RETRO ILL UMINA TION 



67 



stippled glass. The diagram represents a fairly advanced stage of 

 the disease in the centre of the cornea seen by D.I. in the corneal 

 block, the deep face presenting by D.I. an appearance suggestive 

 of the greyness, the unevenness and the dull hueless lustre of the 

 surface of pewter which has become partly corroded ; more 

 towards the periphery of the cornea the changes are less intense, 

 taking the form of discrete punctate nodules which are very 

 readily seen by R.I. against the patch of light on the iris bordering 

 the pupil-edge. The illustration, which is diagrammatic, incorrectly 

 shows the pupil-edge in visual focus which cannot be the case if 

 the microscope is focused on 

 the cornea, particularly with 

 the objectives of higher 

 power. 



Fig. 21, C, corresponding 

 with Fig. 22, d, d', shows 

 how the specularly reflected 

 beam from the posterior cap- 

 sule of the lens may be utilised 

 in retroillumination of the 

 cornea. 



It is not easy to interpret 

 the nature of unknown 

 pathological features seen by 

 R.I. ; thus, it is not possible 



Fig. 38. — Intraepithelial vacuolation 

 of the cornea by retroillumination. 



to say by R.I. if the nodules seen in Fig. 40 are of the nature of 

 convexities or concavities on the corneal face. (For this, recourse 

 was had to the interpretation of the reflections off the rims of the 

 nodules as described under Figs. 19 and 26 (10). 



Fine pathological features occur, more particularly of the cornea, 

 which are visible only by retroillumination and not at all, or only 

 indifferently, by direct illumination. In these cases it is not always 

 easy to localise the depth ; in the first place, the surrounding normal 

 corneal tissue is invisible by retroillumination ; and secondly, the 

 pictures seen by the two eye-pieces, particularly with the high- 

 power objectives, are not always alike, because the visual back- 

 ground is different for each (Fig. 41). Hence stereoscopic relief is 



3—2 



