SENSE OF SIGHT. 



569 



the anterior portion of the muscle causes the increased con- 

 vexity of the lens, and not its displacement backward. 



Schoen's Theory of Accommodation (Fig. 

 347). This writer explains the increased 

 convexity of the lens by assuming that the 

 contraction of the ciliary muscle produces the 

 same effect on the lens as is produced upon a 

 rubber ball when held in both hands and com- 

 pressed by the fingers. The theories of Tscher- 

 ning and Schoen presupposes a stretching of 

 the zonule of Zinn, while that of Helmholtz is 

 based on its relaxation. Recent observations of 

 Hess seem to demonstrate that the change in 

 this structure is one of relaxation rather than 

 increased tension, so that at the present time 

 the theory of Helmholtz may be accepted as 

 the true explanation of accommodation. 



Range of Accommodation. The point 

 nearest to the eye to which objects can be 

 brought and be seen distinctly is the near- 

 point ; while the point farthest from the eye 

 at which distinct vision exists is the far-point 



FIG. 347. To illus- 

 trate Schoen' s theory 

 of accommodation. 



intervening space is the range of accommodation. 



the length of the 



FIG. 348. Scheiner's experiment. In the upper figure the eye is focussed for a 

 point farther away than the needle ; in the lower, for a nearer point. The continuous 

 lines represent rays from the needle, the interrupted lines rays from the point in 

 focus. 



Near-point. This is also called punctum proximum, and is ex- 

 pressed by p.p. It varies in different individuals, but for a normal 

 adult eye is about 12 cm. Objects brought nearer to the eye than 

 this cannot be seen with distinctness, for the refractive media 

 cannot bring the image of such objects to a focus upon the retina. 

 The near-point for any given eye may be determined by Scheiner's 



