OPTICAL SECTIONS 



51 



cornea. Similarly, in Fig. 16a, c, the spot of precipitate is proved 

 to be on the back face of the cornea if it occupies the position 

 drawn when it is in the plane of the lateral face of the block 

 proximal to the observer, whether the observation be made 

 monocularly or binocularly. If, instead of working in a " block " 

 of illuminated tissue, we narrow the slit so that the lateral faces 

 of the block come closer and closer to one another, we narrow the 

 width of the block until it becomes a thin lamina making what 

 Vogt has termed an " optical section." A comparison of corneal 



A B C 



Fig. 18. — A, Nebula of the cornea giving optical appearance of 

 flattening of the anterior face. B, Thin optical section through 

 nebula of the cornea. C, Thin optical section through thin 

 superficial '" bullae " of the corneal epithelium. 



sections w4th the corneal " block " (but with pathological altera- 

 tions) will be seen in Fig. 18, and another corneal section, with 

 pathological features, is shown in Fig. 44, D. It is always best to 

 use such thin optical sections for localisation of features under 

 direct illumination ; their primary detection, if by direct illumina- 

 tion, may be best done in either the wide beam in some cases, or 

 in the thin section in others, according to the nature of the feature 

 and of its immediate surroundings. 



