PROXIMAL ILLUMINATION 



63 



as a factor contributing to the visibility of the K.P. in Fig. 5 

 — by directing the hglit on to the white sclera at the edge of the 

 cornea — can conveniently be classified as a form of proximal 

 illumination ; it can appropriately be called the method of 

 " sclerotic scatter " (10). The light, focused in the region X 

 (Figs. 33 and 34), is scattered by the white sclerotic tissue ; some 

 of it, entering the cornea meridionally rather than radially, passes 

 right across the cornea, unable to make its escape because of suc- 

 cessive interior reflections dej^ending on the critical angle for the 

 medium. Reaching the 

 opposite side of the normal 

 cornea it manifests itself 

 in the sclerotic at the 

 limbus, there forming a 

 faint crescent of light 

 (Y, Figs. 33 and 34 ; and 

 Fig. 44A). If in its course 

 across the cornea the light 

 meets some abnormality 

 such as the scar of an 

 inflammatory focus or of 

 a perforation, or if any 

 part of one of the corneal 

 surfaces is deprived of its 

 optical individuality by 

 adhesion to it of some 



scattering body whose refractive index is not materially different, 

 then the light, escaping from the cornea, becomes visible. This 

 principle is familiar in certain advertisement signs made out 

 of plate glass through which light, unseen save where letters 

 are engraved on the glass, is directed longitudinally. The method 

 is appropriate only for naked-eye or low-power observation, but 

 it is a very useful method for the preliminary detection of 

 features. 



To summarise, w^e thus have the following methods of illumina- 

 tion of transparent or translucent tissues (10) : — 



D.I. : Direct illumination. 



Fig. 32. — Proximal illumination. 



