172 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS $I •Cl't . Figure 12. Direct retro-illumination. Slit filuminator beam set at 45 ø with a 90 ø angle between corneal microscope, focused on cornea, and slit illuminator. Observer looks parallel to reflected light rays. RL • reflected light Figure 13. Direct retro-illumination of normal rabbit eye Sclerotic scatter illumination aids in detecting very slight and early changes in the corneal tissue which only mildly obstruct, but significantly scatter, light. Central circular clouding of the cornea is probably best determined by this method. In indirect illumination, a narrow beam is focused on nontransparent, trans- lucent tissue such as sclera, iris, or leukoma of the cornea. The microscope is focused at the same plane, slightly to the side of the beam. The angle between the corneal microscope and the slit illuminator is 60 ø or more, thus increasing the amount of internal reflection and the amount of light scattered (Figs. 16

TECHNIQUES OF SLIT-LAMP ILLUMINATION 173 $i Figure 14. Sclerotic scatter. Slit illuminator beam is focused at limbus with corneal microscope focused on corneal plane .. , ::•- ::4 :'.% ..: :•?::•: :}'•- &:i: .:.-:•:. '{/•:•- ß .. f' ,.!, :,-•:" • •."F '• .. . . . ß ....•..... :, ?*?':- .5.' "* -:': s: ' :*'" .:...?:.:. ..% ..:½4.. ::• •. :•.'" '5.• z..:•... '".:•1.. ,".-s, •., .. .. : •:.:.. --• •....•..: ..... ,. •. . ß. Figure 15. $cle, otie scatte, of •o•,1 rabbit and 17). A beam of about 1 • wide is used. A narrow slit is focused on one of the t•sues mentioned. The microscope is focused adjacent to the illumi- nated area, This •pe of illu. ination is most usef• in s•dying •e ir•. In specular reflection, a narrow slit beam • focused at the corneal surfa• •d •se reflection produces a cornea] parallelepiped. There • a signific•t difference be•een •e refractive indices of •e air in •e precorheal film and to a l•ser degree be•een the indices of the aqueous and •e endo•elium. •ese differences in refractive indices produce so-called "zones of •scontinu- i•." Regular •d irreg•ar reflections of the light occur at each zone of d•-