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photographic camera, both in structure and in its manner of functioning. At the front in both is placed the lens, with its diaphragm to control the quantity of light which enters the recording chamber, this function being performed in the human eye by the elastic iris, which contracts and expands automatically as the light waxes or wanes. At the back of the camera is the sensitized plate, and at the back of the eye is the infinitely more sensitive retina, overlaid by the optic nerve, with its millions upon millions of minute tentacles, reaching out to seize upon every fleeting color and form that passes before the lens. These little transparent filaments (so infinitely minute that the point of the finest needle is like a fence-post in comparison) are divided into two distinct varieties, known respectively as rods and cones. The rods are straight and
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