Thetransi-tionzonebetweenscleraandcorneais calledthelimbusandhasafuzzyappearance.Theeyecontainsthreebasiclayers:thefibrous(corneo-scleral)coat,theuveaoruvealtract(choroid,ciliarybodyandiris)andtheneurallayer(retina).Thesecoatssurroundthecontentsoftheeye,whicharethelens,theaqueoushumorandthevitreousbody[Forresteretal.2001].Thecornea,withanindex ofrefractionof1.376,is responsibleformostoftherefractive poweroftheeye.Afterthecornea,lightpassesthroughtheaqueoushu-mor(index ofrefraction1.336),thelens(index ofrefractionrangesfrom1.386intheouterlayersto1.406atthecenter)andthevit-reousbody(index ofrefraction1.337)beforeit reachestheretina[Hecht1987]DrawingbyKarenLefohn.Usedwithpermission.2TheHumanEye2.1OverviewThehumaneyeis anorgandesignedforphoto-reception;i.e.con-vertinglightenergytonerve  actionpotentials.Theseactionpo-tentialsaresubsequentlyrelayedtotheopticnerve andthebrain,wherefurtherprocessingoccurs,resultinginconsciousvision.Allstructuresintheeyearesubservienttothisphysiologicalprocess[Forresteret al.2001].Thelargeststructureoftheeyeisthesclera(Figure1),whichis approximatelysphericalwitha radiusofaround11.5mm.Thecorneasitssomewhatinfrontofthisstructureandhasa radiusof7.8mm.Thescleratakesaround56thofthecircumferenceoftheeye,whilethecorneais responsiblefortheother16th.