THE SENSES. 



715 



The experiment (fig. 423 A) is more striking when two candles are 

 used, and the images of the two candles from the front surface of the 

 lens during accommodation not only approach those from the cornea, 



B 



Fig. 423 A. Fig. 424. 



Fig. 483 A. Diagram of Sanson's images. A, when the eyes are not, and B, when they are 

 focussed for near objects. The fig. to the right in A and B is the inverted image from the pos- 

 terior surface of the lens. 



Fig. 424, 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 fig. 423, reflected from the 

 eye under examination when the eye is fixed upon a distant object; the position of the images 

 having been noticed, the eye is then made to focus 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. 



but also approach one another, and become somewhat smaller. (San- 

 son's images.) 



Mechanism of accommodation. The lens having no inherent power 

 of contraction, its changes of outlines must be produced by some power 

 from without; this power is supplied by the ciliary muscle. It is some- 

 times termed the tensor cJioroidece. Its action is to draw forward the 

 choroid, and by so doing to slacken the tension of the suspensory liga- 

 ment of the lens which arises from it. The anterior surface of the lens 

 is kept flattened by the action of this ligament. The ciliary muscle 

 during accommodation by diminishing its tension, diminishes to a pro- 

 portional degree the flattening of which it is the cause. On diminution 

 or cessation of the action of the ciliary muscle, the lens returns to its 

 former shape, by virtue of the elasticity -of the suspensory ligament 

 (fig. 425). From this it will- appear that the eye is usually focussed 

 for distant objects. In viewing near objects the pupil contracts, the 

 opposite effect taking place on withdrawal of the attention from near 

 objects, and fixing it on those distant. 



