THE SENSE OF SIGHT. 661 



is termed the far point or the punctum remotum. This for the normal eye 

 is at a distance of 65 meters or beyond. 1 If the luminous point gradually 

 approaches the eye from a point 65 meters distant, the accommodative 

 power comes into play and gradually increases until it attains its maximum. 

 The nearest point up to which the eye is able to form distinct images of 

 objects is called its near point or punctum proximum. This near point in a 

 healthy boy of twelve years will lie at 2 inches or 7 cm. from the eye, 

 while the same point lies only 8 inches or 20 cm. distant in a man of forty 

 years. Of objects which lie nearer than the punctum proximum the eye 

 cannot form distinct images. The distance between the punctum remotum 

 and the punctum proximum is termed the range of accommodation. 



Force of Accommodation. The increase in curvature of the lens 

 necessary to focalize rays when the eye is directed from the far to the near 

 point necessitates the expenditure of energy on the part of the ciliary muscle. 

 The force expended in the act of accommodation may be measured by 

 a lens the refracting power of which is such as to enable it to produce the 

 same result that is, to give the diverging rays coming from the near point, 

 e.g., 20 cm., a parallel direction. A lens, therefore, which has a focal dis- 

 tance of 20 cm. would be a measure of the force expended ; for such a lens 

 placed in front of the crystalline lens, when in a state of repose, would, 

 with the assistance of the latter, bring diverging rays coming from the near 

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

 refracting power of 5 dioptrics. 



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 by dividing 100 cm. by the focal distance. The re- 

 fracting 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. 



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. 

 At ten years the increase is 14 dioptrics, as the near point is 7 cm. ; at thirty 

 years the increase is but 7 dioptrics, 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. 



1 In practical ophthalmic work a point six meters distant is taken as the far point for the 

 reason that the rays at this distance are practically parallel. 



