[Illustration:  Fig. 130.—­Illustrating the manner in which the Image of an Object is brought to a Focus in a Photographer’s Camera.]

331.  The Refractive Media of the Eye.  The eye is a closed chamber into which no light can pass but through the cornea.  All the rays that enter the eye must also pass through the crystalline lens, which brings them to a focus, as any ordinary lens would do.

Now, if the media through which the light from an object passes to reach the retina were all of the same 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.