258 THE SPECIAL SENSES 



which fills the larger part of the eyeball. It next falls 

 upon the sensitive screen, the retina, which forms the 

 lining coat of the eye. 



Formation of image. In order that we may get a 

 perfect image of an object, it is necessary that the eye 

 should act as a lens to focus upon the retina rays of 



FIG. 139. The formation behind a convex lens of a diminished and inverted 

 image of an object placed in front of it. 



light from the object. The cornea receives the rays and 

 makes them converge enough to bring them to a focus 

 behind the retina. The lens then completes the work 

 by focusing them upon the retina. For this purpose, the 

 lens must be relatively flat if the rays do not require 

 much bending because they come from a distance, or 

 quite convex if they come from nearby objects and so 

 require considerable bending. 



Adjustment of lens. To enable the lens thus to 

 adjust itself, there is a circle of elastic fibres which 

 radiate out from its edges. These are constantly pulling 

 the edge of the lens outward and thereby flattening its 

 front surface. In addition to the elastic fibres, there is 

 a ring of muscle fibres which encircles the edge of the 

 lens. These muscle fibres, by contracting, relieve the lens 

 of the pull of the elastic fibres and so make it more con- 

 vex. The muscle fibres are controlled automatically 



