density as the air, and were also plane
surfaces, an impression would be produced, but the image would not be
distinct. The action of the lens is aided by several refractive media
in the eye. These media are the cornea, the aqueous humor, and the
vitreous humor. By reason of their shape and density these media refract
the rays of light, and bring them to a focus upon the retina, thus aiding
in producing a sharp and distinct image of the object. Each point of the
image being the focus or meeting-place of a vast number of rays coming
from the corresponding point of the object is sufficiently bright to
stimulate the retina to action.[44]

Thus, the moment rays of light enter the eye they are bent out of their
course. By the action of the crystalline lens, aided by the refractive
media, the rays of light that are parallel when they fall upon the normal
eye are brought to a focus on the retina.

If the entire optical apparatus of the eye were rigid and immovable, one
of three things would be necessary, in order to obtain a clear image of an
object; for only parallel rays (that is, rays coming from objects distant
about thirty feet or more), are brought to a focus in the average normal
eye, unless some change is brought about in the refractive media. First,
the posterior wall of the eye must be moved further back, or the lens
would have to be capable of movement, or there must be some way of
increasing the focusing power of the lens. In the eye it is the convexity
of the lens that is altered so that the eye is capable of adjusting itself
to different distances.[45]

[Illustration: Fig. 131.--The Actual Size of the Test-Type, which should
be seen by the Normal Eye at a Distance of Twenty Feet.]

332. The More Common Defects of Vision. The eye may be free from
disease and perfectly sound, and yet vision be indistinct, because the
rays of light are not accurately brought to a focus on the retina. "Old
sight," known as presbyopia, is a common defect of vision in
advancing years. This is a partial loss of the power to accommodate the
eye to different distances. This defect is caused by an increase in the
density of the crystalline lens, and an accompanying diminution in the
ability to change its form. The far point of vision is not changed, but
the near point is removed so far from the eye, that small objects are no
longer visible.

[Illustration: Fig. 132.--Diagram illustrating the Hypermetropic
(far-sighted) Eye.

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