THE OPTICAL DEFECTS OF THE EYE. 21 



Posterior staphyloma is a serious complication in myopia, as the 

 sensibility of the retina becomes more or less impaired in the position 

 of the bulging of the selerotic, and in some cases the retina becomes 

 detached from the choroid. It is the existence of this disease that 

 prevents improvement in cases of myopia, as the eye becomes flat- 

 tened with advanciug age. 



Donders considers that in myopia, the antero-posterior diameter is 

 alone at fault; that is, it is too much elongated, and that the cornea 

 and crystaline lens have usually a normal curvature. 



The characteristics of a myopic eye, are* 



1st. Parallel rays are focussed in front of the retina. 



2nd. The "far" point is at a definite distance and positive. 



3rd. When the eye is in a state of rest it is adapted for divergent 

 rays. 



4th. Concave glasses improve vision. 



Chaptee IV. — Hypeemeteopia. 



Ton will remember that when a normal eye is in a state of rest, 

 and directed to a distant object, parallel rays are brought to a focus 

 upon the retina, and that when a myopic eye is in a state of rest, 

 parallel rays are brought to a focus in front of the retina. When, 

 however, a hyperemtropic eye is in a state of rest, parallel rays would 

 (if continued) form a focus behind the retina. Hypermetropia 

 is, therefore, the reverse of myopia. In myopia, the refractive power 

 of the eye is excessive, and in hypermetropia it is not strong enough. 

 When the accommodation of a myopic eye is paralysed, it has the 

 power of focussing none bat diverging rays upon the retina, but a 

 hypermetropic eye under the same circumstances can focus only con- 

 verging rays upon the retina. The " far" point of a myopic eye is 

 at a definite distance and positive, but the "far" point of a hyper- 

 metropic eye is at a definite distance and negative. Concave glasses 

 improve the vision for a myopic eye, and convex for a hypermetropic 

 one. 



This is an afi'eetioa which has received very little attention until 

 within the last ten years. It was indeed noticed by Dr. McKenzie 

 of Glasgow, in 1841, but it was not until about five years ago that 



• Prom Bonders' system of classification, 



