254 Life and Health 



aiding in producing a sharp and distinct image of the 

 object. Each point of the image, being the focus or 

 meeting place of a vast number of rays coming from the 

 corresponding point of the object, is sufficiently bright to 

 stimulate the retina to action. 



Thus, the moment rays of light enter the eye they are 

 bent out of their course. By the action of the crystalline 

 lens in altering its convexity so that the eye is capable of 

 adjusting itself to different distances, and aided by the 

 refractive media, the rays of light that are parallel when 

 they fall upon the normal eye are brought to a focus on 

 the retina. 



Experiment 114. With a hand mirror reflect the sunlight on a 

 white wall. Look steadily at the spot for a full minute, and then let 

 the mirror suddenly be removed. The "complementary" color a 

 dark spot will appear. 



Experiment 115. To show that impressions made upon the retina 

 do not disappear at once. Look steadily at a bright light for a 

 moment or two, and then turn away suddenly, or shut the eyes. 

 A gleam of light will be seen for a second or two. 



Look steadily at a well-lighted window for a few seconds, and then 

 turn the eyes suddenly to a darkened wall. The window frame may 

 be plainly seen for a moment. 



Experiment 116. Take a round piece of white cardboard the size 

 of a saucer, and paint it in alternate rings of red and yellow, two 

 primary colors. Thrust a pin through the center and rotate it rapidly. 

 The eye perceives neither color, but orange, the secondary color. 



397. Old Sight. " Old sight," known as presbyopia, is 

 - a common defect of vision in advancing years. This is a 

 partial loss of the power to accommodate the eye to differ- 

 ent distances. This defect is caused by an increase in the 

 density of the crystalline lens, and an accompanying dimi- 

 nution in the ability to change its form. The far point of 



