116 CONSCIOUS NERVOUS OPERATIONS 



of the two retinas. The two pictures are not, however, 

 identical. The right eye will see more of one side of an 

 object than will the left, and the left eye will see more of 

 the opposite side. This enables us to form more accurate 

 judgments of form and distance than would be possible 

 with only one eye. Then, too, one eye may be wholly 

 destroyed and a person may still retain distinct vision. 



157. Duration of Sight Sensation. The impression made 

 upon the retina by a flash of light remains for about one 

 eighth of a second, so that if flashes of light follow one 

 another at a shorter interval than that they appear as 

 one continuous impression. Children make a circle of 

 fire by whirling rapidly a lighted stick, and the spokes of 

 a swiftly revolving wheel appear continuous. If one looks 

 at the sun or other bright object and then closes the eyes, 

 he will continue for an instant to see the object. These 

 delusive appearances are due to the fact that the nervous 

 impressions made by light upon the end organs in the ret- 

 ina remain after the removal of the rays which excite them. 



158. Fatigue of the Retina. While the retina is extremely 

 sensitive it is also easily fatigued. If one looks steadily 

 for a time at a bright object and then turns the eye away, 

 he will still see the outline of the bright object, but it will 

 be dark. This is because that part of the retina upon 

 which the light fell from the bright body has become wea- 

 ried and no longer responds to the stimulus of light. If 

 the body looked at is of a bright yellow color, the figure 

 seen when the eye is turned away will be blue, because 

 the retina is no longer able to respond to the stimulus of 

 yellow rays, but is affected by the rays of the complemen- 

 tary color. 



159. Defects of Vision. It is very common to see per- 

 sons wearing lenses, or " glasses," to correct what is called 



