Optics of the harbor porpoise eye in water.

A two-dimensional ray-tracing model for the harbor porpoise eye is constructed from new measurements, mainly on two enucleated eyes, and from data found in the literature. Model calculations show that the crystalline lens has too much refractive power to focus light on the retina. The cornea has a high refractive index and acts as a diverging lens of considerable refractive power. The cornea corrects the eye to near emmetropia for axial and temporal (caudal) directions of view. The eye is approximately 5-D myopic for nasal (frontal) directions of view. The iris serves a dual role as a stop: the iris determines the shapes of bundles of light that enter the lens and the iris blocks light that leaves the lens anterior to its equator.