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Popiilar Science Mvnthhj 



The infra-red world is as strange as t±ie ultra-violet. The sky appears black, foliage a 

 beautiful rich red, and there are long, heavy shadow;s 



With light it is the same. There arc 

 octaves of light which our eyes can 

 nc\-er hope to see. Perhaps the best 

 known of invisible rays are those used in 

 wireless telegraphy; they arc produced 

 1)\- vibrations of far lower frequency 

 than those which we see as sunlight. 



When you strike the middle "C" on 

 a piano you hear a single musical note. 

 And so, when you look at the world 

 about you through a pane of red glass, 

 you sec things in a single light-note, as 

 it were. Change the color of the glass 

 and the world apix-ars different. The 

 same trees, the same flowers, the same 

 houses are there, but with one color 

 details are obscured and with another 

 intensified. 



It is perfectly pf)ssiblc to view the 

 world with in\'isible rays and to learn 

 things about which we never dreamed of 

 in our philosophy — only we must use 

 an eye, which, unlike our own e\-es, will 

 see the unknown world for us and make 

 a jiicturc of it which we can percei\'e. 

 The ordinary photographic camera is 



such an eye. The sensitized plate is 

 extraordinarily responsive to those \ery 

 high-pitched vibrations that do not 

 affect the eye. All that remains is 

 to strike the single note in a given 

 octave of light, with which the world 

 is to be viewed in order to see things as 

 they are but as we never see them. 



In order to see the world with in\isil)le 

 ultra-\iolet rays something better than 

 glass must be employed; for glass is 

 almost as opaque to them as a plate of 

 sheet-iron. Quartz must be used, since 

 (juartz is transparent to them. Hence 

 a quartz lens must be fashioned for the 

 camera. To exclude all but \iolet rays 

 from the lens a filter must be employcxl — 

 a kind of sieve through which only the 

 ultra-\iolet rays will pass, just as only 

 red raN's will pass through red glass. 

 Some fifteen years ago I disco\ered that 

 an aniline dye, called nilroso-dimelhyl- 

 aniline, would exclude all but the iillra- 

 \i()let rays, the elTect of which 1 wished 

 to study. Thin films of siUer are also 

 serviceable, as well as the vapor of 



