MOUNT WILSON SOLAR OBSERVATORY. 277 



the electric furnace to burn apart at a selected point, producing a 

 low-voltage arc with an initial current of 1,000 amperes or higher. 

 Experiments with the improved equipment have confirmed the obser- 

 vations reported in 1913 as to the varying intensity distributions of 

 lines of different elements, the tendency of the tube-arc to show a 

 spectrum resembhngthe spark, and the dissymmetries imparted to many 

 of the stronger lines. The last feature is notable in the iron spec- 

 trum. The stronger arc Unes are generally reversed and distinctly 

 unsymmetrical, with the red side stronger. The degree of this dis- 

 symmetry, which varies from a barely perceptible one-sidedness to 

 a condition where only a trace of the violet side of the line is visible, 

 has been found to be closely related to the behavior of the line under 

 other physical conditions. Thus lines of small dissymmetry in the tube- 

 arc are low-temperature Unes in the furnace and show only moderate 

 sensitiveness to pressure displacement. Lines which are very one-sided 

 in the tube-arc are high-temperature furnace Unes and are strongly 

 displaced by pressure. There are important exceptions, however, to 

 the correspondence between tube-arc dissymmetries and pressure 

 displacements and the similarity is probably to be interpreted as a 

 sensitiveness to displacing agencies of different character. 



The Koch microphotometer has been used in registering a large 

 number of curves for the tube-arc plates, which show the character 

 and the degree of the dissymmetry for different classes of Unes. It was 

 also possible, by measuring the distances from the curves of standard 

 Unes in both the tube-arc and the ordinary arc, to determine the 

 displacement of the intensity maximum of the unsymmetrical Une. 

 Measurements of the curves for strong iron Unes in the blue gave dis- 

 placements ranging from 0.015 to 0.040 A. This displacing action in 

 the tube-arc is diminished if the vapor-density is decreased to such a 

 degree that the Une in question becomes narrow. Changes in vapor- 

 density alone, however, do not give unsymmetrical Unes or measurable 

 displacements, this having been tested by measurements of electric- 

 furnace spectra for large differences of vapor density. The effect 

 appears to depend primarily on the discharge conditions of the tube-arc, 

 but to be magnified if the Une is given greater width through high vapor 

 density. 



Measurements of the microphotometer curves of a group of iron 

 Unes in the green-yeUow show these Unes to be displaced toward the 

 violet in the tube-arc. The same lines have been measured by St. 

 John and Babcock as displaced toward the violet at the pole of the u'on 

 arc, and the relative magnitudes of the dissymmetries for the tube- 

 arc Unes in general show a very close correspondence with their polar 

 displacements. 



In addition to bringing out the enhanced Unes, the tube-arc inten- 

 sifies the diffuse lines of iron, the appearance of the spectrum as a whole 

 being very different from that of the ordinary arc. 



