CORNEA BY SPECULAR REFLECTION 



59 



Fig. 24. — Observation of the endothelial 

 face of the cornea by D.I.S.R. 



to the selected axis of observation. Hence when the axes of the 

 incident sUt-beam and of the microscope are arranged, as in Fig. 24, 

 only a small area of the 

 posterior face of the corneal 

 block will present itself to 

 view by D.I.S.R., and, if 

 we are working, as is usual 

 in clinical work, with the 

 illumination and obser\'a- 

 tion axes lying in a hori- 

 zontal plane, the area of the 

 corneal surface — anterior 

 or posterior — seen thus can 

 only be one whose tangent- 

 plane is vertical (Figs. 23 

 and 25). Hence, for pur- 

 poses of viewing the corneal surfaces by D.I.S.R., the illuminating 

 beam may just as well be cut down to the size and cross-section 



of a cylindrical beam of small diameter 

 (Fig. 25). 



It should be noted that a view through 

 the Czapski microscope by D.I.S.R. 

 cannot be binocular because the axis of 

 the right and left portions of the micro- 

 scope converge on the common point on 

 which each focuses (Fig. 12). Thus if 

 one side of the microscope be directed 

 along the axis O (Fig. 20), the other side 

 will lie along, say, O^ ; so that whilst the 

 first would vield a view bv D.I.S.R., 

 the second would reveal the same area 

 simply by D.I. This fact can be put to 

 use when it is desired to ascertain whether 

 irregularities of contour of the reflecting 

 surface are of the nature of elevations (convexities) or depressions 

 (concavities). The direction of a reflected beam (nO, Fig. 26) off a 

 specular surface (MR) is deflected (nO') in the direction of tilt (XY) 



Fig. 25. — View of the 

 endothehal face of the 

 cornea by D.I.S.R. 



