The refractive index of the cornea of a harbor porpoise eye was measured by two-wavelengths laser-interferometry. In the thickest part of the cornea a refractive index of about 1.53 was found. From this value, the refractive index gradually decreases to about 1.37 at the surfaces of the cornea. Due to its peculiar shape, the cornea contributes significant, negative refractive power to the overall optics of the eye. The combination of a diverging corneal lens with the powerful, converging crystalline lens results in near emmetropia for the harbor porpoise eye in underwater viewing conditions.