Theoretical studies of the human eye as an optical instrument are carried out by means of schematic models, which attempt to represent mathematically the optical components of the eye and reproduce its optical behavior. Due to the complex optical structure of the eye (parameters to be considered include not only obvious features like dimensions, spacing, curvatures, and refractive indices of both cornea and crystalline lenses, but also optical decentration, physiological toricity, index gradients, accommodation state, chromatic dispersion, etc.), schematic eye models greatly vary in their complexity as well, in an unresolved compromise between simplicity of use and fidelity of optical description.
Historically, the first eye model dates back to the mid-19th century with Listing, a student of Gauss, who developed a scheme with a single refracting surface and an index of refraction of 1.33. At the turn of the same century, other models were presented by von Helmoltz, Tscherning (one with three and one with four refracting surfaces), and Gullstrand, who provided two schemes for the unaccommodated eye-one with six surfaces (number 1 or exact model) and a reduced scheme with just three surfaces (number 2 or simplified model). Gullstrand also developed a model for the fully accommodated eye; he was awarded the Nobel Prize in physiology/medicine in 1911 ". . . for his work on the dioptrics of the eye."
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