EARTH AND SUN AS MAGNETS HALE. 155 



the exacting requirements of a difficult investigation. For it must 

 be borne in mind that the problem is veiy different from that of 

 detecting the magnetic fields in sun spots, where the separation of 

 the lines is from 50 to 100 times as great as we may expect to 

 find here. Thus the sun's general field can produce no actual separa- 

 tion 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 suc- 

 cessive strips, when measured under a microscope, should give the 

 streng-th of the magnetic field. 



The above remarks apply strictly to the case when the observer is 

 looking directly along the lines of force. At other angles neither 

 component is completely cut off, and the magnitude of the displace- 

 ment will then depend upon two things: The strength of the mag- 

 netic 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 coin- 

 cide 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. 14). 



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. Fur- 

 thermore, 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 photo- 

 graphs 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 ex- 

 ceedingly small displacements observed, the results must be given 

 with some reserve, though they appear to leave no doubt as to the 

 reality of the effect. Observations in the second order spectrum 

 failed to give satisfactory indications of the field. But with the 

 higher dispersion of the third order 11 independent determinations, 

 made with every possible precaution to eliminate bias, show opposite 

 displacements in the northern and southern hemispheres decreasing 

 in magnitude from about 45° north and south latitude to the equator. 



