38 



MICROSCOPY OF THE LIVING EYE 



itig through the area of tissue through which observation must 

 be made — a difficulty usually much intensified if pathological 

 processes have modified the surfaces or the interior of the cornea 

 — and, secondlv, to the unavoidable illumination of the visual 

 background. Any trained clinical observer knows that he can 

 usually see a spot of white K.P. much better if he holds his con- 

 densing lens so as to focus the light on to the sclerotic near its 

 junction with the cornea, or on to the junction itself (Fig. 5). 

 The reason of this is that the transparency of the area of corneal 

 tissue through which observation (O) is being made is not impaired 

 by the transmission of direct light, and the visual background (1) 



is unilluminated. It 

 mav be asked how 

 the spot of K.P. 

 is illuminated ; it 

 scatters light which 

 reaches it indirectly 

 by two ways, firstly 

 by diffuse reflection 

 from the surface of 

 the iris (Fig. 5), and 

 secondly by light 

 which travels from 

 the sclerotic meri- 

 dionally along the cornea by total internal reflections — the method 

 of " sclerotic scatter " which will be explained later (Figs. 33 

 and 34) . The first of these two factors could therefore be em- 

 ployed exclusively if we dealt with a beam having a very 

 restricted diameter, as in Fig. 6, in which, for illustration pur- 

 poses, the feature for observation has been drawn inside and not 

 behind the cornea. " Retroillumination " (R.I.) seemed an 

 appropriate term for this method (6), but in England tlie term 

 " transillumination " of the Continental writers is usually pre- 

 ferred ; and given the required conditions, retroillumination may 

 afford observation of a bright feature against a relatively dark 

 visual backgroimd (as in Fig. 6) or of a relatively dark, opaque 

 or non-rclucent feature against a light visual background, as in 



Fig. 



-Observation of K.P. by light focused 

 on the hmbus, 



