CHAP, in.] SIGHT. 1155 



cornea ; and the far limit is at a variable but not very great 

 distance, so that the rays of light proceeding from an object not 

 many feet away are brought to a focus in the vitreous humour 

 instead of on the retina. The range of distinct vision is therefore 

 in the myopic eye very limited. In the hypermetropic, or long- 

 sighted eye, the rays of light coming from even an infinite 

 distance are, in the passive state of the eye, brought to a focus 

 beyond the retina. The near limit of accommodation is at some 

 distance off, and a far limit of accommodation does not exist. 

 The presbyopic eye, or eye of advanced years, resembles the 

 hypermetropic eye in the near point of accommodation being at 

 some distance, but differs from it inasmuch as the former is an 

 essentially defective power of accommodation, whereas in the latter 

 the power of accommodation may be good and yet, from the 

 internal arrangements of the eye, be unable to bring the image of 

 a near object on to the retina. When an eye becomes presbyopic, 

 the far limit may remain the same, but since the power of ac- 

 commodating for near objects is weakened or lost, the change is 

 distinctly a reduction of the range of distinct vision. When no 

 effort of accommodation is made, the principal posterior focus of 

 the eye lies in the normal, emmetropic eye on the retina, in the 

 myopic eye in front of it, and in the hypermetropic eye behind it. 



711. By what changes in the eye are we thus able, within 

 the above mentioned limitations, to see distinctly objects at differ- 

 ent distances? In directing our attention from a far to a very 

 near object we are conscious of a distinct effort, and feel that some 

 change has taken place in the eye ; when we turn from a very 

 near to a far object, if we are conscious of any change in the eye, 

 it is one of a different kind. The former is the sense of an active 

 exertion ; the latter, when it is felt, is the sense of relaxation 

 after exertion. 



Since the far limit of an emmetropic eye is at an infinite 

 distance, no such thing as active accommodation for far distances 

 need exist. The only change which need take place in the eye in 

 turning from near to far objects will be a mere passive undoing 

 of the accommodation previously made for the near object. And 

 that no such active accommodation for far distances takes place 

 is shewn by the following facts ; the eye, when opened after being 

 closed for some time, is found adjusted not for moderately distant 

 but for far distant objects ; when the power of the eye to accom- 

 modate is impaired or abolished, as we shall see it may be, by 

 atropin or nervous disease, the vision of distant objects may be 

 unaffected ; and we are conscious of no effort in turning from 

 moderately distant to far distant objects. The sense of effort 

 often spoken of by myopic persons as being felt when they 

 attempt to see things at or beyond the far limit of their range 

 seems to arise from a movement of the eyelids, and not from 

 any internal changes taking place in the eye. 



F. 74 



