TECHNIQUES OF SLIT-LAMP ILLUMINATION 169 thelium (which appears as an anterior bright line), the st-roma (a marblelike area), and endothelium (a posterior thin white line). When an optical section is formed on the cornea, it has four general layers which may be observed. From anterior to posterior, they are: (a) a thin, bright layer representing the precorneal film (b) a thin, dark layer representing the epithelium (c) a thin, granular area representing the stroma and (d) a thin, bright layer rep- resenting the endothelium. Direct illumination is used to study any tissue of the anterior segment. A conical beam is used to detect aqueous flare, which appears as a Tyndall effect and results from light being reflected from cellular or proteinaceous debris in the anterior chamber. In diffuse illumination, the beam is not sharply focused in the plane of the area being studied but is either converging or diverging. The wide aperture illuminates the interorbital area when the microscope is focused in the plane of the area being studied (Figs. 8 and 9). The microscope is directly in front of the eye being examined and the angle between the microscope and the slit illuminator is about 45 ø . Any size beam may be used however, it is usually optimal to have a wide aperture so that a large area is illuminated. The dia- phragm can be adjusted to produce a wide rectangular or circular beam. Lack of focus in the plane of observation and the width of aperture thus produces a large illuminated area. This type of illumination is useful for viewing a large area with a relatively intense uniform illumination and under conditions of stereoscopic magnification. A general view of the cornea, lids, sclera, coniunc- riva, and lacrimal drainage system is seen by diffuse illumination. In retro-illumination, the beam is reflected from a structure (iris or lens) posterior to the plane on which the microscope is focused. The structures in the anterior plane are studied in the reflected light. This type of illumination is most commonly used when the beam reflects from the iris and the micro- scope is focused on the cornea. Unless some structure in the media obstructs or scatters this reflected light, no special details are observed. Scars, pigment, and vessels containing blood are opaque to light and appear dark on the Figure 8. Diffuse illumination. Wide slit illuminator beam not focused in area and plane corneal microscope is

170 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Figure 9. Diffuse illumination of normal rabbit corneal epithelium brighter background when viewed with retro-illumination. Edema and cor- neal precipitates scatter light and appear bright on a darker background when viewed with retro-illumination. There are two types of retro-illumination: indirect and direct. In indirect retro-illumination, the structure, microscope, and reflected light do not lie in the same visual line. The structure is at the side of the path of the reflected light and the microscope is moved away from the path so that the structure is observed against a dark background such as the pupil or the iris. In direct retro-illumination, the structure, microscope, and reflected light all lie in the same visual line. Scars, pigment, and the vessels with blood are best seen with direct retro-illumination. Edema and precipitates are best seen with indirect retro-illumination. A slit 1-2 mm wide may be used to form the parallele- pipcd. For indirect retro-illumination, the microscope is directly in front of the eye being examined and the angle betxveen the illumh•ation and the ob- server is set at 45 ø (Figs. 10 and 11 ). In direct retro-illumination, both the mi- croscope and the illumination system are set at about 45* to the eye being examined. The angle between the microscope and the illuminator is, there- fore, 90 ø (Figs. 12 and 13). Retro-illumination is useful in studying most types of epithelial edema, vacuoles, scars, posterior precipitates, and channels from the blood vessels that infringe upon the cornea. In sclerotic scatter, the beam is focused on the corneal limbus. The scatter- ing or dispersion of the light from the perilimbal sclera produces a halo