686 THE SENSES 



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 focused 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 focused on the retina. But those 

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

 which is less than twenty feet, cannot be distinctly focused. 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 focused 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 

 near 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 

 especially 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 w r ith considerable uniformity from youth to old 

 age. It is not a disease, but a physiological process which every eye under- 

 goes as its owner grows older. It is due to a gradual diminution of elasticity 

 of the lens by a short sclerosis from the center toward the periphery. It 



