760 



THE EYE AND VISION. 



[CH. LV. 



clearer, and approaches the first. If the eye is now adjusted for 

 a far point, the second image enlarges again, becomes less distinct, 

 and recedes from the first. In both cases the first and third 

 images remain unaltered in size, distinctness, and position. This 

 proves that during accommodation for near objects the curvature 

 of the cornea, and of the posterior surface of the lens, remain 

 unaltered, while the anterior surface of the lens becomes more 

 convex and approaches the cornea. 



The experiment is more striking when two candles (represented 

 by arrows in fig. 581) are used, and the images of the two candles 

 from the front surface of the lens during accommodation not only 



Fig. 581. 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 posterior surface of the lens. 



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

 and become somewhat smaller. (Sanson's Images.) Helmholtz's 

 Phakoscope (fig. 582) is a triangular box with arrangements for 

 demonstrating this experiment. 



Mechanism of accommodation. The lens having no inherent 

 power of contraction, its changes of outline must be produced by 

 some power from without ; this power is supplied by the ciliary 

 muscle. Its action is to draw forwards the choroid, and by so 

 doing to slacken the tension of the suspensory ligament 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 proportional 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 suspen- 

 sory ligament (fig. 583). From this it will appear that the eye is 

 usually focussed for distant objects. In viewing near objects the 

 ciliary muscle contracts ; the ciliary muscle relaxes on withdrawal 

 of the attention from near objects, and fixing it on those distant. 



