A simple model of the accommodating lens of the human eye.

THE HUMAN EYE is often discussed as optically equivalent to a photographic camera (2). The iris is compared with the shutter, the pupil to the aperture, and the retina to the film, and both have lens systems to focus rays of light. Although many similarities exist, a major difference between the two systems is the mechanism involved in focusing an object. In a camera, the focal length of each lens is fixed, and changes in focus are brought about by movement of the lenses. However, in the human eye, changes in focus are brought about by changes in the power of the lens by varying its curvature. Functional models of the eye are commercially available. Current prices of such models range from $400 upward. This article describes a simple and inexpensive model to illustrate the change in curvature of the human lens during accommodation. This model attempts to help students appreciate the complexity and elegance of the human eye by mimicking the flexibility of the human lens during a change in focus. Glass lenses therefore, although easily available, are not an option for classroom demonstration of the phenomenon, as their curvatures cannot be changed. An appropriate model of the lens for teaching requires a fluid interior and an elastic exterior. A water-filled latex sheath was used for this purpose. Materials required. The materials required for this model include a latex sheath from a condom or a transparent balloon, a strong support to attach the lens assembly, a transparent box, a source of smoke, elastic bands, nonelastic string, and a pair of laser pointers. Construction of the model. A smooth condom was filled with water and tied with a piece of string at two points so that the area between these knots was roughly spherical when relaxed. The extra latex on both ends was cut away, leaving two long pieces of string on either end for attaching the lens to the support. When stretched, the structure elongates, becoming less convex; when relaxed, the structure becomes more convex. Mounting the lens involved attaching the string on either end to a piece of elastic band (fabricated from a rubber glove). This elastic band was fixed to a stand on both ends. To prevent sagging of the lens due to the weight of the water, a box below provided support. To visualize the passage of light through the lens, two parallel laser beams were passed through the lens assembly. Laser pointers were used for this purpose. When passed through the lens, these beams converge. The beam is not visible unless the light is scattered. To scatter the light, the converged light rays were allowed to pass through a transparent box filled with smoke. A transparent plastic box was used for this purpose. (Any transparent container, such as an inverted empty fish tank, could be used instead.) Incense sticks were used to produce smoke. In this model, the latex sheath represents the lens and the string on either end represents the suspensory ligaments. As the ciliary muscle was not modeled, a cartoon of smooth muscle was fixed over the stretched elastic band. An illustration of the final assembly of this model is shown in Fig. 1. Presentation of the model. This model was presented to first-year medical students in groups of 15. Students had attended lectures on the structure and function of the eye. First, the lens was placed in the stretched position (Fig. 2A). When two parallel laser beams were passed through the lens, they converged at a point. The smoke enabled the students visualize the path of the light rays. The point where these rays converged was marked on the surface of the box. Students were asked to measure the distance of this point from the center of the lens to obtain the focal length of the stretched lens. For a convex lens, parallel rays of light are focused at the focal point. The distance from the center of the lens to the focal point is the focal length. The power of a lens is given by the following formula: power (in diopters) 1/focal length (in m). Students were asked to calculate the power of the stretched lens. The elastic band was then pulled inward. This resulted in the slackening of the pieces of string, making the lens more convex (Fig. 2B). Students were then told that this was how contraction of the ciliary muscle results in slackening of the suspensory