374 CAROLINE JF. BALDWfN. [Vol. III. 



for these : first, near the tip of the negative carbon ; second, 

 half-way between the carbons ; and third, near the positive carbon. 



III. The flame was blown out from between the carbons by 

 means of a horseshoe magnet. Under these circumstances the 

 violet i^art does not change much in its position, but is slightly 

 extended on the side away from the magnet ; while the blue and 

 yellow are blown out nearly three centimeters beyond the edge of 

 the carbons. The appearance of the arc was as shown in Fig. 4. 

 Photographs were taken with the slit extending vertically through 

 this image in nine different places (Fig. 5) : first, at the inner 

 edge of the arc ; second, in the center between the tips of the 

 carbons ; third, in the outer part of the violet ; fourth, at the edge 

 of the carbons ; fifth, outside of the carbons through the blue and 

 yellow ; sixth, in the yellow ; seventh, at the extreme end of the 

 flame ; eighth and ninth, at the edge of the carbons, giving the 

 extent of the flame in the first place at the side toward the nega- 

 tive carbon, and in the second place at the positive carbon. 



One hundred and thirty photographs were taken by these 

 methods, from which the study of the ordinary arc spectrum has 

 been made. 



IV. Metals were introduced into the arc, and similar sets of 

 photographs were taken. 



The metallic salts used were lithium carbonate, sodium nitrate 

 and chloride, potassium chloride, calcium chloride, strontium oxide, 

 barium chloride, copper sulphate, silver nitrate, zinc chloride, and 

 cadmium chloride. Sodium and zinc were studied by all three 

 methods, the others only by the first two. This series included 

 one hundred and fifty photographs. 



All of these photographs were taken in the primary spectrum, 

 as there was less confusion due to overlapping of spectra. The 

 ultra-violet of the second spectrum, which extended to the green 

 of the first, could be easily cut off by means of a glass plate placed 

 in front of the slit. This gave a pure spectrum in the region to 

 which the plates were sensitive. 



In order to have the negatives evenly exposed, and the intensity 

 as nearly as possible the same for different parts of the spectrum, 

 four positions were chosen for photographing. First, the ex- 



