ASTRONOMY: ST. JOHN AND BABCOCK 
131 
VARIABILITY OF SPECTRUM LINES IN THE IRON ARC 
By C. E. St. Johr^ and H. D. Babcock 
MOUNT WILSON SOLAR OBSERVATORY. CARNEGIE INSTITUTION OF WASHINGTON 
Presented to the Academy, January 18, 1915 
Recent progress in the application of spectroscopy to physical and as- 
tronomical problems has depended in large measure upon the determi- 
nation of very minute changes in spectrum lines, involving not only their 
position in the spectrum, but their variations of intensity and shape and 
their alteration by magnetic and electric fields. In the attempt to in- 
terpret observations upon the spectra of heavenly bodies, which in gen- 
eral are produced under the most complex conditions of radiation, it is 
therefore of prime importance to possess detailed information regarding 
the peculiarities of behavior of spectrum lines individually. 
These considerations make clear the need for standard lines whose 
wavelength is accurately known when they are produced under known 
conditions, and whose response to definite variations in the physical condi- 
tions surrounding the source is well estabHshed. For the general purposes 
of precision spectroscopy, the necessity for the fixing of a complete system 
of working standards of wavelength extending throughout the spectrum 
has led to the adoption of the electric arc between iron terminals under 
certain specified conditions as a standard source of radiation. However, 
the determinations by different observers of the wavelengths of lines se- 
lected as standards do not show the accordance which would be expected, 
if the errors were solely those inherent in the method of observation. 
The suspicion thus aroused as to the constancy of these wavelengths, 
when the arc is employed under the specified conditions, coupled with 
the need for extensions to our knowledge of the character of the lines 
under different conditions of radiation has made necessary a careful study 
of the iron arc spectrum, some results of which form the substance of this 
paper. 
A visual observation of the arc made with the help of various colored 
screens brings out the fact that there are considerable differences in 
brightness in different parts of this source. At the negative pole the 
vapor is in general far brighter than at the positive pole, while there is 
an equatorial region which is less bright than either. With a deep red 
screen the outer envelope of the arc appears to surround a comparatively 
dark core which contains two small bright brushes, one emanating from 
each pole. A blue screen, on the other hand, causes the outer parts to 
appear much fainter than the part which appeared dark in red light. 
