50 NATIONAL ACADEMY OF SCIENCES 



fields in sun-spots, where the separation of the lines is from fifty 

 to one hundred times as great as we may expect to find here. 

 Thus the sun's general field can produce no actual separation of 

 the lines. But it may cause a very slight widening, which should 

 appear as a displacement when suitable polarizing apparatus is 

 used. This is so arranged as to divide the spectrum longitudi- 

 nally into narrow strips. The component toward the red end 

 of the spectrum of a line widened by magnetism should appear 

 in one strip, the other component in the next strip. Hence, if 

 the sun has a magnetic field of sufficient strength, the line should 

 have a dentated appearance. The small relative displacements 

 of the lines on successive strips, when measured under a micro- 

 scope, should give the strength of the magnetic field. 



The above remarks apply strictly to the case when the ob- 

 server is looking directly along the lines of force. At other 

 angles neither component is completely cut off, and the magni- 

 tude of the displacement will then depend upon two things: 

 the strength of the magnetic field and the angle between the line 

 of sight and the lines of force. Assuming that the lines of force 

 of the sun correspond with those of a magnetized sphere, and also 

 that the magnetic poles coincide with the poles of rotation, it 

 is possible to calculate what the relative displacement should 

 be at different solar latitudes. These theoretical displacements 

 are shown graphically by the sine curve on the screen (Fig. 15). 



We see from the curve that the greatest displacements should 

 be found at 45 north and south latitude, and that from these 

 points they should decrease toward zero at the equator and the 

 poles. Furthermore, the curve shows that we may apply the 

 same crucial test used in the case of sun-spots: the direction of 

 the displacements, toward red or violet, should be reversed in 

 the northern and southern hemispheres. 



I shall not trouble you with the details of the hundreds of 

 photographs and the thousands of measures which have been 

 made by my colleagues and myself during the past year. In 

 view of the diffuse character of the solar lines under such high 

 dispersion, and the exceedingly small displacements observed, 



