DIOPTRIC MECHANISMS. 749 



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 

 abnormal 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. In the nor- 

 mal emmetropic 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. 



Hypermetropic Eye. Rays coming from a distance focussed too late.] 



623. 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 dif- 

 ferent 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 distance 

 takes place is shown 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 paralyzed by atropine 

 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 in- 

 ternal changes taking place in the eye. 



624. 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, how- 

 ever, showed that accommodation took place as usual when the eye (and 

 head) is immersed in water. Since the refractive powers of aqueous humor 

 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 shown 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 altera- 

 tion in the curvature of the cornea, and pressure on the bulb would act in- 

 juriously by rendering the retina anaemic and so less sensitive. In fact, 

 there are only two changes of importance which can be ascertained to take 

 place in the eye during accommodation for near objects. 



