THE EYE AND VISION. [CH. LV. 



however strong. This, which is termed the near-point, can be 

 determined by the following 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 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 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. 5 84) the lens ft repre- 

 sents the eye ; ef the two pin-holes in the card, nn the retina ; 

 a represents the position of the needle. When the needle is at a 



Fig. 584. Diagram of experiment to ascertain the minimum distance of distinct vision. 



moderate distance, the two pencils of light coming from e and / 

 are focussed at a single point on the retina nn. If the needle is 

 brought nearer than the near-point, the strongest effort of accom- 

 modation is not sufficient to focus the two pencils, they meet at a 

 point behind the retina. The effect is the same as if the retina 

 were shifted forward to mm. Two images h.g. are formed, one 

 from each hole. It is interesting to note that when two images 

 are produced, the lower one g really appears in the position Q, 

 while the upper one appears in the position p. This may be 

 readily verified by covering the holes in succession. 



During accommodation two other changes take place in the 

 eyes: 



(1) The eyes converge owing to the action of the internal rectus 

 muscle of each eyeball. 



(2) The pupils contract. 



The contraction of all of the muscles which have to do with 

 accommodation, viz. of the ciliary muscle, of the internal recti 

 muscles, and of the sphincter pupillse is under the control of the 

 third nerve. 



