168 THE SENSES. 



the cross in Fig. 49, the circular spot will be visible at the same 

 time, though less distinctly. As the book is moved slowly back- 

 ward and forward, a point will be found at which the round spot 



FIG. 49. 



disappears, reappearing as the book is held nearer or farther or as 

 it is inclined in either direction and the image is carried away from 

 the blind spot. 



What part of the retina possesses the most acute vision ? 



The portion directly behind the lens, where the macula lutea (yel- 

 low spot) with its central depression (fovea centralis) is situated. 

 Here are found none of the fibres of the optic nerve, but a great 

 increase in the number of the cones of the layer of rods and cones, 

 as well as an increase in their size. In looking at any object it is 

 upon this spot that its image is reflected by the media, and here 

 the power of the end-organs of the optic nerve is most highly 

 developed. 



What conclusion is drawn from these facts ? 



That the rods and cones (more especially the cones) are the 

 physiological agents for the reception of light stimuli : for upon the 

 blind spot is no layer of the rods and cones, while in the point of 

 sharpest sight the nerve-fibres are wanting, and these elements are 

 especially prominent. 



What is the duration of visual sensations ? 



The duration of a visual sensation is always greater than that 

 of the stimulus which has caused it. However brief the luminous 

 impression, the effect on the retina lasts about one-eighth of a sec- 

 ond. The spokes of a rapidly-revolving wheel for this reason do 

 not appear as spokes, but as a solid mass, each following one 

 another so rapidly that one impression cannot fade before another 

 has replaced it. 



