REFRACTION 293 



REFRACTION 



The refractive power of the eye, by which rays of hght 

 are brought to a focus on the retina, is attributable to the 

 cornea, aqueous humour, lens and vitreous humour. Of 

 these the most important is the cornea. 



Errors of Refraction.— Hypermetropia, or long-sight, is due 

 in children to the eyeball being too small, owing to its 

 having prematurely ceased to grow. Rays of light come 

 to focus behind the retina. This error is corrected by the 

 use of convex glasses. Hypermetropia also occurs in old 

 age owing to failure of accommodation. 



In Myopia, or short-sight, hght comes to a focus in front 

 of the retina. It is due either to the eyeball being too 

 long or to the lens being too highly refractive. The former 

 defect is due to deficient nutrition during the growing 

 period and over-strain, the weakened eyeball being unable 

 to withstand the intraocular pressure. It is for this reason 

 that treatment should not only include the provision of 

 concave glasses, but should also be directed to relieving 

 the general condition. 



Another error of refraction is astigmatism. This is due 

 to the lens not having the same curvature in its horizontal 

 and vertical axes. The consequence is that horizontal and 

 vertical lines cannot be simultaneously focussed. For this 

 defect cyhndrical glasses are used. 



ACCOMMODATION 



When the eye is looking at a distant object the rays 

 of hght coming from that object are practically parallel. 

 These in a normal eye come to a focus on the retina without 

 any accommodation. Rays from a near object, however, 

 diverge as they approach the pupil, and if no change took 

 place in the eye would come to a focus behind the retina. 

 To correct for this the eye undergoes the process of 

 accommodation, which takes place in the following way. 



