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26
Visual Education
of language. We often say that we "see through" a thing instead of asserting that we understand it; our "foresight" means our foreknowledge; if a proposition seems to be favorable we declare it "looks good"; to say nothing of numerous expressions such as "look me up/' "I'll see you later," "seeing is believing," "au revoir" and "auf wiedersehen."
The human eye is an organ whose essential optical parts are shown in axial section in Fig. 1. When light enters the eye it passes, in order, through the transparent membrane c, known as the cornea ; the chamber a, filled with the aqueous humor; the crystalline lens I; and the chamber v, filled with the vitreous humor; and it finally falls upon the retina r.
The eye is much like a camera, with which nearly everyone is somewhat familiar. The lens of the eye corresponds to the lens of the camera, and the retina corresponds to the photographic plate. The analogy goes still further, for, just as a camera is provided with a diaphragm by means of which the amount of light which enters it may be controlled, so the eye has the iris which regulates the amount of light that falls upon the retina. The pupil is simply the aperture through the iris.
If it were not for the lens of the eye, light from every visible part of an object would be scattered over the whole retina and no definite outlines of anything would be observed. The function of the lens is to bring to a focus on the
Figure 1
Figure 2
retina all the rays of light which enter the eye from a given point on an object. Fig. 2 is a photograph of three initially parallel rays which pass through a lens. It is seen that the direction of the central one is not changed, while the other two are bent, or refracted, by the lens and intersect the central ray at the same point. In the case of an ideally perfect lens all the rays parallel to the central ray would intersect it at the same focus. The case of initially parallel rays is that in which the object is at a very great distance. If the object were