360 GENERAL SCIENCE 



Why We Have Two Eyes. Some have said the reason we have 

 two eyes is because nature has provided an extra eye if one of the 

 eyes is injured. This, however, is not true. With two eyes we are 

 able to judge distances more accurately. If you close one eye and 

 try to judge distance, it is very difficult. The eyes do not see from 

 the same point of view. This produces perspective and gives objects 

 depth. An instrument called the stereoscope is used to give a very 

 clear three-dimensional picture of places and buildings. This is 

 obtained by taking two photographs of the same object from two 

 different points corresponding to the eyes. 



If you close one eye, and try to insert a pencil in a hole it will 

 be almost impossible to accomplish the task at the first trial. But 

 the two eyes focus together, producing the image we are accus- 

 tomed to, for each eye sees the object from a different point. 



By looking simultaneously with two eyes we see an object from 

 two slightly different directions, and, when the images made by the 

 two eyes are blended into one sensation in the brain, we get the 

 effect of depth or solidity which makes objects appear to stand 

 forth free from their background. In other words, the eyes give a 

 perspective to things which would otherwise appear flat. 



Hold your hand at arm's length before you and look at it with 

 only one eye open. It will appear to lie against the wall of the 

 room; if it does not actually appear to do so, it is because you know 

 that it is nearer to you than the wall. Open both eyes and notice 

 the change. Notice that the hand stands forth in space far in front 

 of the wall. Look at the hand first with one eye, then with the 

 other, and notice how each eye perceives the hand in a different 

 direction against the wall. 



The Blind Spot. Where the optic nerve enters the retina a blind 

 spot is produced. The mpst sensitive spot on the retina is called 

 the yellow spot, and is slightly removed from the place where the 

 optic nerve enters the eyeball. By closing one eye and looking at 

 the cross in the illustration, gradually bringing the book toward the 

 face, the round spot will disappear. Fig. 247. The image of the 

 dot is now on the blind spot of the eye, just where the optic nerve 

 is not sensitive to light. If the book is brought a little nearer to 

 the face the black dot will again be seen, 



