CH. LVIIL] 



ACCOMMODATION 



825 



cases the first and third images remain unaltered in size, distinct- 

 ness, 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 bright images (repre- 

 sented by arrows in fig. 525) are used ; the two images from the front 

 surface of the lens during accommodation not only approach those 

 from the cornea, but also approach one another, and become 

 somewhat smaller. (Sansoris Images.) Helmholtz's Phakoscope 



Fio. 526. Phakoscope of Helmholtz. At B B' are two prisms, by which the light of a candle is con- 

 centrated on the eye of the person experimented with, which is looking through a hole in the third 

 angle of the box opposite to the window C. A is the aperture for the eye of the observer. The 

 observer notices three double images, represented by arrows, in fig. 525, 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 made to focus a near object, such as a reed pushed up at C ; the images 

 from the anterior surface of the lens will be observed to move as described in the text. 



(fig. 526) is a 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 the tension of this ligament, 

 diminishes to a proportional degree the flattening of which it is the 



