THE SPECIAL SENSES 303 



the room, or out of the window, and the eye be fixed first 

 upon one and then upon the other, it will be found that when 

 the pencil is clearly seen, the further object is indistinct; and 

 when the latter is seen clearly, the pencil appears indistinct, 

 and that it is impossible to see both clearly at the same time. 

 Accordingly, the eye must have the capacity of adjusting itself 

 to distances, which is in some manner comparable to the action 

 of the screw of the opera-glass. 



83. This, which has been called the function of accommoda- 

 tion, is one of the most admirable of all the powers of the 

 eye, and is exercised by the crystalline lens. It consists 

 essentially in a change in the curvature of the front surface 

 of the lens, partly through its own elasticity, and partly 

 through the action of the ciliary muscle. When the eye is 

 at rest- that is, when accommodated for a distant object 



FIG. 69. THK FUNCTION OF ACCOMMODATION 



The nght half of the diagram shows the eye at rest. The left half shows the lens 

 accommodated for near vision 



the lens is flatter and its curvature diminished (see Fig. 69) ; 

 but when strongly accommodated for near vision, the lens 

 becomes thicker, its curvature increases, and the image on 

 the retina is made more sharp and distinct. Since a strong 

 light is not required in viewing near objects, the pupil con- 

 tracts, as is shown in the left-hand half of the diagram. 



83. Function of accommodation ? In what does it consist? How is the function 

 explained ? 



