582 Our Surroundings 



tors to take off and land on small fields close to centers of popu- 

 lation, instead of on large aviation fields located miles away from 

 these centers. 



Infra-red Rays. What would you think of a man who went 

 out to take pictures in a heavy fog? "You're wasting your time," 

 you might exclaim. "Why, in a fog like this you can't even see 

 across the street, to say nothing of taking pictures !" Yet the man 

 might soon return with pictures as clear as those taken in bright 

 sunlight. How is this possible? 



White light, you remember, can be broken up into the seven 

 primary colors red, orange, yellow, green, blue, indigo, and violet. 

 The red rays have the longest wave length and the violet rays 

 have the shortest. There are other similar rays whose wave 

 length is so short or so long that they cannot be seen by the 

 human eye. Those rays whose wave length is shorter than the 

 wave length of violet light are called ultra-violet rays. Those 

 rays whose wave length is longer than the wave length of red 

 light are called infra-red rays. 



One of the most interesting uses of infra-red rays is in the 

 photographing of distant objects. Fog and dust particles in the 

 air interfere with the passage of ordinary light rays, but they 

 have little effect on infra-red rays. By the use of photographic 

 plates and lenses specially adapted to take advantage of infra-red 

 rays, clear pictures can be taken of distant mountains and other 

 objects so far away that they are quite invisible to the naked eye. 

 Pictures may also be taken in foggy weather and even in the 

 dark by making use of infra-red rays. 



The Photoelectric Cell. A man drives his automobile to- 

 ward his garage, and magically the doors open without the touch 

 of a human hand. A burglar kneels in front of a safe, and before 

 he touches the handles of the door an alarm rings in the nearest 

 police station. A thousand cars pass through a tunnel and are 

 accurately counted even though no one may be there to see them 

 pass. These are but a few of the many marvelous things done by 

 the new "electric eye." 



The real name of the electric eye is the photoelectric cell. In 

 appearance, the photoelectric cell looks much like an ordinary 



