ADAPTATION TO VARIOUS DISTANCES. 655 



muscle contracts, and, by such contraction, diminishes the 

 force with which the elastic suspensory ligament of the 

 lens is tending to flatten it. On. the latter supposition, the 

 lens may be supposed to be always in a state of tension 

 and partial flattening from the action of the suspensory 

 ligament ; while the ciliary muscle, by diminishing the 

 tension of this ligament, diminishes, to a proportional 

 degree, the flattening of which it is the cause. On dimi- 

 nution or cessation of the action of the ciliary muscle, the 

 lens returns, in a corresponding degree, to its former 

 shape, by virtue of the elasticity of its suspensory liga- 

 ment. In viewing near objects, the iris contracts, so that 

 its pupillary edge is moved a very little forwards, and the 

 pupil itself is contracted the opposite effect taking .place 

 on withdrawal of the attention from near objects, and fixing 

 it on those distant. 



The range of distances through which persons can adapt 

 their power of vision p. ^ 



is not in all cases 

 the same. Some per- 

 sons possess scarcely 

 any power of adap- 

 tation, and of this 

 defect of vision there 

 are two kinds; one, 

 in which the person 

 can see objects dis- 

 tinctly only when brought close to the eye, having little 

 power to discern distant objects ; another, in which distant 

 objects alone can be distinctly perceived, a small body 

 being almost invisible except when held at a considerable 

 distance from the eye. In the one case the person is said 

 to be short-sighted or myopic : in the other, long-sighted 

 or_presbyopic. Myopia is caused by anything, such as 

 undue convexity of the lens, which increases the refracting 

 power of the eye, and so causes the image of the object 



