214 THE SPECIAL SENSES. 



ther object is indistinct; and when the latter is seen 

 clearly, the pencil appears indistinct ; and that it is im- 

 possible to see both clearly at the same time. Accordingly, 

 the eye must have the capacity of adjusting itself to dis- 

 tances, 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 accom- 

 modation, 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. 



FIG. 52. THE FUNCTION OP ACCOMMODATION. 



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

 accommodated for near vision. 



When the eye is at rest, that is, when accommodated for a 

 distant object, the lens is flatter and its curvature dimin- 

 ished (see Fig. 52) ; but when strongly accommodated for 

 near vision the lens becomes thicker, its curvature in- 

 creases, and the image on the retina is made more sharp 

 and distinct. Since a strong light is not required in view- 

 ing near objects, the pupil contracts, as is shown in the 

 left-hand half of the diagram. 



83. Function of accommodation ? Tn what does it consist ? How is the func- 

 tion explained ? 



