ASTRONOMY: ST. JOHN AND BABCOCK 
135 
ting their areas as ordinates with abscissae corresponding to the points 
at which the intensity curves were taken. Figure 2 shows the energy 
curves thus obtained for the same lines. The apparent decrease in energy 
close to the negative pole is undoubtedly fictitious, being introduced by 
several causes operating during the exposure of the photographic plate. It 
is seen from the curves of Figure 2 that the energy of emission of X 5424 
is 6 times as great near the negative pole as it is at the center, while 
for X 5434 the ratio is 3.2. Since the former line occurs chiefly in the 
core of the arc and the latter is strong in both inner and outer parts, 
viz., throughout a much greater volume of arc vapor, it is possible that 
the effects observed at the negative pole for Hues hke X 5424 are due in 
/ \ 
/ \ 
- Pole + Pole 
Fig. 2. Distribution of energy along the axis of the arc. 
part to the greater proximity of the separate radiating centers; in other 
words, to an effect of density of radiating vapor as distinguished from 
pressure. It is by no means certain that the true pressure effect depends 
entirely upon increased nearness of the centers of emission and not upon 
changes in the surrounding medium. 
From an examination of the variations of wavelength when the light 
is taken from points between the center and the negative pole, as v/ell 
as from a comparison of spectra taken from the centers of two arcs carry- 
ing currents of different strength, we find that the most sensitive lines 
exhibit measurable displacements at considerable distance from the pole 
and are affected by changes of 40% in the current even when observed 
