Visual Education (Jan-Nov 1920)

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Human Eyes and Optical Instruments 27 near, the rays from any point on it would diverge somewhat asvthey entered the lens, and in this case also they would be brought to a focus after passing through it but at a greater distance from it than in the case of the very remote object. Figure 3 shows how the lens L forms an image of the object AB upon the surface S. Every ray from A is brought to a focus at a, the direction of the ray which passes through the center of L being unchanged. Similarly, every ray from B is brought to a focus at b and the direction of the ray through the center of L is unchanged. The image of AB is inverted. Its size depends upon the angle between the central rays and is proportional to the distance from L to S. If the lens L were too convex or too dense, the rays from A would be brought to a focus before they reached S ; they would cross at a focal point, and, diverging again, they would fall upon S in a small circle. The result would be similar for rays from every other point of AB. The small circles would overlap and no sharp image would be obtained. If L were too flat or of too low density, the rays from AB would converge toward a focus beyond S. The result in this case also would be a series of overlapping circles with the result that there would be no sharp images. The foregoing is a brief outline of the method of formation of optical images by simple refracting lenses, but a number of qualifications must be made. In the first place, if the surfaces of L were sectors of perfect spheres and if L were of the same density throughout, the rays from a point of AB would not all be brought to an exact point on S. Those which passed through the margin of L would be brought to a focus nearest the lens. But if the first surface of L were less convex than the second, then this error, known as spherical aberration, would not be so serious as it would be otherwise. The lens of the eye satisfies this condition for small spherical aberration because its first surface is less convex than the second. If the center of the lens were denser than the marginal parts, a condition that can not be secured in artificial lenses, the spherical aberration would also be reduced. It is a remarkable fact that the central part of the lens of the eye actually is denser than the outer parts. For these reasons the normal eye has only small and unimportant defects of this type. In case of a fixed lens L, the nearer the object AB is to L the more distant is its image ab. In the case of the camera, the nearer the object the more the