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BELL SYSTEM TECHNICAL JOURNAL 



Whether or not Hohveck's measurements are accurate enough to fix 

 the point of greatest opacity, it is certain that somewhere between 

 300A and 1200A the eclipse begins to pass off. Fluorite commences 

 to transmit at about 1200A, quartz and gelatine at about 1800A (each 



Fig. 1 — .Absorbing power of celluloid and of nitrogen plotted versus wave-length 

 in the region of greatest opacity. (Annates de Physique.) 



of these was, for reasons of experimental technique, long the limit of 

 the explored region). Air begins to let through the light at about 

 1800A; the atmosphere indeed arrests the rays of the sun and stars 

 as far along as 2000, but this is ascribed to ozone in the upper strata. 

 Henceforward the absorption of radiant energy in gases consists 

 mainly in shifting the valence-electrons of the atoms froin one level 

 to another, or in altering the amplitude of \ibration of atoms built into 

 molecular groups. The characteristics of individual atoms become 

 steadily less influential; the groupings of the atoms into molecules, 

 crystals, liquid or solid continua determine the amount of absorption. 

 The question whether a jiarticular solid is a conductor or an insulator, 

 entirely irrelevant at high frequencies, eventually becomes the only 



