356 SCIENCE PROGRESS 



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sommables, Bull. Soc. Math. France, 48, 1920. 

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ASTKONOMY. By H. Spencer-Jones, M.A., B.Sc, The Royal Obser- 

 vatory, Greenwich. 



The Extension of the Ultra-violet Spectrum. — When the solar 

 spectrum is observed with prisms and lenses of glass, the violet 

 limit appears to the normal eye to occur at a wave-length of 

 about 4,000 angstroms, near the Faunhofer H lines. By the 

 use of the photographic plate and the employment of optical 

 apparatus made of quartz, the spectrum can be extended 

 without difficulty to a wave-length of about 2,000 angstroms. 

 The absorption of the air and of the lenses and prisms then 

 becomes important, and the extension of the spectrum to 

 wave-lengths much shorter than this value becomes difficult. 

 The region of wave-lengths from 4,000 to 2,000 angstroms is 

 commonly referred to as the ultra-violet region. 



The pioneer work in the extension of the spectroscopy into the 

 extreme ultra-violet {i.e. into wave-lengths shorter than 2,000 

 angstroms) was performed by Victor Schumann about thirty 

 years ago. Schumann was a successful business man who was 

 over forty years of age before he had sufficient leisure to devote 

 to scientific work. When he took up the pursuit of spectrum 

 analysis, he soon perceived the important parts played by the 

 absorption of the gelatine film on the photographic plate and 

 by the absorption of the air in the apparatus. By employing 

 special photographic plates with an emulsion nearly free from 

 gelatine, using lenses and prisms of fluorite — which is more 

 transparent than quartz to short waves — and devising a 

 spectroscope which could be used in a vacuum, he was able to 



