828 THE SENSE OF SIGHT 



ophthalmometric measurements, giving the radius of curvature of the 

 chief refracting media: 



Near vision Far vision 



Radius of curv. of cornea 8 mm. 8 mm. 



Radius of curv. of ant. surface of lens 10 mm. 6 mm. 



Radius of curv. of post, surface of lens 6 mm. 5.5 mm. 



The lens of a relaxed eye measures 3.025 to 4.43 mm. in thickness, 

 average 3.6 mm. On near vision, its anterior surface is carried for- 

 ward through a distance of 0.36 to 0.44 mm.; hence, it will be seen 

 that the thickness of the lens, when accommodated for near objects, is 

 increased by about 0.4 mm., i.e., on the average from 3.6 mm. to 4.0 

 mm. This implies that its anterior surface is then situated at a dis- 

 tance of only 3.2 mm. behind the cornea, while this same distance in 

 the eye at rest measures 3.6 mm. Its posterior surface lies 7.2 mm. 

 behind the cornea during far vision and retains this position, at least 

 practically so, during near vision. 



This change in the shape of the lens is dependent upon its inherent 

 elasticity, and especially upon that of its capsular investment. This 

 property it is permitted to bring into play as soon as the tension under 

 which it is ordinarily held by the structures of the zonula Zinnii, is 

 diminished. Since its refractive power is increased thereby, the enter- 

 ing rays of light must be rendered more convergent. Under ordinary 

 conditions we express the refractive power of a lens in terms of its 

 principal focal distance. A lens possessing a focal distance of one 

 meter, is said to have a refractive power of one diopter (D.). Taking 

 this value as a unit, a lens with a focal distance of 50 cm. possesses 

 a refractive power of 2D., one with a focal distance of 10 cm., a re- 

 fractive power of lOD., and conversely, one with a focal distance of 

 10 m., a refractive power of O.lD. {]^qD.). 



Range of Accommodation. — The ciliary mechanism fulfills the 

 purpose of bringing any object in space to a precise focus upon the 

 retina, but, naturally, it cannot simultaneously produce a sharp image 

 of two objects which are situated at different distances from it. We 

 can readily convince ourselves of this fact by looking at a distant 

 object through a network of fine wire held near our eyes. If we 

 glance at the distant object, the wire network loses its clear contours. 

 Contrariwise, if we look at the network, the object becomes blurred. 



The distant point in space at which an object is still clearly dis- 

 cernible, is called the far-point or punctum remotum. Quite similarly, 

 the point nearest the eye at which an object still produces a perfectly 

 clear impression, is known as the near-point or punctum proximum. 

 In between these two extremes lies the range of distinct vision, or 

 range of accommodation. Any object situated beyond the far point 

 or inside the near point, cannot be brought to a precise focal point 

 upon the retina and must, therefore, appear blurred. 



The Limit of Accommodation of the Normal Eye. — Inasmuch as 

 the normal or emmetropic eye, when at rest, is adjusted so as to focus 



