22 



THE OPTICAL DEFECTS OF THE EYE. 



Prof. Bonders, of TJtrecbt, from his elaborate researches on this 

 subject, first pointed out how common this afi'ection is, and how 

 frequently it is the sole cause of that peculiar weakness of sight 

 (formerly so little understood) called asthenopia. 



Donders believes that this condition of the eye depends more upon 

 a shortening of the antero-posterior diameter of the eye, than upon 

 a too low degree of its refractive power ; that the cornea and crys- 

 taline lens have a normal degree of .curvature, and that parallel 

 rays would form a focus at the normal distance behind the lens, 

 were the retina far enough back to receive it. 



A very good illustration of a hypermetropie eye is one in which the 

 crystaline lens has been removed in the operation for cataract. To 

 enable such an eye to see distinctly, even distant objects, it is neces- 

 sary to place in front of it a strong convex lens of about four inches 

 focus, called a cataract glass. The eye having too low a refractive 

 power to convei'ge rays to a focus, on the retina, it is necessary to give 

 rays falling upon the eye, a preliminary degree of convergence ; the 

 eye having sufficient power to complete their refraction to a point 

 upon"the retina. We do the same thing in relieving cases of hyper- 

 metrophia. 



Fig. 10 represents a hypermetropie eye in a state of rest. P P are 

 parallel rays which are focussed behind the retina at f. L, Fig. 11, 

 is a convex lens which changes the parallel rays to convergent ones, 

 at c, c, as if they came from the direction a b and d e, which again 

 are refracted by the eye, and brought to a focus upon the retina at F. 



When a hvpermetropie eye is in a state of rest, and directed to dis- 

 tant objects, it is adjusted for convergent rays ; images upon the 

 retina will consequently be ill defined, and vision will be indistinct. 

 To remedy this, it is necessary for the eye to increase its refractive 

 power by increasing the antero-posterio diameter of the crystaline 

 lens, so as to bring parallel rays to a focus on the retina. 



