Those who know the rudiments of dioptrics and something of the eye's remarkable structure must know that the sun's rays undergo refraction in the crystalline lens and in the other humors, and that they then converJe on the retina or optic nerve, which covers the entire fundus of the eye just as the sun's rays, after passing through a magnifying glass or convex lens, converJe at the focus or the burning point of the lens, two, three, or four inches from it, inversely proportional to its convexity.