e 



Figure 228. — How we see the Pencil. 



SENSE ORGANS 217 



the eye (optic) nerve. When these two conditions bake 

 place, we see. 



As we have learned, the stimulus for the eye is always 

 light. In physics we learn that the rays of light brave] in 

 straight lines. This fact explains why we cannot 

 round a corner. When the rays of light are made to pass 

 through a glass lens, the rays which pass through the thin 

 edges of the lens are bent and do not travel to the same place 

 they would have reached had they not passed through the 

 lens. In the same way light rays from an object pass 

 through the lens in our 

 eyes and are bent. This 

 results in the image of 

 the object, the pencil in 

 this instance, being in- 

 verted on the retina. 

 The light rays of the pencil stimulate the nerve cells in 

 the retina, and this stimulus, after being carried to the 

 brain, is interpreted to us as a pencil, though we do not 

 know how stimuli travel on nerves. The inverted image 

 of the picture on the retina is made to look natural to us 

 because we are used to seeing everything in inverted imag 



Care of the Eyes. — The eyes are our most precious 

 sense organs, and as such they should receive the best of 

 care. Certain imperfections in the lens or other parts of 

 the eye can be helped by the use of glasses. If your «\ ea 

 annoy you, or if you cannot see objects as clearly as your 

 schoolmates, have a competent oculist examine and treat 

 them. 



The Ear. — The ear is a sense organ for the reception 

 of the stimuli which we interpret as sounds. The ear of 

 man consists of the outer, middle, and inner ear. The 

 first two carry the stimuli to the third, where the}' are 

 received by nerve cells and carried to the brain. 



