Page:Elementary Text-book of Physics (Anthony, 1897).djvu/437

§ 345]|REFLECTION AND REFRACTION.|423}} the image which a given lens will produce, or the focal length of a lens necessary to produce an image of a certain size.

345. The Eye as an Optical Instrument.—The eye, as may be seen from Fig. 113, which represents a section by a horizontal plane, is a camera obscura. $$a$$ is a transparent membrane called the cornea, behind which is a watery fluid called the aqueous humor, filling the space between the cornea and the crystalline lens. Behind this is the vitreous humor, filling the entire posterior cavity of the eye. The aqueous humor, crystalline lens, and vitreous humor constitute a system of lenses, equivalent to a single lens of about two and a half centimetres focus, which produces a real inverted image of external objects upon a screen of nervous tissue called the retina, which lines the inner surface of the posterior half of the eyeball. The retina is an expansion of the optic nerve. The light that forms the image upon it excites the ends of the nerve, and, through the nerve-fibres leading to the brain, produces a mental impression, which, partly by the aid of the other senses, we have learned to interpret as the characteristics of the object the image of which produces the impression. For distinct vision the image must be sharply formed on the retina; but as an object approaches, its image recedes from a lens, and if, in the eye, there were no compensation, we could see distinctly objects only at one distance. The eye, however, adjusts itself to the varying distances of the object by changing the curvature of the front surface of the crystalline lens. There is a limit to this adjustment. For most eyes, an object nearer than fifteen centimetres does not have a distinct image on the retina.

We may here consider the means by which we estimate the distance and size of an object. The retina is not all equally sensitive. The depression at $$b,$$ called the yellow spot, is much more sensitive than the other portions, and a minute area in the centre of that depression is much more sensitive than the rest of the yellow spot. That part of an image which falls on this small area