ILLUMINATING BEAM 



.37 



the source the smaller the diameter of the illuminating beam, 

 and in days gone by many a teaeher of ophthalmology, often 

 not clearly as to the actual reasons, would teach the student that 

 to see by oblique illumination many of the pathological features 

 commonly met with in the front half of the eye, he should 

 remove his source of light as far away as was compatible with its 

 intensity. The reason that this gives better visibility is that the 

 observer is usually able to manipulate for his effects with a beam 

 of relatively smaller diameter ; but when the 12 mm. diameter of 

 the cornea is considered it may be appreciated that the beam of 

 light, by such simple methods, can seldom be of less diameter 

 than the cornea, particularly 

 with the sources of light which 

 were available to within a few 

 years ago. (The arc-lamp does 

 not seem to have been tried in 

 the past, doubtless partly from 

 fear of danger to the living 

 eye.) 



Let such a relativelv wide 

 beam of light (I, Fig. 4), as 

 through a simple condensing 

 lens from an ordinary lamp, 

 be focused through the cornea 

 for detection and observation of a spot of K.P. on the back 

 face ; the precipitate is illuminated, but the observer, taking 

 liis view along the direction O, is first obstructed visually by 

 (a) the dazzle of specular reflection from some part of the curved 

 corneal surface under illumination, and by (h) some diffuse 

 reflection from all the anterior corneal surface (and also the pos- 

 terior corneal surface) under illumination, and also by (c) the 

 internal illumination of the tissue owing to its relucency ; secondly, 

 he is handicapped because the visual background of the lens (not 

 drawn), or it might be the iris, is inevitably illuminated as well. 

 To summarise, the visibility of the spot of K.P. thus illuminated 

 is poor, owing, firstly, to impairment of the transparency of the 

 normal corneal tissue in front caused by the entering light pass- 



FiG. 4. — Observation of K.P. illu- 

 minated by a wide beam of light. 



