No. 5.] STUDY OF ARC SPECTRA. 375 



treme ultra-violet, including the first bright group (\ = 2263 to 

 \ = 3600) ; second, the region of the three principal bright 

 groups (X, = 3450 to X. = 42ii); third, from the brightest of the 

 groups to the D lines, or from X. = 3800 to X= 5900 ; 2L\\d fottrth, 

 with a very long exposure, the region between \ = 45(X) and 

 X = 6400. The exposures varied in length from thirty seconds 

 in the violet to five or ten minutes in the extreme ultra-violet, and 

 fifteen to twenty minutes in the red of the spectrum. 



On account of the motion of the arc it was necessary to have 

 a movable screen, by means of which the light could be cut off 

 whenever the arc moved so as to throw the wrong part of the 

 image upon the slit. This movement of the arc caused so much 

 trouble that it was difficult to obtain good photographs of the 

 different sheaths in the regions of the spectrum which required 

 long exposure. Hence the study of the spectra of different parts 

 of the arc has been confined to the region between X = 30io and 

 X=550o; and toward the ends of this region photographs were 

 taken only in the violet, blue, and yellow sheaths of the image. 

 A slightly longer exposure was given in the outer part of the arc 

 than was needed in the violet and blue portions. 



The length of the arc used was about 1.4 cm. 



I. 



Results obtained with the Ordinary Arc. 



The distinctive features of the arc spectrum are the bright 

 groups 1 from X = 3520 to ^ = 3590, \ = 38oo to \ = 3885 men- 

 tioned by Kayser and Runge, Drude and Nunst, Nichols and 

 Franklin, etc.; one from X = 4400 to X = 46o4; the carbon bands ^ 

 given by Kayser and Runge and others, A. = 4680 to ^. = 4737, 

 \ = 4746 to \=5i65, and X=5530 to X=5635; together with 

 a host of periodically placed fine lines. The bright groups 

 appear to be due to a crowding together of these numerous fine 

 lines. The strong lines of the various metals, which are present 



1 Snow, Physical Review, Vol. I., p. 28 ; Kayser and Runge, Wied. Ann., 

 XXXVIII., p. 81, 1889; Drude and Nunst, Wied. Ann., XLV., p. 460, 1S92 ; Nichols 

 and Franklin, Am. J. (3), p. 106, 1SS9. 



- Kayser and Runge, Ueber die Spectren der Elemente, Zweiter Abschnitt. 



