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•-

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