DIRECT ILLUMINATION 



47 



surface of the cornea. If the incident and reflected beams, I 

 and sr, make a wide angle to one another, the observer may 

 equally avoid dazzle from the reflected beam by taking a line 

 of observation across its aerial reflected path, i.e., allowing sr 

 to cut across the observation axis to the other side of O ; or 

 he may arrange the incident beam radially along the normal 

 to the surface so that the reflected beam comes back along the 

 incident path of the entering light I. The method depicted in 



Fig. 14, — Fincham pattern of Gullstrand slit-lamp and Czapski binocular 

 microscope, made by Messrs. Clement Clarke & Co., Ltd. 



Fig. 15 may be called that of observation by " direct illumina- 

 tion " (D.I.) (6 and 10). 



It has transpired that in this method the clear illumination of 

 the feature under observation, say the spot of K.P. of Figs. 15 

 and 16, is not the only valuable result of working with a sharply 

 restricted illuminating beam : owing to the relucency of the 

 transparent cornea and lens, the traversing beam defines its own 



