ACUTENESS OF VISION 263 



connected with them into activity. As a result, multi- 

 tudes of nervous impulses pass through their nerve fibres 

 and thence along the optic nerve to the centre of sight 



FIG. 144. To find blind spot, hold in front of right eye and close left eye; looking 

 steadily at cross, move book nearer and farther from eye until white circle is 

 made to disappear. 



in the cortex of the brain. Here they cause the activity 

 of the nerve cells and as a result we have sensations 

 which we call sight. 



Acuteness of vision. When we consider the extent 

 of the field of vision pictured upon the small surface 

 of the retina, the delicacy of detail which we perceive 

 seems marvellous. For example, a white thread can be 

 seen on a black blackground at a distance of sixty feet. 

 Since the combination of cornea and lens is equivalent 

 to a lens of about three-quarters of an inch focus, the 

 image of the white line upon the retina must be about 

 one two-thousandth of a millimeter in diameter. The end 

 organs of the nerves upon which the image falls must 

 therefore be of extreme fineness and efficiency. 



Defects of vision. Although the eye serves us so 

 wonderfully well, yet as an optical instrument it has 

 certain marked defects, which are more exaggerated in 

 some persons than in others and are especially noticeable 

 in those who continually use their eyes for close work. 

 In a faint light, as in twilight, when the pupil is large, 



