of an inch in diameter and its central portion is only a fourth of an inch thick. Adjusted for infinite distance, the front curvature has a radius of about four tenths of an inch, while for near objects the radius is only about three tenths of an inch. A curious experiment is looking at a minute object through a pinhole in a bit of paper or cardboard, when the object appears highly magnified. This is because the nearer the object is to the eye, the larger it appears. The shortest normal distance of distinct vision is about five inches; but in looking through a pinhole we can see at a distance of less than an inch, using a very small part of the central portion of the crystalline lens. Accommodation for very near objects is assisted, also, by contraction of a little band of fibers in the iris, about a fiftieth of an inch in width, immediately surrounding the pupil.
The most wonderful thing about the formation of a perfect image upon the retina is the mechanism of correction for form
Fig. 4.—Section of the Lens showing the Mechanism or Accommodation. The left side of the figure (F) shows the lens adapted to vision at infinite distances. The right side of the figure (N) shows the lens adapted to the vision of near objects. (After Fick.)
and color. In grinding lenses for the microscope, for example, it is mechanically easy to make a very small convex lens with perfectly regular curvatures—that is, each curvature being a portion of a perfect sphere; but in such a lens the focus of the central portion is longer than that of the parts near the edge; and when an object is in focus for the center it is out of focus for the periphery. This is a fatal objection to the use of uncorrected lenses of high power; but in microscopes it is corrected by combinations of lenses, reducing the magnifying power, however, about one half. This is not all. When white light passes through a simple lens it is decomposed into the colors of the spectrum. This is called dispersion, and it surrounds the object with a fringe of colors. The dispersion by concave lenses is exactly the opposite of the dispersion by convex lenses, so that this may be corrected by a combination of the two; but when this is done with lenses made of precisely the same material, the magnifying power
