48 



MICROSCOPY OF THE LIVING EYE 



path in so clear a manner as to demarcate a sectional area of tissue 

 affording a very precise means of localising the level of the K.P. 

 and of other such features. This sectional path of the beam is 

 shown in Fig. 15, and in perspective in Figs. 16, 18, 25, 44, 46, 49, 

 50, etc ; but it must be understood from the beginning that this 

 sharp-cut clear sectional illumination occurs only at the precise 

 focal region of the beam ; i.e., if the observer wants this effect in 

 the cornea or in the anterior part of the lens or in the posterior 



part of the lens, he 

 must so adjust the 

 beam that its focal 

 region intersects the 

 area concerned. This 

 is shown in Fig. 15, 

 where the cornea is 

 intersected by the 

 focal region ; but in 

 Figs. 6 and 7 this 

 focal property of the 

 illuminating beam 

 has not been de- 

 picted. A sharp-cut 

 illumination effect, 

 as depicted in 

 Fig. 16, in both, e.g., 

 the cornea (C) and 

 lens (L) at the same 

 time, is not obtained 

 in practice any more 

 than focused observation through the microscope is obtain- 

 able at more than one level of tissue at the same time. The 

 beginner should grasp this essential fact from the outset and 

 learn to manipulate the focusing adjustment of the lamp, 

 finding and maintaining his levels, just as he docs when 

 focusing for observation through a microscope ; in slit-lamp 

 microscopy one hand is always controlling the directing and focus- 

 ing of the illumination and the other hand is controlling the 

 microscope. The illumination-effect produced in cither cornea or 



Fig. 15. — Illumination-beam traversing cornea 

 and lens ; observation of a spot of K.P. by 

 direct illumination. 



