CH. LYIII.] ACCOMMODATION 827 



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. 528) the lens b represents the refractive apparatus of the eye ; 

 e and / 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 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 accommodation 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 



FIG. 528. Diagram of experiment to ascertain the minimum distance of distinct vision. 



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 pupillae, is under the control of the 

 third nerve. It should further be noted that although the act is a 

 voluntary one, the fibres of the ciliary muscle and of the sphincter 

 pupillse are of the plain variety. 



The account of accommodation as given in the preceding pages is true for man 

 and other mammals, birds, and certain reptiles. 



Beer has, however, shown that in many animals lower in the scale, the 

 mechanism of accommodation varies a good deal, and is often very different from 

 that just described, consisting, in fact, in a power of altering the distance between 

 the lens and the retina. 



In bony fishes, the eye at rest is accommodated for near objects ; in focussing 

 for distant objects the lens is drawn nearer to the retina by a special muscle called 



