SEC. 4. IMPERFECTIONS IN THE DIOPTRIC 

 APPARATUS. 



545. 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 im- 

 perfect 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 eye- 

 ball. 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 refrac- 

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

 before they fall on the cornea, sufficient divergence 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, before they fall on the cornea, 

 sufficient convergence to enable the eye to bring them to a focus 

 on the retina. 



870 



