is lost. Newton supposed that it was an impossibility to construct a lens corrected for color which would magnify objects; but since the discovery (in 1753 and 1757) of different kinds of 
Fig. 5.—Achromatic Lens. glass having the same refractive power but widely different dispersive powers, perfect lenses have been possible,
In human eye, a practically perfect image, with no alteration in color, is produced by a mechanism which human ingenuity can not imitate. There is a slight error in the cornea, which is corrected by an opposite error in the crystalline lens; the iris plays the part of the diaphragm of optical instruments and shuts off the light from the borders of the crystalline lens, where the error is greatest, particularly in near vision; the curvatures of the lens are not perfectly spherical, but are such that the form of objects is not distorted; and while such curvatures are theoretically calculable, their construction is practically impossible, as experience has shown; different layers of the crystalline lens have different dispersive powers; and thus a practically perfect image, with no appreciable decomposition of white light, is formed on the retina.
Another wonderful thing about the eye, which adapts it most beautifully to our requirements, is the division of the sensitive parts of the retina into a very small area for distinct vision, which we use for reading, for example, and a large surrounding area in which vision is indistinct. If we saw with equal distinctness with all parts of the retina, the vision of minute objects would be confused and imperfect. As it is, the area of distinct vision is very small, probably less than one thirty-sixth of an inch in diameter. In this area, the distance between the separate sensitive elements is not more than one thirty-five-hundredth of an inch; while, if we pass from this only eight degrees, the distance is increased a hundred times. Still, in looking at any one object in the center of distinct vision, the imperfect forms of surrounding objects are appreciated, warning us, perhaps, of the approach of danger.
The mechanism of distinct and indistinct vision has been understood only since 1876. The sensitive parts of the retina are little rods and cones forming a layer by themselves. In 1876, Boll discovered that in frogs kept in the dark the rods of the retina were colored a dark purple; but on exposure to light the color faded, becoming first yellow and then white. Since that time, physiologists have been carefully investigating visual purple and visual yellow. Just outside the layer of rods and cones are the dark cells which render the greatest part of the interior of the eye almost black. In the dark, these cells send little filaments be-
