348 



SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. S2 



These displacements have been studied by Adams, who has utilized 

 the facilities offered by the Snow telescope and the i8-foot spectro- 

 graph to carry out what is probably the most accurate spectroscopic 

 investigation of the solar rotation hitherto accomplished. In the 

 earlier investigations of Duner and Halm, both of which exhibit a 

 high degree of precision, visual observations were employed, and as 

 all of the measures had to be made at the telescope, the observers re- 

 stricted themselves to the use of only two lines. The advantages of 

 photography are obvious when it is remembered that in a single short 

 exposure a portion of the spectrum from 15 to 20 inches long, show- 

 ing opposite limbs of the Sun and containing thousands of lines 

 suitable for measurement, can be recorded upon a sensitive plate. 

 The work on Mount Wilson is limited to making the photographs, 

 which are afterwards measured in the Computing Division at Pas- 

 adena, with measuring machines which give the positions of the 

 lines within about one-thousandth of a millimeter. Since iron, cal- 

 cium, carbon, sodium, hydrogen, and other elements are represented 

 on the plates, it is possible, by measuring the displacements of the 

 corresponding lines, to determine the velocity of rotation of the 

 vapor due to any one of these elements. 



The lines measured by Adams (assisted by Miss Lasby) include 

 some for each of the following elements: iron, manganese, nickel, 

 titanium, lanthanum, carbon, chromium, and zirconium. The fol- 

 lowing table gives the values obtained for different latitude? :^ 



It will be seen that, as in the case of Sun-spots, the period of the 

 Sun's rotation increases from the equator toward the poles. The- 

 oretical investigations suggest that this remarkable law of rota- 



' Adams : Contributions front the Solar Observatory, No. 20, Astrophysicat 

 Journal, vol. xxvi, pp. 203-224, November, 1907. 



