No. 5.] 



STUDY OF ARC SPECTRA. 



373 



resting on the negative carbon. The central portion (i), which is 

 of a violet tint, connects the two bright points of the carbons. 

 Outside of this is a sheath of dull blue (2), which is 

 most brilliant at the negative carbon, where it extends 

 across in such a way as to hide the violet portion. The 

 outer sheath (3) is yellow, shading into orange at the 

 cooler outer edge ; this last part includes the greater 

 portion of the flame of the arc, and extends well up 

 around the positive carbon. The different divisions are 

 shown in Fig. 2. 



The investigation may be considered as divided into four 

 parts : 



I. Photographs were taken with the slit extending vertically 

 through the arc in six different positions : first, through the center 

 of the violet portion ; second, at the line of separation between 



Fig. 2. 



Fig. 3. 



Fig. 4. 



Fig. 5. 



the violet and blue ; third, in the blue ; fourth, between the blue 

 and the yellow ; fifth, in the yellow ; and sixth, at the extreme 

 outer edge of the yellow (see Fig. 3). 



II. Photographs were also taken with the slit extending horizon- 

 tally through the image of the arc. Three positions were chosen 



