THE SENSE OF SIGHT 713 



point to a focus on the retina. A lens of this character is said to have a 

 refracting power of 5 dioptrics. 1 



The refracting media of the human eye in repose have collectively a 

 refracting power of about 64 dioptrics, the reciprocal of its anterior focal 

 distance. The refracting power of the corneal surface alone is equivalent to 

 42 dioptrics. The crystalline lens by reason of its relations and situation 

 in the optic media has a refracting power of about 20 dioptrics. 



The capability of the lens to increase its refraction during accommodative 

 efforts beyond the 20 dioptrics varies considerably at different periods of life. 

 From youth to old age, the elasticity of the lens steadily declines, and the 

 range of accommodation diminishes from the recession of the near point. 



The following table shows the decrease in the accommodative power with 

 increasing years, the shortening of the range 'of accommodation and the 

 distance to which the near point has receded from the eye. 



Convergence of the Eyes during Accommodation. When the eyes 

 are at complete rest and directed to some far distant object the visual lines 

 are parallel and the optic axes are directed outward at an angle of about 

 5 degrees, which is known as the angle alpha. If, however, it is desired to 

 see distinctly with both eyes, any object within the range of accommodation, 

 the eyeballs must be converged toward the median line, the object being to 

 enable the rays of light emanating from a point to fall directly into the foveae 

 so that the two retinal images shall give rise to but a single impression and 

 thus prevent double vision or diplopia which would otherwise result. Con- 

 vergence of the eyeballs, therefore, increases as the visual axes are directed 

 toward objects which are placed progressively nearer the eyeballs. This is 

 accomplished by the conjoint and harmonious action of the internal recti 

 muscles balanced by the action of their physiologic antagonists the external 

 recti muscles. Convergence of the eyeballs is always accompanied by in- 

 crease in the accommodation, the two being necessary for distinct binocular 

 vision. 



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 



1 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 flint 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, 10 dioptrics respectively, obtained by dividing 100 cm. by 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, and using a corresponding negative figure. Thus a lens which diverges parallel rays in such 

 a way as to make them appear to radiate from a point 20 centimeters behind itself is said to have a 

 refractive power of minus 5 dioptrics. 



