7 88 A MANUAL OF PHYSIOLOGY 



as compared with 10 mm. when the eye is directed to a 

 distant object and there is no accommodation. When the 

 lens has been removed for cataract, fairly distinct vision 

 may still be obtained by compensating for its loss by convex 

 spectacles of suitable refractive power (10 diopters* for 

 distant vision, and 15 diopters for the distance at which a 

 book is usually held), but no power of accommodation 

 remains. The person does indeed contract the pupil in 

 regarding a near object, just as happens in the intact eye ; 

 the most divergent rays are thus cut off and the image made 

 somewhat sharper, and there may appear to be some faculty 

 of accommodation left. But the loss of the whole iris by 

 operation does not affect accommodation in the least ; the 

 iris, therefore, takes no part in it. That no change in the 

 antero-posterior diameter of the eyeball, caused by its 

 deformation by the contraction of the extrinsic muscles, 

 can have any share in accommodation, as has been suggested, 

 is clearly proved by the fact that atropia, which does not 

 affect the action of these muscles, paralyzes the mechanism 

 of accommodation. To the consideration of that mechanism 

 we now turn. 



The Mechanism of Accommodation. While everybody is 

 agreed that the main factor in accommodation is the altera- 

 tion in the curvature of the lens, there is by no means the 

 same unanimity as to the manner in which this is brought 

 about. Helmholtz's explanation, which has long been the 

 most popular, is as follows : In the unaccommodated eye 

 the suspensory ligament and the capsule of the lens are 

 tense and taut, the anterior surface of the lens is flattened 

 by their pressure, and parallel rays (or, what is the same 

 thing, rays from a distant object) are focussed on the retina 

 without any sense of effort. In accommodation for a near 

 object, the meridional or antero-posterior fibres of the 



* A diopter (i D) is the unit of refractive power generally adopted in 

 measuring the strength of lenses, and corresponds to a lens of I metre 

 focal length. A lens of 2 diopters (2 D) has a focal length of % metre, 

 a lens of 4 diopters (4 D) a focal length of metre, and so on. The 

 diverging power of concave lenses is similarly expressed in diopters, with 

 the negative sign prefixed. Thus, a concave lens of i metre focal length 

 has a strength of i D and will just neutralize a convex lens of i D. 



