THE SEMI-CENTENNIAL ANNIVERSARY 49 



of the sun to be compared with that of the earth, give 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 6o-foot tower 

 telescope in 1908, but the results were too uncertain to command 

 confidence. 



Thanks to additional appropriations from the Carnegie Insti- 

 tution of Washington, a new and powerful instrument was 

 available on Mount Wilson for a continuation of the investiga- 

 tion in January, 1912. The new tower telescope has a focal 

 length of 150 feet (Fig. 13). 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 

 enclosed 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 mem- 

 bers 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 supports the instru- 

 ments. The plan has proved to be successful, permitting obser- 

 vations demanding the greatest steadiness of the solar image to 

 be made. 



The arrangements are similar to those of the 6ofoot tower. 

 The solar image, i6J inches in diameter, falls on the slit of a 

 spectrograph (Fig. 14) 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 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 instruments are well suited for the exacting requirements 

 of a difficult investigation. For it must be borne in mind that 

 the problem is very different from that of detecting the magnetic 



