LOCALISATION BY DIRECT ILLUMINATION 49 



lens by the slit-beam is shown in Fig. 16a, and by the small 

 round beam in Fig. 16b, it being understood that the sharp foeal 

 effect shown would not })c obtainable in both cornea and lens 

 at the same time. 



Dealing now with 

 the cornea, it will 

 be seen that the area 

 traversed bv the 

 small round focal 

 beam has the form 

 of a clear-cut solid 

 cylinder (Fig. 16b : c) 

 with front and back 

 ends well defined by 

 the anatomical faces 

 of the cornea ; when 

 the focused slit-beam 

 is used the sectionally 

 illuminated area ap- 

 pears as a solid 

 curved four - sided 

 block (Fig. 16a : c) ; 

 and the relationship 

 of the spot of K.P. 

 to this traversing 

 cylinder or block, 

 particularly w h e n 

 judged with the 

 added aid of binocu- 

 lar stereoscopic 

 observation, affords a very precise means of localisation of the level 

 of the K.P. The corneal block is diagrammatically represented in 

 Fig. 17 ; the sharpness of the actual outlining is not exaggerated, 

 but the diagram differs from the reality in not showing uniform 

 internal illumination of the relucent area enclosed bv the outlines. 

 The block has two coronal faces, abdc, anatomically anterior or 

 superficial, i.e., proximal to the observer, and efhg, anatomically 



Fig 16 — A : Wide slit-beam ; B : Small circular 

 beam, illuminatinij a spot of K.P. on the deep 

 face of the cornea. Beam in cornea (c) and 

 lens (l). 



