680 THE SENSES 



connections. In life this slackening of the tension of the suspensory liga- 

 ment of the lens is brought about by the contractions of the fibers of the ciliary 

 body. This allows the anterior surface of the lens to become more convex, 

 by its own elastic powers, thus focusing entering rays of light from a near 

 object upon the retina, figure 460. It therefore appears that when the eye 

 is at rest it is focused for distant objects, in as much as the suspensory liga- 

 ment is taut and the anterior surface of the lens more flattened. The nor 

 mal eye is passive when in focus for distant objects. It is the active contrac- 

 tion of the muscles of accommodation that focuses for near objects. The 

 iris acts in co-ordination with the accommodative contractions of the ciliary 

 muscles. In viewing near objects the pupil contracts, and upon viewing 

 distant ones it dilates. 



Range of Distinct Vision. Near-point. In every eye there is a limit to 

 the power of accommodation. If a book be brought nearer and nearer to the 



FIG. 460. Diagram Representing by Dotted Lines the Alteration in the Shape of the Lens 

 on Accommodation for Near Objects. (E. Landolt.) 



eye, the type at last becomes indistinct, and cannot be brought into focus 

 by any effort of accommodation, however strong. This limit, which is 

 termed the near-point, can be determined by the experiment of Schemer. 

 Two small holes not more than 2 mm. apart are pricked in a card with a pin; 

 at any rate their distance from each other must not exceed the diameter of the 

 pupil. The card is held close in front of the eye, and a small needle viewed 

 through the pin-holes. At a moderate distance it can be clearly focused, 

 but when brought nearer, within a certain point, the image appears double 

 and more or less blurred. This point where the needle ceases to appear 

 single is the near-point of vision. Its distance from the eye can of course be 

 readily measured. It is usually about five or six inches, 12 to 15 cm. In the 

 accompanying figure, 461, the lens b represents the eye; e, /, the two pin- 

 holes in the card, nn the retina; a represents the position of the needle. When 

 the needle is at a moderate distance, the two pencils of light coming through 



