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106 vision up to this point, but no further. We know that a distinct image is formed upon one or other of the delicate coats of the eye, but the manner in which the sensation is conveyed to the brain is an inscrutable mystery. "It is the boast of science," says Herschel, "to have been able to trace so far the refined contrivances of this most admirable organ, not its shame to find something still concealed from scrutiny; for, however anatomists may differ on points of structure, or physiologists dispute on modes of action, there is that in what we do understand of the formation of the eye so similar, and yet so infinitely superior to a product of human ingenuity; such thought, such care, such refinement, such advantage taken of the properties of natural agents, used as mere instruments for accomplishing a given end, as force upon us a conviction of deliberate choice and premeditated design, more strongly, perhaps, than any single contrivance to be found, whether in art or nature, and renders its study an object of the greatest interest."

The Cyclops had each a single eye stuck in the centre of the forehead, but we are provided with a pair of these matchless instruments. Each eye receives an impression of an object, nevertheless we do not see the object double. So long as each image falls exactly on the same part of each sensitive surface, the mind will perceive but one object, and the muscles which move the eyes act in such perfect unison that this result is constantly attained.