SEC. 5. IMPERFECTIONS IN THE DIOPTRIC 

 APPARATUS. 



732. Imperfections of accommodation. The emmetropic eye, 

 in which the principal posterior focus lies on the retina, may, as we 

 have said, be taken as the normal eye. The myopic, in which the 

 principal posterior focus lies in front, and the hypermetropic eye, 

 in which it lies beyond the retina, may be considered as imperfect 

 eyes, though the former possesses an advantage over the normal 

 eye in so far that it can see minute objects more distinctly than 

 can the normal eye, since these can be brought so near the eye as 

 to give a relatively large retinal image and yet remain within 

 the limits of accommodation. An eye may be myopic from too 

 great a convexity of the cornea, or of the anterior surface of the 

 lens, or from permanent spasm of the accommodation-mechanism, 

 or from too great a length of the long axis of the eyeball. The 

 last appears to be the usual cause. Similarly, the cause of 

 hypermetropism is in most cases the possession of too short a 

 bulb. In presbyopia the failure or loss of accommodation may 

 be due either to a loss of elasticity of the lens, or to increasing 

 weakness of the ciliary muscle, or to the parts becoming rigid ; 

 the first appears to be the more common cause ; the change, 

 which may affect not only normal but also other eyes, generally 

 begins in the fifth decade of life. 



These several defects may be remedied by the use of appro- 

 priate lenses, by wearing proper spectacles. The myopic eye 

 needs for distant objects the rays of which fall parallel on the 

 cornea (or at least so little divergent that they still are brought 

 to a focus in front of the retina) a concave glass, of such a 

 refractive power, of such a focal length, as to give to parallel rays 

 sufficient divergence before they fall on the cornea to enable the 

 dioptric mechanism of the eye to bring them to a focus on, and no 

 longer in front of, the retina. 



The hypermetropic eye needs a convex glass of such a focal 

 length as will give to parallel rays sufficient convergence before 

 they fall on the cornea to enable the eye to bring them to a 

 focus on the retina. 



The presbyopic eye similarly needs a convex glass the focal 



