716 HANDBOOK OF PHYSIOLOGY. 



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 eye, the type at last becomes indistinct, and cannot be 

 brought into focus by any effort of accommodation, however strong. 

 This, which is termed the near-point , can be determined by the follow- 



Fig. 425. Diagram representing by dotted lines the alteration in the shape of the lens on ac- 

 commodation for near objects. (E. Landolt.) 



ing experiment (Scheiner). Two small holes are pricked in a card with 

 a pin not more than a twelfth of an inch (2 mm.) apart, at any rate 

 their distance from each other must not exceed the diameter of the pu- 

 pil. 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 

 focussed, but when brought nearer, beyond a certain point, the image 

 appears double or at any rate blurred. This point where the needle 

 ceases to appear single is the near-point. Its distance from the eye can 

 of course be readily measured. It is usually about 5 or 6 inches (13 

 cm.). In the accompanying figure (fig. 426) the lens b represents the 



Fig. 426. Diagram of experiment to ascertain the minimum distance of distinct vision. 



eye; e/the two pin-holes in the card, nn the retina; a represents the po- 

 sition of the needle. When the needle is at a moderate distance, the 

 two pencils of light coming from e and/, are focussed at a single point on 

 the retina nn. If the needle be brought nearer than the near-point, the 

 strongest effort of accommodation is not sufficient to focus the two pen- 



