260 THE SPECIAL SENSES 



bringing the rays of light from nearby objects to a focus 

 upon the retina. 



Far- and near-sight. Flatness of the lens, which 

 brings the rays of light from near objects to a focus 

 beyond the. retina, gives rise to far-sight. It can be 



FIG. 141. Section of front part of eyeball showing the change in the form of the 

 lens when near and distant objects are looked at. a, c, b, cornea ; A, lens when 

 near object is looked at ; B, lens when distant object is looked at. 



corrected by convex glasses, which make up the deficiency 

 of the eye and bring the rays to a focus on the retina. 

 In case the eyeball itself is so long that the rays of light 

 from distant objects are focused before they reach the 

 retina, we have a condition of near-sight. Near-sight 

 requires for its correction a concave lens, in order to 

 throw the focus back to the retina. While some eyes 

 are near-sighted from birth, others acquire this con- 

 dition by being used too much upon near objects in 

 childhood. Too early and too long continued attempts 

 at sewing and reading are often responsible for the 

 development of this unfortunate condition. 



Adjustments to intensity of light. Varying dis- 

 tance is not the only condition to which the eye must 

 be adjusted. Light itself is ever varying in intensity, 

 and the eye must be at once so sensitive as to see ob- 



