192 THE HISTOKY AND DEVELOPMENT OF THE MICROSCOPE 



FIG. 154. 



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, 

 fig. 155, have an 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, 1^ 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. 2 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 

 2 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- 

 imposed. All mirrors should 

 be so mounted as to admit 

 of this rotation. 



The present Editor is 

 greatly in favour of the em- 

 ployment of a rectangular 

 prism cut with care and precision. We get by this means total 

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

 can be obtained by its means than with the plane mirror. It may 

 be mounted in the place of the plane mirror that is to say, the 

 concave mirror may be as usual in its cell and in the other cell, 

 which would have received the plane mirror, the rectangular prism 

 may be mounted and be capable of rotation as the plane mirror 

 would have been. 



It should, however, be noted that this applies only when the 



FIG. 155. 



F IG 156 



