THE SENSE OF VISION. 



759 



it, and will therefore necessitate a constantly increasing amount of change in 

 the lens to enable it to focus the rays on the retina. We find, accordingly, that 

 all objects more than two meters distant from the eye can be seen distinctly at 

 the same time L e. without any change in the accommodative mechanism 

 but for objects within that distance we are conscious of a special effort of 

 accommodation which becomes more and more distinct the shorter the distance 

 between the eye and the object. 



Myopia and Hypermetropia. There are two conditions of the eye in 

 which the range of accommodation may differ from that which has just been 

 described as normal. These conditions, which are too frequent to be regarded 

 (except in extreme cases) as pathological, are generally dependent upon the 

 eyeball being unduly lengthened or 

 shortened. In Fig. 223 are shown 

 diagram matically the three conditions 

 known as emmetropia, myopia, and 

 hypermetropia. In the normal or 

 emmetropic eye, A, parallel rays are 

 represented as brought to a focus on 

 the retina; in the short-sighted, or 

 myopic, eye, -B, similar rays are 

 focussed in front of the retina, since 

 the latter is abnormally distant; while 

 in the over-sighted, or hypermetropic, 

 eye, (7, they are focussed behind the 

 retina, since it is abnormally near. 



It is evident that when the eye is 

 at rest both the myopic and the hy- 

 permetropic eye will see distant ob- 

 jects indistinctly, but there is this 

 important difference : that in hyper- 

 metropia the difficulty can be cor- 

 rected by an effort of accommodation, 

 while in myopia this is impossible, 

 since there is no mechanism by which 

 the radius of the lenticular surfaces can be increased. Hence an individual 

 affected with myopia is always aware of the infirmity, while a person with 

 hypermetropic eyes often goes through life unconscious of the defect. In this 

 case the accomodation is constantly called into play even for distant objects, and 

 if the hypermetropia is excessive, any prolonged use of the eyes is apt to be 

 attended by a feeling of fatigue, headache, and a train of nervous symptoms 

 familiar to the ophthalmic surgeon. Hence it is important to discover this defect 

 where it exists and to apply the appropriate remedy viz. convex lenses placed 

 in front of the eyes in order to make the rays slightly convergent when they 

 enter the eye. Thus aided, the refractive power of the eye at rest is sufficient 

 to bring the rays to a focus upon the retina and thus relieve the accommoda- 



FIG. 223. Diagram showing the difference between, 

 normal, myopic, and hypermetropic eyes. 



