

 point is reached where all 

 the images will be super - 

 i 1 1 1 p< >sed. All mirrors should 

 lie so mounted as to admit 

 of this rotation. 



The present Editor is 



156. greatly in favour of tlte em- 



/iltu/uient of a rectangular 



prism CU\ with care and precision. \\> M- t , T l, v this means 'total 

 reflexion and no double reflexions; and he believes that finer images 



111 be obtained by its ans than with the plane mirror. It may 



mounted in th.- place o/ the plane mirror that is to say, the 



mirror may U- as usual in its cell -and in the other cell, 



ould have received the pi.-,,,,, ni j m ,r, the rectangular prism 



mounted and be capable of rotation as the plane mirror 



would nave 'MTU. 



lid, however. l> ( . noted il, ; ,t this applies only when the 



B 



FIG. 155. 



