168 
ASTRONOMY: G. E. HALE 
Proc. N. a. S. 
INVISIBLE SUN-SPOTS 
By Gkorge E. HAI.K 
Mount Wilson Observatory, Carnbgis Institution of Washington 
Communicated, May 26, 1922 
The vortex hypothesis, proposed in 1908, assumes that a sun-spot re- 
sembles a vast tornado in which electrified particles, due to ionization in 
the solar atmosphere, are rapidly whirled. The invariable presence of 
a magnetic field, caused by the revolving charges, confirms this view,^ 
which is also supported by various other results of observation with the 
spectrograph and spectroheliograph. Subsequently it was found that 
most sun-spots are associated in pairs, of opposite magnetic polarity, 
in which the preceding (western) spot is usually the larger of the two. 
61 per cent of 970 spots examined in the years 1915-1917 were of the bi- 
polar type, while 33 per cent were unipolar. All but 11 per cent of the 
unipolar spots, however, showed a tendency toward the bipolar type, 
indicated by trains of calcium flocculi following (less often preceding) 
the single member.^ Frequently such groups oscillate between the uni- 
polar and bipolar types, one or more small spots appearing or disappearing 
within the mass of calcium flocculi. This peculiarity has led to a search 
for invisible spots, regarded as vortices giving appreciable magnetic fields, 
in which the cooling due to expansion is insufficient to cause perceptible 
darkening of the sun's surface. 
Preliminary attempts to increase the contrast by the exclusive use of 
ultra-violet light did not prove successful, and a second method of detect- 
ing the slight difference of radiation by a heat-measuring instrument 
(bolometer, thermopile, or radiometer) or a photo-electric cell has not yet 
been tried. A third method, however, has given very satisfactory results. 
This consists of a simple device for rendering weak magnetic fields visible 
by their Zeeman effect. 
The iron line X 6173, observed in the second order of the 75-foot spectro- 
graph of the 150-foot tower telescope, appears as a wide triplet in the strong 
magnetic fields of large sun-spots. In very small spots, where the field 
is weak, it is merely widened, but either edge can be cut off by a Nicol 
prism and quarter- wave plate mounted above the slit of the spectrograph. 
When searching for invisible spots a superposed half-wave plate, oscillated 
back and forth by a small electric motor, is used to reverse the action of 
the quarter- wave plate when it comes before the slit. While the motor 
is running, a promising region on the sun (marked by faculae or calcium 
flocculi) is caused to pass slowly across the slit, and the X 6173 line is care- 
fully watched. The presence of an invisible spot is betrayed by the 
apparent oscillation of the corresponding part of the* line, due to the al- 
