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ASTRONOMY: ST. JOHN AND BABCOCK 
the image from one pole of the arc to the other, although each Kne image 
represents an integration of all the Hght in the line of sight. These pho- 
tographs display the differences between different classes of Hnes in a 
striking manner, showing the variations in width, intensity, dissymmetry, 
etc., from point to point along the arc axis. For deriving from these 
plates curves showing the variation of intensity across the spectrum line 
at any desired point, a registering microphotometer was employed. 
This instrument automatically draws a curve which can be transformed 
by the aid of the photometric scale into an intensity curve. By com- 
paring these intensity curves for a given line taken at a series of points 
along the axis of the arc, an idea is had of the actual shape of the Hne in 
— Pole Center 
Fig. 1. Comparative intensities at pole and center. 
X 5424 X 5434 
dift'erent parts of the arc. Figure 1 shows typical intensity curves taken 
from near the negative pole and the center of the arc respectively for 
two lines. The dotted curves refer to X 5424, which shows a large shift 
toward the violet at the negative pole, while the other curves belong to 
X 5434, whose wavelength is unchanged. Both Hnes are seen to be more 
intense at the negative pole than at the center, but the difference is 
greater for X 5424. On account of the greater widening and dissymmetry 
shown by this line in the vicinity of the pole, it is important to compare 
the total energy of radiation at center and pole for each line. This has 
been accomplished by taking intensity curves at short intervals along 
the line from one pole to the other, integrating these curves and plot- 
