Editing Eye (section)

====Refractive cornea====
[[File:Human eye, lateral view.jpg|thumb|A refractive cornea type eye of a human. The cornea is the clear domed part covering the [[Anterior chamber of eyeball|anterior chamber of the eye]].]]
In the [[Mammalian eye|eyes of most mammals]], [[Bird vision#Anatomy of the eye|birds]], reptiles, and most other terrestrial vertebrates (along with spiders and some insect larvae) the vitreous fluid has a higher refractive index than the air. In general, the lens is not spherical. Spherical lenses produce spherical aberration. In refractive corneas, the lens tissue is corrected with inhomogeneous lens material (see [[Luneburg lens]]), or with an aspheric shape. Flattening the lens has a disadvantage; the quality of vision is diminished away from the main line of focus. Thus, animals that have evolved with a wide field-of-view often have eyes that make use of an inhomogeneous lens.<ref name=Land1992/>

As mentioned above, a refractive cornea is only useful out of water. In water, there is little difference in refractive index between the vitreous fluid and the surrounding water. Hence creatures that have returned to the water—penguins and seals, for example—lose their highly curved cornea and return to lens-based vision. An alternative solution, borne by some divers, is to have a very strongly focusing cornea.<ref name=Land1992/>
[[File:Closed human eye, superior view.jpg|thumb|[[Eyelid|Eyelids]] and [[Eyelash|eyelashes]] are a unique characteristic of most mammalian eyes, both of which are evolutionary features to protect the eye.]]
A unique feature of most mammal eyes is the presence of [[Eyelid|eyelids]] which wipe the eye and spread [[tears]] across the cornea to prevent dehydration. These eyelids are also supplemented by the presence of [[Eyelash|eyelashes]], multiple rows of highly innervated and sensitive hairs which grow from the eyelid margins to protect the eye from fine particles and small irritants such as insects.