ASTRONOMY: HALE AND BABCOCK 
125 
row, and if the Stark effect, not yet determined for the elements repre- 
sented by these triplets, even approaches in order of magnitude the 
values indicated by the hydrogen and helium lines, the electric field at 
the level in question must be of very low intensity.^ This point will be 
followed up as soon as the Stark effect can be observed for iron, chro- 
mium, nickel, titanium, manganese, vanadium and other elements 
whose Hues are resolved in sun-spots. 
The hydrogen lines are shown by our photographs to be weakened 
and narrowed in spot spectra. Thus they offer no indication of an 
electric field, but they will be carefully studied for possible traces of 
polarization phenomena. 
As the magnetic fields in sun-spots seem to point so plainly to the 
existence of electric fields, the negative evidence of the Stark effect so 
far found in this quarter is not promising for researches in other parts 
of the sun. But it must not be forgotten that the establishment of a 
definite upper limit of intensity for electric fields at many different 
levels in the solar atmosphere is of the utmost importance, and this can 
be secured even if no pos tive evidence of the Stark effect can be de- 
tected. In sun-spots the lines of force of the electric field would pre- 
sumably be tangential to the surface, making the center of the sun the 
best point for studies of the Stark effect. For the sun as a whole, on the 
contrary, the lines of electric force would be radial, so that evidence 
of a general electric field should be sought in the behavior of lines near 
the limb. These are well known to be unresolved and the only effect 
to be anticipated is a very slight widening of the lines, which should 
have plane polarized edges. 
The marked widening and displacement of solar fines near the limb, 
found here some years ago to be a general phenomenon of much impor- 
tance, is not due to the Stark effect, if we may judge from the fact that 
the lines cannot be reduced by a Nicol prism to their normal width 
at the center of the sun. We must determine, however, whether there 
may remain a very minute effect of widening, such as can be detected 
only with the most refined methods of observation. Three requirements 
must be met: 
1. High resolving power and Hnear dispersion in the spectrograph, 
suppHed in our experiments by a large Michelson grating, giving theo- 
retically perfect resolution. As used in the 75-foot spectrograph of 
the 150-foot tower telescope the linear dispersion in the second order 
is about 3 mm. to the angstrom imit, or slightly greater than in Row- 
land's map of the solar spectrum, which was enlarged from negatives of 
