Abstract
Two principles are realized in the accommodation of an eye that is constructed as a “camera obscura”: (1) Change of curvature of refracting surfaces, principally the lens; (2) change of distance between refracting mediums and image screen, principally distance between lens and retina.
1. There is only increase of curvature, principally of the anterior surface of the lens, during active accommodation. We observe it in mammals, birds, and reptiles (lizards, crocodiles, turtles, a few snakes). Experiments were made before the society to show the increase of curvature of the lens in the eye of the water-turtle—proof of Helmholtz's theory of accommodation.
2. Accommodation by change of the distance between lens and retina is possible in two directions: (a) In cephalopods and fishes, which are normally shortsighted, accommodation for objects at a distance is effected by a movement of the lens toward the retina. In the eye of the fish there is a muscle Musculus retractor lentis (Beer) which draws the lens toward the retina; (b) in amphibia and most of the snakes, the lens is moved toward the cornea, away from the retina, by changes of the intraocular pressure.
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