ADVBRTISHMHNT 



This work forms part of Volume XXIX of the Smifhsoyiiaii Contrihitiom to 

 Knowledge, aud gives an account of researches, aided by grants from the Hodgkius 

 Fund, on the emission and absor{)tion of the gases of atmospheric air in the ultra- 

 violet spectrum. 



Within the last fifteen yeai's our knowledge of I'adiation has been greatly 

 increased, and now embraces wide ranges of the spectrum heretofore unknown. 

 Without assigning any place to the numerous kinds of "rays" whose discovery has 

 been associated in the public mind first with the work of Rontgen, and later with 

 that of the Curies, I am speaking here rather of the extensions of the spectrum in 

 wave-lengths which are actually measurable and known. Thus beyond the red 

 the spectrum has now been studied in pi-actical continuity to a wave-length of 

 nearly 100 microns; and at a great I'emove beyond this is another known region 

 embi'acing the so-called Hei'tzian, or electric, waves now employed in wireless 

 telegraphy. 



Beyond the violet, progress has been, relatively speaking, less rapid, unless 

 indeed it shall prove that the Rontgen and other radiations fall in this region. 

 But a great step in advance has been made by the unwearied and able investigations 

 of the author of the present work, Doctor Schumann. 



The difficulties hindering research in the ultra-violet are gi-eat, and consist 

 chiefly in the opacity of the usual optical media to the short wave-length rays. 

 Quartz, for a long time considered best in this part of the spectrum, is found to be 

 too opa(pie, and has been largely superseded in Doctoi- Schumann's investigations 

 by fluorspar for prisms and plates. Air, even in layers of a few millimeters 

 thickness, is almost wholly opaque, and other gases absorb strongly. It has 

 therefore been necessary to employ a spectroscope from which the air is exhausted 

 to the highest practicable degree ; and this and other necessary apparatus Doctor 

 Schumann has designed and constructed with bis own hands, though aided by 

 grants from the Hodgkins Fund of the Smithsonian Institution. Commercial 

 photographic plates were found to have films too opaque for the recording of these 



