THE SENSES 



4 25 



means of delicate ligaments. In front of the lens is a small cavity 

 filled with a watery fluid, the aqueous humor, while behin<! it jg 

 the main cavity of the eye, filled with a transparent, 

 almost jelly-like, vitreous humor. The lens itself is 

 elastic. This circumstance permits of a change of 

 form and, in consequence, a change of focus upon 

 the retina of the lens. By means of this change in 

 form, or '^accommodation,''^ we are al^le to distin- 

 guish between near and distant ol^jects. 



Defects in the Eye. — In some eyes, the lens is in 

 focus for near objects, but is not easily focused upon 

 distant objects; such an eye is said to be near- 

 sighted. Other eyes which do not focus clearly on 

 objects near at hand are said to be far-sighted. 

 Still another eye defect is astigmatism, which causes 

 images of lines in a certain direction to be indistinct, while images 

 of lines transverse to the former are distinct. Many ner\'ous 

 troubles, especially headaches, may be due to eye strain. 



Diagram hHow- 

 ing how the 

 loiiH chanKc^ 

 its form. 



Diagram to show how an image is formed in the eye ; a, object ; h, lens; c, image up<.n retina. 



How we See. — Suppose an object be held in front of the eye; 

 rays of light pass from everj^ part of the object and are bnni.^lit 

 to a focus on the retina by means of the transpaivnt len.^. You 

 can form an object in the same manner l)y using a reading ghis.s. 

 a box with a hole in one end, and a piece of white pai>er. Notice 



