CHAP. XVII.] OFFICE OF THE IRIS. 49 



distant, they exhibit a characteristic winking (//,i;w, connivo). 

 Myopia occurs in adolescence, and is accompanied with a too great 

 refracting power of the media so that the image is formed anterior 

 to the retina. In order, therefore, to throw back the image on 

 the retina, the object is brought very close to the eye; or the 

 convergence of the rays of light may be diminished by the use of 

 a concave lens. 



It seems probable that the state of myopia may be acquired by 

 the habit of looking intently at small and near objects, and that the 

 common practice of remedying the inconvenience by the use of con- 

 cave glasses tends to increase the defect. Frequent exercise of the 

 eyes on remote objects has, no doubt, the effect of making them far- 

 sighted. It is a common error to say that myopia disappears natu- 

 rally in advanced life. 



Presbyopia. Others again imperfectly distinguish near objects, 

 whilst they see distant ones very plainly, and the distance at which 

 they can see distinctly is sometimes very great. Persons thus 

 affected cannot read small print with the eyes unassisted, and they 

 prefer holding the book at a distance. This condition of vision is 

 connected with a too flat cornea, a deficient aqueous humor, or a 

 flattening of the lens ; and it is in a great degree accounted for by 

 the diminution in the refracting power thence resulting, so that 

 the focus is behind the retina. It belongs to the advanced periods 

 of life. It may be corrected by convex glasses, which increase the 

 convergence of the rays of light. 



It is manifest that neither of these defects is dependent on the 

 muscular apparatus of adjustment, but rather on the curves of the 

 refracting media, which throw the organ in one direction or the 

 other beyond the range of the adjusting power with which it is 

 provided. When the refracting media are optically corrected, as 

 by the use of glasses, the adjusting faculty can be exercised. 



In the eye, considered as an optical instrument, there are. other 

 powers besides those already named, which serve to make it more 

 perfect, and to place it in favourable contrast with the most suc- 

 cessful creations of human ingenuity. 



One important office of the iris is to prevent the passage of rays 

 through the circumferential part of the lens, and thus to obviate 

 that indistinctness of vision which would arise from spherical aberra- 

 tion, or that unequal refraction which results from the difference in 

 the angle of incidence of the several rays on a curved surface. In 

 this respect it resembles the diaphragms used in optical instruments. 

 By its position, close to the surface of the principal lens, and 



VOL. II. E 



