FAR SIGHT AND NEAR SIGHT. 



beyond it, and consequently the image on the retina would be 

 indistinct. 



22. It follows, therefore, that eyes which see distant objects 

 at the greater class of distances would see indistinctly all 

 objects at less distances, unless there were in the eye some 

 means of self- adjustment, by which its convergent -power may 

 be augmented. Such means of self- adjustment are provided, 

 which operate within certain limits, by which we are enabled so 

 to accommodate the eye to the divergence of the pencils proceed- 

 ing from near objects, that the same eyes which are capable of 

 seeing distinctly objects so distant as to render the rays of the 

 pencils sensibly parallel, are also capable of seeing with equal 

 distinctness objects at distances varying from ten to twelve inches 

 and upwards. 



23. By what means the convergent power of the humours is 

 thus varied is not certainly known, but that such means of self- 

 adjustment exist may be proved by the following experiment : 



Let a small black spot be made upon a thin transparent plate 

 of glass, and let it be placed at a distance of about twelve inches 

 from the eye. If the eye be directed to it, the spot will be seen. 

 as well as distant objects visible through the glass. Let the 

 attention be earnestly directed to the black spot, so that a distinct 

 perception of its form may be produced. The objects visible at a 

 distance will then be found to become indistinct. 



But if the attention be directed more to the distant objects, so as 

 to obtain a distinct perception of them, the perception of the black 

 spot on the glass will then become indistinct. It is evident, 

 therefore, that when the eye accommodates itself so as to form. 

 upon the retina a distinct image of an object at twelve inches 

 distance, the image produced by objects at great distances will 

 become indistinct ; and that, on the other hand, when the eye 

 so accommodates itself as to render the image produced on the 

 retina by distant objects distinct, the image produced by an 

 object at two inches distance will become indistinct. 



24. It is evident, therefore, that the power of the eye to refract 

 the pencils of light incident upon it, is to a certain extent under 

 the control of the will ; but by what means this change in the 

 refracting power of the organ is made is not so apparent. Various 

 hypotheses have been advanced to explain it. According to some, 

 the form of the eye-ball, by a muscular action, is changed in 

 such a manner as to increase the length of the optic axis, and 

 thus to remove the posterior surface of the retina to a greater 

 distance from the crystalline, when it is necessary to obtain a 

 distinct view of near objects ; and, on the contrary, to elongate 

 the transverse diameter of the eye, and shorten the optic axis so 



57 



