CHAPTER XII 



DEVICES FOR SEEING BETTER, FARTHER, AND LONGER. 



Eyes are bold as lions, roving, running, leaping, here and there, 

 far and near. EMERSON. 



When men observe a sequence of events in nature that is 

 constant, the statement of such a constant sequence is called 

 a law of nature. While we realize in general that nature con- 

 forms to law, yet we daily see repeated many phenomena or fre- 

 quently make use of commonplace appliances without any 

 appreciation of the laws that underlie their operation 'or even 

 without a realization that there are laws governing such operation. 

 One sees it grow light long before the sun is visible, and the 

 strange fact does not challenge attention; or one plays a flute, 

 turns on the electric lights, or uses the telephone, and yet is 

 not even curious in regard to the laws that make such acts 

 possible. 



But the appreciation of some laws is so vital to our existence 

 that they force themselves on our attention. We know them 

 in practice, at least, even if we do not formulate them in words. 

 Such is the law that light travels in straight lines. Very familiar 

 experiences need only be recalled to make one realize the truth 

 of this statement. When you see an object you want you reach 

 straight for it, and you do not expect to see around corners 

 unless a mirror is employed. The hunter sights along the 

 straight arrow or gun barrel, and lets fly his missile at the animal 

 he desires to kill. Nearly everyone has observed the straight 

 beam of light revealed by the dust particles in its course in a 

 partly darkened room. If you look at some object like a candle 

 flame through holes punched in each of two cards held a foot 

 apart, the flame and the holes must be in the same straight 



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