ERRORS OF REFRACTION 197 



Presbyopia is a gradual failure of the accommodation 

 for near work. Man becomes presbyopic between forty 

 and fifty years of age. It is due to a gradual hardening 

 of the lens, which fails to respond to the action of the 

 ciliary muscle. This error is corrected by placing before 

 the eye a plus spheric lens. 



Emmetropia. — An emmetropic eye is one that is 

 normal as far as any refractive error is concerned. 

 The focus of all distant objects fall upon the retina with- 

 out any effort of the cihary muscle. 



Major-General F. Smith has examined 100 horses' 

 eyes, and found that only i per cent, were emmetropic, 

 3 per cent, were hyperopic, 6 per cent, had mixed 

 astigmatism, and 90 per cent, were myopic. 



It is no doubt due to this fact that so many horses shy, 

 as distant objects are not clear until they come suddenly 

 upon them. Dogs and cats were formerly thought to be 

 hyperopic, but recent investigators have found them to 

 be myopic. A large number of wild animals' eyes have 

 been examined and have been found to be hyperopic. 



Method Used to Determine the Refractive Error.— 

 For diagnostic purposes retinoscopy may be used. 

 The retinoscope is a small, circular, plane mirror with a 

 small hole in the center. A Kght is placed near the right 

 side of the head, shading the eyes, and a reflection of this 

 light is thrown, at one meter's distance, into the pupil 

 through the refractive media to the retina. The exam- 

 iner looks through the central opening, and moves the 



