KNOWT.EDGE OF THE SUN — ST. JOHN 181 



ponent in one strip ami cuts out tho rod and transmits tlic blue 

 component in the following? strip and so on alternately, producing 

 tho stajrirerod eirpct. In tho case shown, the li'j^ht from the si)ot was 

 not viewed exactly parallel to the lines of the magnetic field, hence 

 there is a wetdc middle component, while in the laboratory observa- 

 tion there is no central coini)onont. tho line of si<;ht in this case being 

 parallel to the lines of the magnetic field. As the separation of tho 

 components observed in the laboratory for a given line is propor- 

 tional to the strength of the magnetic field, tho intensity of the field 

 in a spot is deducible from the separation of the components of the 

 same line in its spectrum. The magnetic fields in sun spots are 

 attributed to the rapid rotation of electrically charged particles in 

 deep-seated vortices in which the gases are in cyclonic rotation. 

 The low temperature, some 2,000° C. below that of the general sur- 

 face, was fii-st shown by interpreting the difTerencos between the 

 spectra of the photosphere and the spot umbra in terms of laboratory 

 observations, and was later deduced from the theory of ionization. 

 The polarity of the loading spot in a bipolar group in the North- 

 ern Hemisphere is opposite to that of the leading si)ot in the South- 

 ern Hemisphere, during a given 11-year cycle. The polarities re- 

 verse at the beginning of each new spot-cycle, the complete magnetic 

 period being double that between spot minima. The general mag- 

 netic field of the sun is relatively weak but was detected by methods 

 similar to those applied to spots, although still more refined measures 

 were necessary. It has an intensity at low levels in the reversing 

 layer of 50 gausses. The magnetic axis is 4° from the sun's axis of 

 rotation and revolves around it in 31.5 days. 



DOPPLER EFFECTS 



Lockyer's use of the Doppler principle to explain the distortions 

 of bright chromospheric lines was the first introduction of it into 

 solar investigation, where it has played a preeminent role. 



It has been applied to determining the rotation of the sun by 

 photographing in juxtaposition the spectra of the east and west 

 edges of the sun. (PI. '2, lig. 2.) The center strip is from the east 

 limb and the two outer from the west limb. A careful examination 

 will show that the Fraunhofer lines from the east and west limbs 

 are nmtually displaced. The displacement at the cast limb to shorter 

 wave length indicates that the eastern equatorial edge of the sun is 

 approaching the observer with a velocity of 2 km/sec. and that 

 therefore the sun rotates on its axis once in about 25 days. 



A solar spectrum shows dark lines due to absorption by oxygen 

 and water vapor in the earth's atmosphere. These atmospheric lines 



28U95— 31 13 



