54 MICROSCOPY OF THE LIVING EYE 



Fig. 19, let E to P represent rays of a beam of light incident on a 

 highly polished mirror-like surface AB ; let d, d, d represent areas 

 of defective reflection due, say, to impairment of the polish ; 

 let c, c, c represent small elevations or depressions whose polish is 

 unimpaired. Rays of light such as F, H, K, M, O falling on areas 

 of normal polish and contour will be specularly reflected along the 

 prescribed axes Q, R, S, T, Y, respectively, and if such regular 

 reflection predominates from the surface as a whole, an observer, 

 taking his view along the general axis represented by Q, R, S, T, V, 

 will see a shining patch ; i.e., he will be viewing an area of the 

 surface AB in the axis of its specular reflection. Rays such as 

 E, G and J, falling on the areas of defective reflection d, d, d, will 

 not be specularly reflected, but either diffusely reflected or 

 absorbed, and in either case the areas d, d, d will appear as dark 

 spots in the shining patch. The rays L, N and P will be specularly 

 reflected in the directions X, Z and W respectively, every ray 

 falling on the areas c, c, c being reflected in a direction different to 

 that of the observation axis ; with the exception that the rays 

 from a small area of the summit of the concavity, or bottom of the 

 convexity, both of which conform to the general disposition of the 

 surface AB, wdll be reflected along the observation axis. Hence 

 the areas c, c, c will also appear as dark spots on the mirror surface 

 with probably (depending in the shape of the areas c, c, c) a minute 

 bright glint in the centre of each. It can be said that the area 

 AB is being viewed, and certain peculiarities detected, by " direct 

 illumination with observation along the axis of specular reflec- 

 tion " (" D.I.S.R." (10) ). This principle may be applied to study 

 the two corneal and two lental surfaces. Let MP (Fig. 20) be a 

 plane mirror-surface, M'C a concave mirror-surface and WV a 

 convex mirror-surface on to an area of which (x) the illuminating- 

 beam is directed along the axis IX. Observation of the area x 

 along any such axes as 02, Ol, 03 will be by DI, but only along 

 the axis O will observation be by D.I.S.R., and the picture of the 

 area x in this last instance will be entirely different to that in the 

 other instances ; observation along O affords very sensitive means 

 of detecting various minute changes and irregularities on the 

 surface. 



