RETRO ILL UMINA TION 



65 



beam traversing the lens-tissue behind it. Any arrangement of the 

 illuminated background suffices to render visible a truly opaque 

 feature like a lump of dark pigment set in transparent tissue, so 

 long as it lies in the path between the observer's eye and the 

 illuminated background ; but many features which are visible, 

 and visible either mainly or only, by retroillumination, are of the 

 nature not so much of opacities as of sites of refractile disturbance 

 of the tissue, e.g., little vacuoles formed by discrete drops of fluid 

 within the substance of the lens-cortex ; or minute punctate 

 nodules or excres- 

 cences on the deep 

 face of the cornea. Re- 

 troillumination shows 

 these up because each 

 acts as a minute lens, 

 and it is desirable to 

 have a sharp contrast 

 effect behind them — 

 the meeting of light 

 and darkness along a 

 sharply defined mar- 

 gin ; then the view of 

 the margin, displaced 

 by refraction as it is 

 transmitted through, 

 say, the tiny vacuole, is seen in the vacuole geometrically out 

 of line with the corresponding effect in the general illuminated 

 background, as in Fig. 36, which shows vacuoles in the anterior 

 lens-cortex viewed by R.I., against a sharply defined cylindrical 

 beam of small diameter in the manner of Fig. 35. If the 

 vertically extended slit-beam were used here instead, and 

 the area of retroillumination were thus spread over a wider 

 vertical area, the visibility of the vacuoles would be far less. 

 In a similar way, vacuoles, invisible by direct illumination, are 

 readily seen in the corneal epithelium by R.I. when the small 

 circular beam is used (Fig. 37). Another source of retroillumina- 

 tion for the cornea is from a patch of light focused on the iris as 



Fig. 35. — Vacuole in the anterior lens-cortex 

 viewed by retroillumination. 



R.A.M. 



