372 
ASTRONOMY: A. S. KING 
Further use of the tube-arc has now been made in studying the spec- 
trum of iron from X 3600 to X 6600, special regard being paid to the 
dissymmetries produced and the manner in which certain types of Hnes 
are intensified. 
The tube-arc shows dissymmetries in a very pronounced degree 
which are present in a more or less incipient state in the ordinary arc, 
especially in the region near the negative pole. Such dissymmetries 
are naturally most conspicuous in the case of reversed lines, and but 
few of these show a fully symmetrical structure in the tube-arc spectrum, 
the red side usually being stronger than the violet. 
The unsymmetrically reversed iron Hnes fall into two main groups, 
and these groups show a close connection with the behavior of the lines 
at various temperatures of the electric furnace and with their pressure 
displacements. The first group is made up of lines whose diss)nnmetry 
in the tube-arc is small or moderate in amount. These appear in the 
electric furnace at low temperature. The arc under pressure gives 
them approximately sjrmmetrical and with small or moderate pressure 
displacements. The lines of the second group are reversed very unsym- 
metrically in the tube-arc, the red side being often 6 to 8 times as strong 
as the violet side. They are relatively much stronger in the arc than 
in the furnace. Under pressure they are very unsymmetrical and show 
large pressure displacements. In the ordinary iron arc the lines of the 
second group show a decided dissymmetry near the pole as compared 
with the center of the arc. 
A similar concordance of behavior is shown for a group of strong 
lines in the green-yellow. These widen toward the violet near the pole 
of the iron arc and are displaced toward the violet by pressure. In 
the tube-arc these are unreversed and nearly symmetrical, but show 
strong displacements toward the violet. Reversal with the violet 
side strongest has not been observed among the iron lines, but this 
structure is shown by the tube-arc for the H and K Hnes in the calcium 
spectrum. 
The measurement of these tube-arc Hnes, difiicult and uncertain with 
a micrometer microscope, is made much more accurate by the register- 
ing micro-photometer, the curves of which show the structure of the 
lines and permit the interval between neighboring lines to be measured. 
A number of favorably situated lines have been thus measured, the 
distance being taken from a line approximately symmetrical to one 
showing strong dissymmetry. The latter Hne in each case was found 
to be displaced by amounts ranging from 0.02 to 0.04 A. An interest- 
ing feature is that when a line is made narrow by the presence of very 
