CHAP. IL] SIGHT. 497 



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. In the normal emme- 

 tropic eye, when no effort of accommodation is made, the principal 

 focus of the eye lies on the retina, in the myopic eye in front of it, 

 and in the hypermetropic eye behind it. 



Mechanism of Accommodation. 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 accommodation for near objects; 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 that will 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 facts that the eye, when opened after being 

 closed for some time, is found not in medium state but adjusted 

 for distance ; that when the accommodation mechanism of the eye 

 is paralysed by atropin or nervous disease, the accommodation for 

 distant objects is unaffected; and that 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. 



What then are the changes which take place in the eye, when 

 we accommodate for near objects ? It might be thought, and 

 indeed once was thought, that the curvature of the cornea was 

 changed, becoming more convex, with a shorter radius of curvature, 

 for near objects. Young, however, shewed that accommodation 

 took place as usual when the eye (and head) is immersed in water. 

 Since the refractive powers of aqueous humour and water are very 

 nearly alike, the cornea, with its parallel surfaces, placed between 

 these two fluids, can have little or no effect on the direction of 

 the rays passing through it when the eye is immersed in water. 

 And accurate measurements of the dimensions of an image on 

 the cornea have shewn that these undergo no change during 

 accommodation, and that therefore the curvature of the cornea 

 is not altered. Nor is there any change in the form of the bulb ; 

 for any variation in this would necessarily produce an alteration 

 in the curvature of the cornea, and pressure on the bulb would 

 act injuriously by rendering the retina anaemic and so less 

 sensitive. In fact, there are only two changes of importance 

 p. 32 



