RADIANT ENERGY 



391 



ation is one of the greatest achievements of modern science and 

 was anticipated by Faraday. The name given to the spectrum 

 implies that electricity, magnetism, and radiation (light) are 

 one. Faraday went further, for he had hopes of coupling such 

 physical forces as electricity (voltaic and static), gravity, heat, 

 cohesion, and magnetism. He discovered the principle of 

 electromagnetic induction, and we now speak of the princi- 

 ples of electromagnetism. He saw the connection between 

 magnetism and hght, and we now speak of magnetooptics. 

 He realized the bearing of electricity upon chemistry, and we 

 now have electrochemistry. 



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Fig. 166. — The spectrum of electromagnetic radiation (wave lengths are in 

 centimeters on a logarithmic scale). 



Let us briefly consider the electromagnetic spectrum (Fig. 166), 

 starting with the hertzian waves of longest length. These elec- 

 tric or radio waves undoubtedly affect living matter, but we 

 know Uttle of their influence. The somewhat shorter heat 

 waves constitute a w^orld of their own and will not be considered 

 here. Their effect is known to all in a general way. 



Visible light comes next; it appears to be homogeneous, but 

 actually it is made up of many colors which can be separated 

 by passing white Hght through a glass prism. Composite white 

 light is diffracted into the infinite colors of the spectrum of 

 which violet, indigo, blue, green, yellow, orange, red are charac- 

 teristic. The longest visible rays are 7,500 A. U. (0.75 At) in 

 length; the central yellow ones are 6,000 A. U.; and the shortest 

 visible violet ones, 3,900 A. U. long. Many systems, Hving and 

 nonhving, are selectively sensitive to the rays of the spectrum. 

 Most photographic negatives are sensitive to the blue end and 

 practically insensitive to the red. Certain negatives are now 

 made sensitive to the red end of the spectrum. Light rays of 



