58 



MICROSCOPY OF THE LIVING EYE 



of the single-layered flat hexagonal endothelial cells (Fig. 25). This 

 view of the endothelium by D.I.S.R. is to be seen almost at the same 

 time as, and — in the course of the illuminating beam — close behind, 

 the area of the epithelial surface also under D.I.S.R. ; but the two 

 views are not quite coincident because the radii of curvature of the 

 two corneal surfaces are not quite equal. Thus it follows that 

 under conditions in which a small area of the epithelial (anterior) 

 surface is being correctly viewed by D.I.S.R., the corresponding 

 area of the endothelial (posterior) surface in the illuminating beam 



WCRO' \ I 



Fig. 23. — Specular reflection of an incident slit-beam, I, from the 



endothelial face of the cornea. 



just falls short of being so disposed as to be seen effectually by 

 D.I.S.R., the axes of the reflected rays from the two being not quite 

 parallel ; and vice versa when an area of the endothelial surface is 

 under examination by D.I.S.R. This fact facilitates viewing of 

 the endothelial surface, since the dazzling from the bright anterior 

 surface would be very confusing if it were unavoidably seen to full 

 effect at the same time. Although specular reflection occurs from 

 all the area of the posterior (Fig. 23) and anterior corneal surface 

 upon which the focused beam is incident, the area seen by D.I.S.R. 

 at any one time is never a large one because the curvature of the 

 cornea prevents rays, specularly reflected off any more than a 

 small area, from collectively following a mean path approximating 



