174 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS $I Figure 16. Indirect illumination. Corneal microscope is focused in same plane as slit ilhnninator beam but slightly to side Figure 17. Indirect illumination of normal rabbit iris coutinuitv. As a result of regular (specular) reflections, a reflcctiug light age of the illumiuating leus of the slit lamp is formed by the couvex, nfirror- like anterior corneal surface. Because of the irregular reflection, details of the reflecting surface itself can be seen, but only when the eye of the observer is in the direct path of the reflecting beam. Irregularities of the surface absorb much light, and therefore, appear as dark spots on the background of the dazzling reflex produced by the regularly reflected light. The microscope is focused on the image produced by a regular reflection. The angle between the corneal microscope and the slit illuminator is 60 ø with a slit beam of 1-2 mm

TECHNIQUES OF SLIT-LAMP ILLUMINATION 175 Figure 18. Specular reflection. Slit illuminator beam focused on corneal surface with corneal microscope focused on reflected light image of iris Figure 19. Specular reflection of normal human eye wide being used (Figs. 18 and 19). The vertical height of the slit is reduced to about 4-5 mm. When the slit beam travels through the cornea, it forms an illuminated corneal area, an illuminated iris area, and a reflected light image of the illuminated lens in the aqueous. If the microscope is on its common axis, its own specular reflection comes into view. Specular reflection is most useful for viewing surface irregularities of the precornea! film, corneal epithelium, and corneal endothelium. Blinking helps to differcntiate between fixed and movable particles. The endothelium mosaic appears as a dark depressed spot and endothelium deposits can be seen. Folds or tears of Descements mem- brant' can be seen bccause they disturb the endothelial contour.