In a camera, the equivalent of accommodation is accomplished by varying the distance between the lens and the film so that the sharpest possible picture will be focused at the plane of the film. In the human eye, however, this type of focusing action is impossible, since the distance between the lens and the retina is fixed. Instead, the focusing apparatus of the eye depends on changing the refractive (light-bending) power of the lens. The crystalline lens of the human eye is an elastic structure enclosed in a capsule of transparent membrane. When a person with a normal eye looks at a distant object, the capsule is pulled tight by contraction of the ciliary muscles, flattening the lens somewhat. This decrease in curvature reduces the refractive power of the lens and parallel rays are brought into focus on the retina.
When a closer object is brought into view, the ciliary muscles relax, allowing the lens to assume a more nearly spherical shape. The increase in curvature (or convexity) of the lens increases its refractive power, and divergent
rays from close objects are brought into focus on the retina. The ability of the eye to accommodate is at its greatest in the child, and decreases steadily as a part of the normal aging process. Young children can usually see clearly objects held only an inch or two in front of their eyes. This is why the very finest embroidery, engraving, and other highly detailed close work at one time was done by children. With aging, however, the lens gradually becomes less elastic and changes its shape with greater diffculty. The ability to accommodate, therefore, decreases, and with each year, the near point (the closest point of clear vision) moves further away.
A pictorial representation of what is called “the accommodation ot the Here the camera has been made to focus on what we wish to see, in this case on the red object in the background on the left, and on the blue form in the foreground on the right. But—unlike the camera, which must be manipulated mechanically—the normal eye will focus on the mind’s choice automatica!fy.