208 THE SPECIAL SENSES. 



but nearer or further than that, it will be plainly seen. 

 The other eye may be also tried, with a similar result : if 

 the gaze be directed to the right-hand circle, the left one 

 will seem to disappear. The experiment may be 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 persist 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 turning away the eyes. A stick lighted at one 

 end, if whirled around rapidly in the dark, presents the 

 appearance of an unbroken 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 pro- 

 duce this sort of lasting impression, it is invisible, as in 1 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 ihey thus endure, that the act of winking, which 

 takes place so frequently, but so quickly, is not noticed and 

 does not interrupt 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, 



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



71. What further illustration? Winking, why it is not noticed. Ease wit' 

 which the retina is fatigued or deprived of sensibility ? How shown ? 



