The quality of an image formed on a vertebrate’s retina is largely determined by the cornea and the lens. In air, the cornea is the principal refracting agent of light rays and is responsible for approximately two-thirds of the refractive power of the eye, which, in humans, amounts to approximately 40 diopters (40 D). This is because the cornea is bordered on its inner surface by the aqueous humor, with a refractive index of 1.33, and on its outer surface by air, with a refractive index of 1.0. The cornea of birds plays an important role in accommodation (Schaeffel and Howland, 1987; Glasser et al., 1994). In the chicken Gallus gallus, changes in corneal curvature account for up to 9 D of the total 15–17 D of accommodation (Schaeffel and Howland, 1987). The focusing power of the cornea in air is a function of its curvature, and corneal accommodation is brought about by curvature changes: the more curved it is, the greater its power (Howland et al., 1997). The refractive power of the cornea underwater is virtually lost, as the media bathing its inner and outer surfaces (the aqueous humor and water, respectively) are of similar refractive indices. In a submerged eye, the lens becomes the sole agent for accommodative adjustments and must provide for the refractive power lost by the cornea if image quality i
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