TRANSPARENT TISSUES 



69 



the enterino^ slit-beam, is bv no means so readily visible as a small 

 white-looking " lens-opacity " thus vividly illuminated ; yet the 

 pigment may be for all practical purposes completely obstructive 

 to the passage of light and the " opacity " often but slightly so. 

 Certain forms of fluffy white " lens-opacities," whose visibility is 

 very bright indeed under the direct illumination of the slit-lamp, 

 scarcely show to any marked extent by the transmitted light of 



Fig. 40. — A bilateral chronic affection of the endothelial face of the 



cornea of elderly persons. 



retroillumination. It is a common fault for beginners to look 

 upon tissue-areas (e.g., the zones of disjunction of the lens) which 

 may be more relucent than the surrounding tissue (the general lens- 

 substance) as being more " opaque." The normal cornea by D.I. 

 is very relucent ; but it is not opaque. An ideally transparent 

 medium has unit opacity of which the density, the logarithm of 

 the opacity, is zero. Because in the optical section certain features 

 may appear vividly white, i.e., very visible, very relucent, it is 

 inaccurate to say that they are very " opaque " ; the word opacity 

 should not be used, scientifically, for features whose optical density 



