36 MICROSCOPY OF THE LIVING EYE 



In the transparent media of the anterior half of the eye there are 

 four very definite smooth surfaces — the anterior and posterior 

 corneal, and the anterior and posterior lental — at each of which, 

 when light enters or leaves, these phenomena occur ; but within 

 the substance of the corneal and lental tissues the traversing light 

 does not behave in quite the same way that it does when passing 

 through glass. Glass is physically an optically homogeneous 

 medium, so that the passage of light within it, between the 

 proximal and distal surfaces (P and D, Fig. 2), is invisible under 

 ordinary circumstances (Fig, 3, 3). Transparent living tissues, 

 on the other hand, are not optically homogeneous : for example, 

 the cornea is built up of histological cellular elements so 

 arranged as to admit of as high a degree of transparency as 

 possible, yet each element is in contact with interspaces containing 

 nutrient lymph-fluid whose refractive index probably differs a 

 little from that of the solid elements Some scatter of light occurs 

 at these multiple microscopic surfaces, the result being a visible 

 internal illumination of the tissue throughout the path of the beam. 

 No term existed in physics which might express this phenomenon, 

 and the word " relucency " has been suggested (6 and 10) for 

 that property of a transparent non-homogeneous substance in 

 virtue of which the path of a traversing beam of light is normally 

 visible, be it due to diffuse internal reflections, scatter, fluores- 

 cence, diffraction or any of the other causes which may modify 

 the light so as to render it visible in its interior course ; it is not 

 necessary, for clinical purposes, to distinguish between these 

 different contributing phenomena, save that the term relucency 

 should be understood not to include that of specular reflection 

 (sr, Fig. 2), as by drawing this distinction the term has more 

 practical usefulness. It may here be added that, for obvious 

 reasons, the term " visibility " is no substitute for the term 

 " relucency," visibility being merely dependent on such abstract 

 qualities as contrast ; an area almost non-relucent might be highly 

 visible in virtue of contrast with a relucent surrounding area. 



The usual simple method of illuminating the eye for observation 

 is to focus obliquely on it, by means of a simple condensing 

 lens, a beam of light from a distant source ; the more distant 



