THE SPECIAL SENSES 297 



repeated by using two black buttons on the marble top of a 

 bureau, or on some other white surface. The blind spot does 

 not practically interfere with vision, since the eye is seldom 

 fixed immovably on an object, and the insensitive parts of the 

 two eyes can never be directed upon the same object at the 

 same time. 



70. Impressions made upon the retina are not at once lost, 

 but continue a measurable length of time, and then gradually 

 fade away. Thus, a bright light, or color, gazed at intently, 

 cannot be immediately dismissed from sight by closing or turn- 

 ing away the eyes. A stick lighted at one end, if whirled 

 around rapidly in the dark, presents the appearance of an un- 

 broken luminous ring; and the spokes of a rapidly revolving 

 carriage-wheel seem to be merged into a plane surface. If an 

 object move too rapidly to produce this sort of lasting impres- 

 sion, it is invisible, as in the case of a cannon-ball passing 

 through the air in front of us. 



71. If a card, painted with two primary colors as red and 

 yellow be made to rotate swiftly, the eye perceives neither 

 of them distinctly ; but the card appears painted with their 

 secondary color orange. The average duration of retinal 

 images is estimated at one-eighth of a second ; and it is because 

 they thus 'endure, that the act of winking, which takes place 

 so frequently, but so quickly, is not noticed and does not inter- 

 rupt the vision. The retina is easily fatigued or deprived of 

 its sensibility. After looking steadfastly at a bright light, or 

 at a white object on a black ground, a dark spot corresponding 

 in shape to the bright object, presents itself in whatever direc- 

 tion we look. This spot passes away as the retina resumes its 

 activity. 



72. If a bright color be gazed at intently, and the eyes then 

 be turned to a white surface, a spot will appear ; but its color 

 will be the complement of that of the object. Fix the eye 

 upon a red wafer upon a white ground, and on removing the 



70. Duration of impressions upon the retina ? How illustrated ? 



Tl. What further illustration? Winking, why it is not noticed ? Ease with which the 

 retina is fatigued or deprived of sensibility ? How shown ? 



72. How further shown ? How is the result accounted for ? " Color-blindness " ? 



