THE EYE 337 



Field of view. A person can clearly recognize 

 objects in only a small part of the field of view just in 

 front of the eyes, while the rest seems to be only indistinct 

 shadows. To be distinct, the image must fall upon the 

 part of the retina less than -^ of an inch across, which is 

 situated directly behind the cornea. In reading a book, 

 the eye can distinctly see two or three words at once, but 

 by rapid and unconscious movements of the eyes sidewise, 

 we cover a larger field of view. 



610. Duration of sensations. The sensation of sight is 

 produced almost instantly when the eyes are directed 

 towards an object, but the image persists for ^ of a 

 second. If a succession of pictures of a moving object 

 are thrown upon a screen at that rate, the object will seem 

 to go through its motions without interruption. Birds 

 flying and waves dashing upon the beach may be thus 

 shown absolutely true to life. A point of light swung 

 about a circle seems to be a shining ring. If two colors 

 are revolved at that rate, the eye no longer sees either one, 

 but a mixture of the two. Thus a blue and a yellow spot 

 side by side, when revolved before the eye, seem a single 

 green spot. 



611. Color blindness. Sometimes the nerves of the 

 retina are unable to recognize certain colors. In the 

 most usual form of color blindness red is supposed to 

 be green or gray. In locomotive engineers and sailors 

 color blindness may be a serious defect, for they are 

 guided by different colored signals, especially those con- 

 taining red. 



612. Exhaustion of the retina. When the eyes look 

 steadily at objects for a long time the vision becomes 

 blurred. If one gazes steadily at a bright colored object, 

 the retina is fatigued by that color. A white object looked 



OV. PHYSIOL. 22 



