850 ACCOMMODATION. [BOOK in. 



at an infinite distance. The ' range of distinct vision' therefore 

 for the emmetropic eye is very great. In the myopic, or short- 

 sighted eye, the near limit is brought much closer (5 or 6 cm.) 

 to the 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 infi- 

 nite 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 accommodating 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 princi- 

 pal 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 hyper- 

 metropic eye behind it. 



533. 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 ; we are conscious of no effort 

 in turning from moderately distant to far distant objects ; and 

 when the power of the eye to accommodate is impaired or 

 abolished, as we shall see it may be, by atropin or nervous dis- 

 ease, the vision of distant objects may be unaffected. The sense 



