DEVELOPMENTS IN ASTRONOMY—PLASKETT. 259" 
Prof. Hale, that the umbra of sun spots is at a lower temperature 
than the rest of the sun’s surface, and that in sun spots, as first 
found by Prof. Fowler, of London, we have the spectra of some 
chemical compounds, such as titanium oxide, magnesium and cal- 
cium hydride, further showing that the temperature is sufficiently 
reduced to allow the formation of such compounds, which do not 
appear in the normal solar spectrum. Again, we have the discovery 
of Evershed, of Kodaikanai, India, of radial motions of the vapors 
around sun spots, and the final discovery by Hale that many, if 
not all, sun spots are surrounded by whirls, and that electrically 
charged particles, which, it has been further shown, are negatively 
charged, are carried around by these whirls and produce the mag- 
netic field which is shown to exist around sun spots. 
At the high temperature of the sun, magnetism as we know it 
can not exist, and the field must be produced by such whirls or 
vortices. The manner in which the magnetic field in sun spots was 
detected and proved is a splendid example of experimentation to 
test scientific deductions and a full justification of the expenditure 
on the powerful apparatus needed for such work. The whirls sur- 
rounding sun spots are shown on photographs of part of the sun’s 
surface in the light given by the red line of hydrogen. Such pho- 
tographs are made by the spectroheliograph, an instrument which 
enables us to photograph the sun’s surface in the light of different 
- gases or vapors, and hence records the distribution of these vapors. 
‘The great resemblance between these whirls and the lines of force 
around a magnet, as shown by iron filings, led Prof. Hale, the in- 
ventor of the spectroheliograph and the discoverer of this effect, to 
suspect the presence of a magnetic field; and the next question was 
to verify this suspicion. It was found several years ago by Zeeman 
that.if a luminous vapor is produced between the poles of a magnet, 
many of the lines of its spectrum are widened. Prof. Hale found 
that the spectrum of a sun spot, with the high dispersion available 
on Mount Wilson, showed some of the same lines widened, strength- 
ening his suspicion. Furthermore, when the widened lines produced 
by a magnetic field in the spectrum of a luminous vapor are examined 
through a polarizing apparatus, many of the lines are split up into 
doublets, triplets, quadruplets, or even sextuplets; and a similar test 
applied to a sun-spot spectrum gave a similar, though much weaker, 
effect, conclusively proving the presence of a magnetic field. Com- 
parison showed that its strength was about one-quarter of that 
needed to saturate iron, too weak to produce any magnetic disturb- 
ance on‘the earth, and therefore incapable of explaining the frequent 
coincidence of magnetic storms and large sun spots. 
The fact that sun spots are at a lower temperature than the rest of 
the photosphere has been corroborated by the work of Prof. Abbot, 
