154 ANNUAL REPOET SMITHSONIAN INSTITUTION, 1913. 



perturbations due to currents in the solar atmosphere must delay the 

 most effective application of these methods, though they promise 

 much future knowledge of the magnetic field at high levels in the 

 solar atmosphere. 



Of the field at low levels, however, they may tell us little or notli- 

 ing, for the distribution of the electrons may easily be such as to 

 give rise to a field caused by the rotation of the solar atmosphere, 

 which may oppose in sign the field due to the rotation of the body of 

 the sun. To detect this latter field, the magnetic field of the sun as 

 distinguished from that of the sun's atmosphere, we must resort to 

 the method employed in the case of sun spots — the study of the 

 Zeeman effect. If this is successful it will not only show beyond 

 doubt whether the sun is a magnet ; it will also permit the polarity 

 of the sun to be compared with that of the earth, gives a measure of 

 the strength of the field at different latitudes and indicate the sign 

 of the charge that a rotating sphere must possess if it is to produce a 

 similar field. 



I first endeavored to apply this test with the 60-foot tower tele- 

 scope in 1908, but the results were too uncertain to command con- 

 fidence. 



Thanks to additional appropriations from the Carnegie Institute 

 of Washington, a new and powerful instrument was available on 

 Mount Wilson for a continuation of the investigation in January, 

 1912. The new tower telescope has a focal length of 150 feet (fig. 

 12). To prevent vibration in the wind, the coslostat, second mirror, 

 and object glass are carried by a skeleton tower, each vertical and 

 diagonal member of which is inclosed within the corresponding 

 member of an outer skeleton tower, which also carries a dome to 

 shield the instruments from the weather. In the photograph we 

 see only the hollow members of the outer tower. But within each 

 of them, well separated from possible contact, a sectional view would 

 show the similar but more slender members of the tower that sup- 

 port the instruments. The plan has proved to be successful, per- 

 mitting observations demanding the greatest steadiness of the solar 

 image to be made. 



The arrangements are similar to those of the 60-foot tower. The 

 solar image, 16^ inches in diameter, falls on the slit of a spectro- 

 graph (fig. 13) in the observation house at the ground level. The 

 spectrograph, 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 in- 

 vestigation, 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 instruments are well suited for 



