68 



MICROSCOPY OF THE LIVING EYE 



not available for localisation, the observer's retinal disparation 

 being too gross, though it may or may not play a part in revealing 

 the mutual relationship of the multiple features to one another. 

 The level of features seen in the cornea bv retroillumination is 

 best ascertained by the relationship to some chance feature on a 

 surface visible at the same time, e.g., a tear " bubble " on the 

 epithelial surface, or a spot of pigment on the back of the cornea 

 after senile detachment from the iris. One other means is available : 

 to arrange a close angle of the illumination and observation axes 

 and to endeavour to produce within the same field at the same 

 time both a view by R.I. and also an illuminated pencil or D.I. 



A B 



Fig. 39. — A, Correct and B, incorrect focusing of a patch 

 of light for retroilhiniination. 



"block" of the corneal tissue. Thus, in Fig. 42, the view d2, 

 seen by retroillumination along 02, would be compared with the 

 view dl seen by direct illumination along 01. This method is 

 more easily applicable near the periphery of the cornea where the 

 iris is closer to it than is the case near the centre. 



The term " opacity," which is properly defined as the ratio of 

 the intensity of incident light to the intensity of light transmitted, 

 has been partly misused in ophthalmology, as when we focus a 

 light by ordinary "oblique " illumination direct upon an area of 

 pathological change in, e.g., the cornea or lens, and, judging from 

 its visibility as it is thus seen by direct illumination, we call the 

 area an " opacity." To make a simple comparison ; a splash of 

 dark pigment on the anterior lens-capsule, in the direct course of 



