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, winch do not 

 appear in the normal solar spectrum. Again, we have the discovery 

 of Evershed, of Kodaikanal, 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 wliirls, and that electrically 

 charged particles, which, it has been further shown, are negatively 

 charged, are carried around by these wliirls 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 wliirls 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 fines 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 fines 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, 



