THE EYE AS AN OPTICAL INSTRUMENT. 



203 



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 ob- 

 jects 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 magni- 

 fied. 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 OF 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 com- 

 binations 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 oppo- 

 site of the dispersion by convex lenses, so that this may be cor- 

 rected by a combination of the two ; but when this is done with 

 lenses made of precisely the same material, the magnifying power 



