The lenses of the eye model are equipped with handles, which allow them to be easily inserted into the water. The handles of the plastic lenses are marked with their focal lengths in air. Two of the lenses are cylindrical lenses for causing and correcting astigmatism in the model; these can be identified by notches on their edges that mark the cylindrical axes.
WARNING: DO NOT wipe or rub the lenses with a cloth or tissues to dry or clean them. They are plastic and easily scratched. We will provide blow-off guns you can use to dry them off. The crystalline lens, which is supported in the slot labeled SEPTUM, can be replaced with different lenses to accommodate, or focus, the eye model at different distances. (The label refers to the septum, or partition, formed by the lens and other tissues, that separates the aqueous and vitreous humors.) Two other slots behind the cornea, labeled A and B, can hold additional lenses to simulate changing the power of the crystalline lens. A cylindrical lens can be placed in slot A or B to give the eye astigmatism. The pupil aperture can also be placed into slot A or B to demonstrate the effect of a round or “cat-shaped” pupil. Two slots in front of the cornea, labeled 1 and 2, can hold simulated eyeglasses lenses to correct for near-sightedness, far-sightedness, and astigmatism.
A circle marked on the retina screen represents the fovea, and a hole in the screen represents the blind spot. The retina screen can be placed in three different positions (labeled NORMAL, NEAR, and FAR) to simulate a normal, near-sighted, or farsighted eye.
Optics of the Eye
The corneal lens and crystalline lens together act like a single, convergent lens. Light entering the eye from an object passes through this lens system and forms an inverted, real image on the retina. The eye focuses on objects at varying distances by accommodation, or the use of muscles to change the curvature, and thus the focal length, of the crystalline lens. In its most relaxed state, the crystalline lens has a long focal length, and the eye can focus the image of a distant object on the retina. The farthest distance at which the eye can accommodate is called the far point for distinct vision. For a normal eye, the far point is infinity. When muscles in the eye contract and squeeze the lens, the center of the lens bulges, causing the focal length to shorten, and allowing the eye to focus on closer objects. The nearest distance at which they eye can accommodate is called the near point for distinct vision. The near point of a normal eye is about 25 cm.