W. B. STOKES ON MULTIPLE IMAGES IN MIRRORS. 



323 



position when the mirror is revolved in the plane of A. They 

 cannot. The mirror A B has parallel surfaces. Microscope 

 mirrors and most plate-glass mirrors are not parallelised, but 

 are, at the best, " optically" flattened, and may be regarded as 

 wedges. 



It is then easily seen how images approximate and retire 

 when the mirror is revolved. 



A. 



B. 



Let us give surfaces A and B an inclination of 1° (Fig. 2). 

 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. 



There is another image nearer the letter A, but, as it follows 

 the same laws apparently as the others, save that it is a real 

 double reflection, we need not consider it. 



If this mirror is revolved in the plane of A, of course "No. 1 

 image will remain still, and No. 2 and subsequent images will 

 revolve with the mirror round ISTo. 1. 



If we exaggerate this wedge shape of our mirror, we can see 

 that at a particular angle these images can be made to superim- 

 pose. 



B 



Let the signs be as before (Fig. 3) and the images whose 



