THE EARTH AND SUN AS MAGNETS 121 



(Fig. 14) in the observation house at the ground level. The spectro- 

 graph, of 75 feet focal length, enjoys the advantage of great stability and 

 constancy of temperature in its subterranean vault beneath the tower. 

 In the third order spectrum, used for this investigation, the D lines of 

 the solar spectrum are 29 millimeters apart. The resolving power of the 

 excellent Michelson grating is sufficient to show 75 lines of the iodine 

 absorption spectrum in this space between the D's. Thus the instru- 

 ments are well suited for the exacting requirements of a difficult inves- 

 tigation. For it must be borne in mind that the problem is very 

 different from that of detecting the magnetic 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 pro- 

 duce no actual separation of the lines. But it may cause a very slight 

 widening, which should appear as a displacement when suitable polar- 

 izing apparatus is used. This is so arranged as to divide the spectrum 

 longitudinally 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 microscope, should give the strength 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 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 displace- 

 ment should be at different solar latitudes. These theoretical displace- 

 ments 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. Further- 

 more, 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 exceed- 

 ingly 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 



VOL. LXXX1II. — 9. 



