ADAPTATION TO VARIOUS DISTANCES. 511 



tances of the images of an object at such a distance that the 

 rays are parallel, and of one at the distance of four inches, is 

 only about 0.143 of an inch. On this calculation, the change 

 in the distance of the retina from the lens required for vision 

 at all distances, supposing the cornea and lens to maintain the 

 same form, would not be more than about one line. 



It is now almost universally believed that Helmholtz is right 

 in his statement that the immediate cause of the adaptation of 

 the eye for objects at different distances is a varying shape of 

 the lens, its front surface becoming more or less convex, accord- 

 ing to the distance of the object looked at. The nearer the 

 object, the more convex does the front surface of the lens be- 

 come, and vice versd; the back surface taking little or no share 

 in the production of the effect required. Of course, the lens 

 has no inherent power of contraction, and therefore its changes 

 of outline must be produced by some power from without ; and 

 there seems no reason to doubt that this power is supplied by 

 the ciliary muscle. The exact manner, however, in which, by 

 its contraction, the ciliary muscle effects a change in the shape 

 of the crystalline lens is doubtful. The most probable expla- 

 nation of the phenomenon, however, is that in adapting the 

 eye for viewing near objects the ciliary 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 ac- 

 tion of the suspensory ligament ; while the ciliary muscle, by 

 diminishing the tension of this ligament, 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, in a corresponding degree, to its former shape, by 

 virtue of the elasticity of its suspensory ligament. In view- 

 ing near objects, the iris contracts, so that its pupillary edge 

 is mbved a very little forwards, and the pupil itself is con- 

 tracted 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 is not in all cases the same. Some per- 

 sons possess scarcely any power of adaptation, and of this de- 

 fect of vision there are two kinds ; one, in which the person 

 can see objects distinctly 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 



