SURFACES OF CORNEA AND LENS 



55 



The two corneal and the anterior lental surfaces are convex ; 

 the posterior lental surface is concave. The anterior corneal 

 surface is a highly efficient mirror — or rather the normal film 

 of moisture on its surface is such — because it is very smooth 

 and also because there is a relatively high difference between 

 the refractive index of the moisture on its surface and that 

 of the air in contact with it. The posterior corneal surface is a 

 less efficient mirror for the main reason that the difference between 

 the refractive indices of the cornea and the aqueous fluid is less 

 pronounced ; so we get a 

 dazzling bright view of any 

 area of the anterior corneal 

 surface seen bv D.I.S.R., and 

 a much duller view of the 

 posterior surface by D.I.S.R. 

 The anterior lental surface 

 viewed by D.I.S.R. is mode- 

 rately bright, but although 

 the lens- capsule is very smooth 

 the reflecting properties are a 

 little complicated probably 

 by the fact that immediately 

 beneath the very thin capsule 

 lie the living cells and close 

 next to these the lamellae of 

 lens-fibres, which doubtless 

 makes the surface reflection a composite one ; so that even the 

 normal anterior capsule viewed by D.I.S.R. presents a charac- 

 teristic patterning. The normal posterior lens-capsule has a fairly 

 bright uniform coppery or golden lustre by D.I.S.R. 



Of all these four surfaces the two which most nearly approach 

 mirror-perfection are thus the anterior corneal and the posterior 

 lental, the one convex and the other concave. The former forms 

 mirror images behind it, and the latter in front of it. When the 

 observer is searching about w4th the focusing adjustment of his 

 microscope to view one of these two surfaces by D.I.S.R., he will 

 frequently come upon a mirror-image of the diaphragm-aperture 



Fig. 20. — Observation of a small 

 focally illuminated area, x, of a 

 plane or curved surface along 

 different axes, that of O being 

 along the axis of specular reflection. 



