432 ORGANS OF TEE SENSES. 



by increasing the converging power of the eye : hyperme- 

 tropics, therefore, make use of a convex glass, for the purpose 

 of shortening the ocular cone, as do also far-sighted persons 

 when they desire to see objects near at hand, their power of 

 accommodation or adaptation being insufficient for this pur- 

 pose. 



The study of these various degrees of converging power in 

 the eye, and of the artificial means by which its defects are 

 remedied, will enable us to understand how accommodation is 

 effected in the normal condition. The use of glasses, of which 

 we have just spoken, is a sort of artificial accommodation, 

 especially in the case of far-sighted persons. It is, therefore, 

 probable, that in physiological accommodation, something 

 similar takes place ; in other words, the converging power of 

 this organ is modified. 



It was for a long time supposed that the mechanism of 

 accommodation consisted in a change in the form of the eye, 

 modifying, not the ocular cone, but the position of the retina; 

 which would then be placed at the summit of the cone, caus- 

 ing the eye, when looking at objects at a distance, to shorten 

 in antero-posterior diameter under the influence of the recti 

 muscles of the eye, .and to lengthen under that of its oblique 

 muscles, when looking at objects close at hand. This func- 

 tion of the motor muscles of the eye is, however, entirely 

 hypothetical, and the theory is contradicted by the anatom- 

 ical arrangement of these muscles, as well as by physiological 

 experiments. 



It has been also supposed that the crystalline lens can be 

 moved backwards or forwards, and can act in the same man- 

 ner as we use the objective in a microscope when we desire 

 to bring an object into focus ; a knowledge of anatomy, how- 

 ever, shows that this is impossible, and direct experiments 

 have shown that there is no foundation for such a theory. 



Direct experiment shows that accommodation, as our study 

 of artificial adaptation would lead us to suppose, consists in 

 a change of curve, and, consequently, a change in the con- 

 verging power of one only of the media of the eye, the crystal- 

 line lens. The experiment is made by studying the images 

 furnished by the different surfaces of the media of the eye, 

 these surfaces acting like mirrors. Thus we may easily ob- 

 serve that objects are reflected in the surface of the cornea, 

 as well as in the anterior and posterior surfaces of the crys- 

 talline lens; so that, if we place a light before an eye (Fig. 

 117), we shall find in the eye three images of the flame : the two 



