THE OPTICAL SYSTEM OF THE EYE 525 



chamber of the eye is, as we have seen, approximately, 2-6 mm. only. This brings us to 

 the two modern theories, that of Helmholtz and Tscherning. The theory of the former 

 (which is the one most generally accepted) supposes that the lens when removed from 

 the eye is strongly convex and is accommodated for near vision. When in the eye 

 however, it is caused to become flatter through the traction of the zonula of Zinn (suspen- 

 sory ligaments) on the edges of its capsule, and is therefore focussed for distance. But 

 when the ciliary muscle contracts, it removes the tension on the zonula and therefore 

 allows the lens to return by its elasticity to its more spherical form. Before describ- 

 ing the rival theory it would be well to examine the principal evidence on which 

 Helmholtz' theory has been v based. During near vision measurement by means of 

 the ophthalmometer shows that the anterior surface of the lens advances slightly and 

 becomes at the centre of much greater curvature (10 mm. radius for distant vision, to 

 6 mm. for near). There can thus be no question that the change in the curvature 

 of the lens is responsible for accommodation. The posterior surface is found to change 



FIG. 265. Diagram to show the changes in the position and shape of the eye structures 

 during accommodation. Thin line at rest ; thick line during accommodation. 



but little ; almost the whole range is therefore produced by the anterior surface. The 

 changes found to occur in the lens may therefore be summarised as follows : 



Distance Near 



Radius of anterior surface . . . . .10 . . 6 



Radius of posterior surface . . . . .6 . . 5-5 



Thickness of lens 3-6 4 



Focus of lens in mm. . . ' . . .44 . . 30 



Focus of lens in dioptres . . . . 23 .. 33 



If the lens in near vision becomes more spherical owing to the relaxation of the 

 zonula, as Helmholtz supposed, we should expect a lens removed from the eye to be 

 more spherical still, that is in a state of strong accommodation. Tscherning stated that 

 the changes in the curvature of the lens are much more complex than those given above. 

 During accommodation not only is there, he says, an increase in the curvature near the 

 centre of the lens, but at the same time a decrease in the curvature at the periphery. 

 This view he supported by quoting the careful measurements which Young made by 

 means of his optometer, and which have been confirmed by other observers. There 

 is found to be a zone about 1-4 mm. from the centre of the lens where the curva- 

 ture does not change appreciably. Inside this zone the curvature increases during 

 accommodation, whereas outside the lens becomes flatter. 



Tscherning supposed that these changes of curvature are produced by increase in the 

 tension of the zonula during accommodation, in other words exactly the opposite action 

 to that which Helmholtz supposed to occur. This question as to whether contraction 

 of the zonula is associated with near or distant vision can therefore be used as a criterion 

 between the rival theories. Experiments haye shown tha,t the choroid moves 



