50 



MICROSCOPY OF THE LIVING EYE 



posterior or deep, i.e., distal from the observer, both these faces 

 being curved in conformity with the curves of the cornea ; and 

 two more faces, both of them lateral, eacg, the lateral face 

 proximal to the observer, and fbdh, the lateral face distal from the 

 observer. It will be noted that the superficial face abdc and the 

 proximal lateral face eacg, together with all their bounding 

 edges, are seen in unobstructed view ; but the remaining faces 

 efhg and fbdh, together with their remaining bounding edges, 



particularly the edge fh, are necessarily 

 seen less distinctly because they are viewed 

 through the illuminated substance forming 

 the " block " of the moment. 



It has been said above that the block 

 is uniformly illuminated throughout its 

 substance ; in the case of the human 

 cornea the relucency of the tissue within is 

 not quite so intense as the relucency of the 

 front and back surfaces ; hence the front 

 and back faces of the block are a little 

 brighter than the area which they enclose 

 between them. Pathological features seen 

 in the block stand out stereoscopically 

 with great precision in their relation to one 

 another when viewed under magnification 

 through the binocular microscope ; and 

 their absolute localisation as regards their 

 depth within the tissue — a very important matter in clinical 

 ophtlialmology — is very largely evident by their stereoscopic 

 relationship to the four-sided block ; but it is not desirable for 

 purposes of precise localisation to estimate depth solely on this 

 evidence. The exact estimation of depth should be made when the 

 observed feature is brought just within the proximal lateral face of 

 the block (eacg) ; thus, if a vessel is seen emerging from this face 

 of the block at a point just one-third of the distance between 

 front and back edges of the face, then, whether viewed mono- 

 cularly or binocularly, this estimation represents the exact level 

 of the vessel at this point between the front and back face of the 



Fig. 17. — Edges and 

 faces of the illu- 

 minated corneal 

 block. 



