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.

176 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS NORMAL RABBIT EYE The rabbit has been the animal of choice for the pharmacological and toxi- cological evaluations carried out in our laboratories. However, other species, such as the monkey, dog, rat, and mouse may be used for experimental stud- ies. For the purpose of this presentation and because a grading scale was de- signed for the rabbit, it will be utilized for discussion. In order to become proficient in the use of the slit lamp for animal experi- mentation, one must become familiar with the normal anatomy of the animal eye as it appears with the aid of the slit lamp. The techniques which are dis- cussed for the rabbit are applicable to other species. In test conditions, ocular changes from normal to most severe may be visualized. The first structure to be observed with the aid of the slit lamp is the cornea (Fig. 7). When utilizing direct illumination, the microscope is focused on the surface of the cornea. Epithelial defects are easily observed with this type of illumination. Switching to retro-illumination, the observer quickly looks at the layers of the cornea for any changes with respect to transparency, hydration, or epithelial defects. In switching to specular illumination, the endothelium pattern of the cornea is observed. At first, the mosaic pattern of the endotheli- um may not be recognized by the observer but once observed it is recognized easily. The anterior chamber, which is posterior to the cornea, is the next area to be observed. The normal anterior chamber is invisible to light of the slit lamp because there are no discrete cellular units or reflective characteristics therein (Fig. 20). Merely by localizing the beam, i.e., restricting the beam to a small, conical unit, the integrity of the anterior chamber can be observed. In the ab- normal anterior chamber, the presence of light which reflects (Tyndall effect) is indicative of a pathological change. The next structure to be visualized in the routine examination is the iris (Fig. 17). This can be accomplished by utilizing indirect illumination. In the albino rabbit eye, blood vessels may be observed quite easily with all magnifi- cations of the slit lamp due to the lack of iridal pigmentation. However, if the observer looks at another species, such as the dog or cat, pigmentation of the iris usually obscures the presence of small vessels of the iris. It is fortunate that the albino rabbit has easily seen vessels, since congestion of the iris blood vessels is indicative of a pathological change. In very young rabbits, the ves- tigial ends of blood vessels may be seen protruding into the pupillary area. Normally, although the iris of the rabbit will appear pink, these blood vessels will not be congested. In very young rabbits, slight amount of congestion of iridal vessel is normal. Pupillary response to light may be observed by noting changes in pupillary size with the slit image on the pupil. Within the pupillary perimeter, the anterior capsule of the lens is the next structure to be examined using direct and indirect illumination. The anterior capsule will appear as a slightly opaque, rough-textured surface. The observer