142 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914. 
solar spectrum if one takes the light from the east and west limbs 
of the sun. There is a displacement of the lines of the two spectra 
with respect to one another, depending upon the fact that the one 
side of the sun is approaching the earth and the other side receding, 
by virtue of the rotation of the sun on its own axis. 
It had long been known that the sun rotated upon its axis, be- 
cause of the behavior of sun spots, which march across the disk of 
the sun in a period of about 14 days.1| Duner, Halm, Adams, and 
others have observed the rotation of the sun by means of the dis- 
placement of the spectral lines. The curious fact that the surface 
of the sun rotates with unequal velocities, largest at the equator 
and smaller as we approach the poles of the sun in either direction, 
had been noted from sun spot observations. This peculiar rotation 
behavior of the sun’s surface was investigated much more thoroughly 
by Adams, who followed the rotation of the sun up to solar latitude 
of 75°. He found that the period of rotation, as determined by the 
majority of the spectrum lines, varied from 24.6 days at the equator 
to 33.1 days at latitude 75°. However, the element hydrogen, which 
is situated high up in the solar atmosphere, indicated a much more 
nearly equal velocity of rotation at differing latitudes. The values 
range from 23.7 days at the equator to about 26 days at latitude 75°. 
Another cause of the displacements of the spectral lines is in the 
pressure which exists in the solar envelope. This was investigated 
first by Humphreys and Mohler at Baltimore. It has since fur- 
nished a valuable means of measuring the pressure which ‘exists in 
the solar envelope. For the element iron it is found to be about 
five times the atmospheric pressure at the surface of the earth? 
Still another interesting displacement of spectral lines was found 
by Zeeman to be due to the presence of a magnetic field. Spec- 
trum lines are broken up in the presence of a magnetic field into 
doubles or triples or still more complex groups, whose complexity 
of arrangement depends upon the situation of the spectroscope with 
respect to the magnetic field, and on the strength of the magnetic 
field in which the light is produced. This peculiarity was taken 
advantage of by Hale, who has recently proved the existence of a 
magnetic field in sun spots, and still more recently the existence 
of a general magnetic field over the whole surface of the sun, analo- 
gous in many respects to the magnetic field which exists over the 
surface of the earth. . 
A brilliant invention of Hale’s earlier years of investigation was 
that of the spectroheliograph. This is an instrument for observing 
the sun’s disk in the light of a single line of a single chemical element. 
1 This exceeds the half-period of the sun’s rotation because of the advance of the earth in its orbit at the 
same time. 
2 Recent work of Evershed and of St. John indicates that this estimate must be revised, and that pres- 
sures of one atmosphere or less exist where the iron lines are formed. 
