416 LIFE ON THE EARTH 



This rough comparison is by no means a description of 

 the eye, for it is a most complex and wonderful organ, vastly 

 superior in construction to a camera. A technical descrip- 

 tion would, however, be out of place here. The impres- 

 sion made on the retina remains for an instant; and so if 

 successive pictures (about twelve a second) are taken of a 

 moving object and projected on a screen at the same rate 

 the eye will not distinguish the intervals between the pic- 

 tures and the object will appear to be in motion. This is 

 the way in which moving pictures are produced. 



Sometimes the lens is not able to focus a picture distinctly 

 on the retina, and then it is necessary to aid the lens of the 

 eye with artificial lenses, or glasses. Silly notions about 

 one's personal appearance in glasses should never stand in 

 the way of wearing glasses when they are necessary. If 

 there is a strained feeling when the eyes are used, or if 

 headaches result from continued use of the eyes, reliable 

 advice should be sought. 



The eye is so important for our usefulness and happiness 

 that the greatest care should be taken of it. One should 

 not read when he is lying on his back, when the light is 

 either poor or glaring, or when the book cannot 

 be held steadily. The eye may be infected from 

 public washbowls, public towels, or even by 

 rubbing with one's own fingers. Any infection 

 of the eye demands skillful treatment and should 

 not be trifled with. 



FIGURE 133 Sound and Hearing. Experiment 136. Arrange 

 a large, wide-mouthed bottle with a small bell sus- 

 pended in it from the stopper and a delivery tube extending through 

 the stopper. (Figure 133.) Attach the delivery tube by a thick- 

 walled rubber tube to an air pump and exhaust the air from the 



