Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 121

In aquatic species, the cornea is in contact with water rather than air. Because of the very small (∼0.003) difference between the refractive indices of the cornea and water, the cornea of these species has virtually no refractive power. In fact, because the anterior corneal surface has lower curvature than the posterior surface, under water the cornea acts as a weak divergent lens. Fish are forced to compensate for the absence of corneal refraction by increasing the refractive power of other ocular structures, usually the lens. For this reason, as noted earlier, the lenses of fish eyes are very spherical. Their increased curvature results in significantly larger refractive power.

The problem of refraction under water is further complicated in species that move in and out of water because it is physically impossible for an eye to be emmetropic both in air and under water. Eyes that are emmetropic in the air will be hypermetropic under water because the refractive power of the cornea is lost due to its submersion in water. Therefore, species that live and function in both habitats must “choose” whether they will be emmetropic in the air or under water.