DEFECTS IN THE OPTICAL APPARATUS 651 



further illustrated in figure 465 which represents the position / t shown in 

 figure 464. 



Myopia. This is that refractive condition of the eye in which parallel 

 rays are brought to a focus in front of the retina, 4, figure 466. It is due 

 either to an abnormal elongation of the eyeball, antero-posteriorly, or to an 

 increase in the convexity of the refracting surfaces, or to both of these con- 

 ditions. Parallel rays are focussed in front of the retina, and, crossing, 

 form circles on the retina. Thus, the images of distant objects are blurred 

 and indistinct. The eye is, as it were, permanently adjusted for a near point. 

 Rays from a point near the eye are exactly focussed on the retina. But those 

 which issue from any object beyond a slight distance, the myopic jar-point, 

 which is less than twenty feet, cannot be distinctly focussed. This defect 

 is corrected by concave glasses, which cause parallel rays entering the eye 



FIG. 465. Diagram of Character of Retinal Images in Astigmatism. (John Green.) 



to diverge. Such glasses of course are needed only to give a clear vision of 

 distant objects. For near objects they are not required. 



Hypermetropia. This is that refractive condition of the eye in which 

 parallel rays are brought to a focus behind the retina, 3, figure 466. It is the 

 opposite of myopia, and is due either to an abnormal shortening of the eye- 

 ball, antero-posteriorly, or to a decrease in the convexity of the refracting 

 surfaces, or both. Parallel rays entering the eye at rest are focussed behind 

 the retina. An effort of accommodation is therefore required to focus parallel 

 rays on the retina. When the rays are sharply divergent, as in viewing a very 

 near object, the accommodation is insufficient to focus them. Thus, both 

 ne2r and distant objects require an effort of accommodation, and the eye 

 is under a constant strain which produces in the end various nervous, as well 

 as ocular, disorders. This defect is obviated by the use of convex glasses, 

 which render the pencils of light more convergent. Such glasses are espe- 

 cially needed for near objects, as in reading, etc. They are also required for 

 distant vision to rest the eye by relieving the ciliary muscle from constant work. 



Presbyopia. Presbyopia is a condition of diminished range of accom- 

 modation. It takes place with considerable uniformity from youth to old age. 



