THE SENSE OF SIGHT. 619 



ing power of 5 dioptrics. This measure of the energy expended in 

 accommodation holds true only for emmetropic eyes and at about 

 forty years of age. 



Since lenses of the same curvature made from different materials 

 have different refracting powers, it becomes necessary to have, for 

 purposes of comparison, some unit of measurement. The unit now 

 accepted is the refracting power of a glass lens which is sufficient 

 to focalize parallel rays at a distance of 100 cm. or i meter. This 

 amount of refracting power is termed a dioptry. Lenses which 

 would focalize parallel rays at a distance of 50, 20, or 10 cm. are 

 said to have a refractive power of 2, 5, or 10 dioptrics, respectively, 

 obtained^ dividing into 100 cm. the focal distance. The refracting 

 power of a biconcave lens is determined by prolonging backward in 

 the direction the parallel rays have come, the rays which have been 

 rendered divergent by the lens. 



The refracting media of the human eye in repose have collectively 

 a refracting power of about 52 dioptrics, the reciprocal of its focal 

 length. The refractin^ppwer of tEe~corneal surface alone is equiva- 

 lent to 32^iop!nesr From the position occupied by the Ien%it would 

 require jor parallel rays a refracting power of 14 dioptrics. But 

 owing to its position in a medium denser than air and to the con- 

 verging direction of the rays which enter it, it has been calculated 

 that its refracting power is about 20 dioptrics. 



The capability of the lens to increase its refraction during accom- 

 modative efforts beyond the 20 dioptrics varies considerably at 

 different periods of life. At ten years the increase is 14 dioptrics, 

 as the near point is 7 cm.; at thirty years the ^increase is but 7 dipj> 

 tries, as the near point is 14 cm.; at sixty the increase is but i dioptry, 

 and the near point 100 cm.; at seventy it is zero. From youth to old 

 age, the elasticity of the lens steadily declines, and the range of accom- 

 modation diminishes from the recession of the near point. 



Convergence of the Eyes during Accommodation. In binocu- 

 lar vision the eyes are turned inward and the optic axis of each a 

 line passing through the center of the cornea and the center of the 

 eye turned' toward the median line during accommodation. So 

 long as the eyes are directed toward the far point, 65 meters or beyond, 

 the optic axes are parallel. When the eyes are directed to any point 

 within 65 meters the optic axes are converged, the convergence in- 

 creasing steadily as the near point is approached. In this way the 

 fovea of each eye is directed to the same point and single vision made 

 possible. Were this not the case, double vision would result. 



Functions of the Iris. For purposes of distinct vision it is essen- 

 tial that the quantity of light entering the interior of the eye shall 

 be so adjusted that the formation and subsequent perception of the 

 image shall be sharp and distinct. This is accomplished by the iris, 



