A PHOTOGRAPHIC STUDY OF ARC SPECTRA. I. 



By Caroline Willard Baldwin. 



Introduction. 



IN the course of his investigation of the Infra-Red Spectra of 

 the Alkalies, Professor Snow ^ has shown the remarkable effect 

 produced upon the arc spectrum when the alkalies were present 

 in the carbons. He found that the curve obtained from his 

 bolometric measurements was materially changed by the metals. 

 While in the ordinary arc he had several very strong maxima 

 in the ultra-violet and in the visible spectrum, he discovered that 

 upon introducing the metals, these intense regions disappeared 

 and strong maxima were now observed only in the infra-red. 



The maxima in the violet and ultra-violet are the well-known 

 characteristic, bright groups of the arc spectrum. These are 

 produced by a peculiar crowding together of fine lines in the 

 regions ^=3450 to ^-=3590, \= 3700 to X = 3885, and X = 4030 

 to X = 42ii. The disappearance of these and other similar groups 

 would amount to a practical obliteration of the arc spectrum and 

 substitution in its stead of the line spectrum of the particular 

 metal used. 



The phenomena seemed worthy of further study, and as the 

 bright groups lie in the regions to which the photographic plate 

 is most sensitive, it was thought worth while to try the applica- 

 tion of photography to determine whether the disappearance noted 

 was complete, or whether the bright regions were only so much 

 reduced in intensity as to escape detection by bolometric obser- 

 vation. 



As a matter of fact, the work was extended beyond its original 

 limit, and in the end it assumed the form of a photographic study 

 of the spectrum obtained from different regions of the arc under 



1 B. W. Snow, Physical Rkvikw, Vol. I., p. 28. 



