THE MECHANISM OF ACCOMMODATION 



679 



posterior surface of the lens, remain unaltered, while the anterior surface of 

 the lens becomes more convex and approaches the cornea. 



The experiment, figure 458, is more striking when the two prisms of the 

 phakoscope which form two images of the candle are used. The pair of 

 images of the candle from the front surface of the lens not only approach 

 those from the cornea during accommodation, but also approach one another, 

 and become somewhat smaller, Sanson's images. 



FIG. 459. 



FIG. 458. Diagram of Sanson's Images. A, When the eyes are focused for far 

 objects, and B, when they are focused for near objects. The figure to the right in A and 

 B is the inverted image from the posterior surface of the lens. 



FIG. 459. Phakoscope of Helmholtz. At B, B', are two prisms, by which the light 

 of a candle is concentrated on the eye of the person experimented with at C. A is the 

 aperture for the eye of the observer. The observer notices three double images, as in 

 figures 457 an( ^ 45^, reflected from the eye under examination when the eye is fixed upon a 

 distant object; the position of the images having been noticed, the observed eye is then 

 focused on a near object, such as a reed pushed up by C; the images from the anterior 

 surface of the lens will be observed to move toward each other, in consequence of the lens 

 becoming more convex. 



The Mechanism of Accommodation. The mechanism of accommo- 

 dation depends primarily upon the inherent tendency of the lens to approxi- 

 mate the shape of a sphere. When the eye is at rest the intra-ocular tension 

 is such as to put stress on the suspensory ligament around its equator, which 

 compresses the elastic lens in its antero- posterior dimension. The elasticity 

 of the lens can make itself apparent when the tension of the suspensory liga- 

 ment is relaxed. This takes place completely after a division of the fibers 

 of the zonula. When we remove the lens from the eye of a young person, 

 we see it assume the spherical shape immediately upon the division of its 



