"The eyes of the Lord are upon the righteous, and his ears are open unto their cry."—Psalm xxxiv.
971. Why, having two eyes, and each eye receiving a reflection upon its retina, does the brain experience only one impression of an object?
Because, besides those optical laws which bring upon the two retinas the exactly corresponding images of the same objects, the optic nerves meet before they reach the brain, and blend the impulses which they convey.
972. Why are the eyes provided with eyelids?
Because the eyes require to be defended from floating particles in the air, and to be kept moist and clean. The eyelids form the shutters of the eye, defending it when waking, by closing upon its surface whenever danger is apprehended, moistening its surface when it becomes dry, and covering it securely during the hours of sleep.
973. Why are the eyelids fringed with eyelashes?
Because the eyelashes assist to modify the light, and to protect the eye, without actually closing the eye-ids. When the eyelids are partially closed, as in very sunny or dusty weather, the eyelashes cross each other, forming a kind of shady lattice-work, from the interspaces of which the eye looks out with advantage, and sees sufficiently for the guidance of the body.