784 THE EYE AND VISION [GIL LVI. 



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. 586. 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 

 pupillae 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 

 the retractor lentis. In cephalopods the same occurs, but the retractor lentis is 

 absent ; here the approach of the lens to the retina is brought about by an alteration 

 of intra-ocular tension. In Amphibia and most snakes, the eye at rest is focussed 

 for distant objects; in accommodating for near objects the lens, by alteration of 

 intra-ocular tension, is brought forward, that is, the distance between it and the 

 retina is increased. There appear to be not a few animals in all classes which do not 

 possess the power of accommodation at all. Indeed, Barrett states this is so for 

 most mammals. 



DEFECTS IN THE OPTICAL APPARATUS 



Under this head we may consider the defects known as (1) 

 Myopia, (2) Hypermetropia, (3) Astigmatism, (4) Spherical Aber- 

 ration, (5) Chromatic Aberration. 



The normal (emmetropic) eye is so adjusted that at rest parallel 



