MYOPIC AND PKESBYOPIC EYE. 



271 



In the myopic or short-sighted the visual point is so close, that objects 

 cannot be seen unless brought near the eye. This defect is owing to 

 too great a refractive power in the 

 transparent parts of the organ; or to Fig. 1 19. 



too great a depth of the humours ; or 

 it may be caused by unusual convexity 

 of the cornea or crystalline ; or from 

 the retina being too distant from the 

 latter. From any one or more of 

 these causes, the rays of light pro- 

 ceeding from distant objects, are Myopic Vision, 

 brought to a focus before they reach - 



the retina, and the objects consequently are not distinctly visible. 

 (Fig. 119.) To see them distinctly, they must be placed close to the 

 eye, in order that the rays may fall more divergently; and the focus 

 be thrown farther back so as to impinge upon the retina. The defect 

 may be palliated by the use of concave glasses, which render the rays, 

 proceeding from the object, more divergent. It is by no means unfre- 

 quent in youth; and the myope has been consoled with the common 

 belief, that, in the progress of life, and in the alterations which take 

 place in the eye from age, he is likely to see well without spectacles, 

 when others of the same age may find them essential. It is probable, 

 however, that this is, in many cases at least, a vulgar error; as we 

 have known different myopic sexagenarians, who have not experienced 

 the slightest improvement. 



The presbyope, presbytic, or long-sighted labours under an opposite 

 defect. The visual point is much more distant than the average ; and 

 he is unable to see an object unless 



it is at some distance. This con- Fig. 120. 



dition is owing to too feeble a re- 

 fractive power in the transparent 

 parts of the eye; to insufficient 

 depth of the eyeball ; to too close an 

 approximation between the retina 

 and crystalline; or to too little con- 

 vexity of the cornea or crystalline ; Presbyopic Vision, 

 so that the rays of light proceeding 



from a near object are not rendered sufficiently convergent to impinge 

 upon the retina ; but fall behind it. This defect, which is experienced 

 more or less by most people after middle age, is palliated by the use of 

 convex glasses, which render the rays proceeding from an object more 

 convergent, and enable the eye to refract them to a focus farther for- 

 ward, or on the retina. 



Although the presbyopic eye is unusual in youth, it is sometimes met 

 with. A young friend, at ten or twelve years of age, was compelled 

 to employ spectacles adapted to advanced life ; and this was the case 

 with several of the members of a family, to whom the arts have been 

 largely indebted in this country. One of them, at twenty, was com- 

 pelled to wear spectacles which were almost microscopes. 



Both the myopic and the presbyopic conditions exist in a thousand 



