ACCOMMODATION 



31 



There is thus a certain leeway which the focus of the optical image 

 may have without its becoming blurred in the consciousness. This lee- 

 way is in fact so great that without any change in the dioptric structures 

 of the eye, an object can approach from the horizon to a distance of 

 about twenty feet* without its image moving back far enough to get out 

 of the visual-cell layer and into the insensitive chorioid. The image in 

 the eye is so very small compared with the object that since the move- 

 ment of the image, either laterally or forward and backward, is minified 

 to a high degree, the movements of the image over the surface of the 

 retina (especially through its thickness) are almost microscopic. Conse- 



Fig. 14 — The mechanism of human accommodation. 



The left half of the diagram shows the structures in relaxation. The thickness of the lens 

 capsule has been exaggerated one hundred times to bring out its local variations. On the 

 right, accommodation; by reference to the angular scales, the movements of the various parts 

 can be discerned. Note that the contraction of both the radial and circumferential portions 

 of the ciliary muscle has stretched forward the smooth orbicular region of the ciliary body 

 (to which most of the zonule fibers attach) and has bunched up the coronal region (bearing 

 the ciliary processes, whose profiles are indicated by the dotted lines). The relaxation of the 

 zonule fibers has permitted the elastic lens capsule to mold a bulge of sharpened curvature 

 on the anterior surface of the lens. Note also that the sphinrter muscle of the iris has 

 contracted, closing down the pupil in its 'accommodation reflex'. 



quently, the object may recede from twenty feet to infinity without its 

 image coming forward more than the length of the rods and cones — 

 a small fraction of a millimeter (see Fig. 19, p. 43). 



Within twenty feet, however, the refracting power of the media must 

 somehow be increased to keep the image in the visual-cell layer of the 

 retina. In the human, the anterior surface of the lens is sharpened in 



*It is really a bit more, but so variable that for the didactic purposes of this book, twenty 

 feet is arbitrarily taken as standard. 



