MOUNT WILSON OBSERVATORY. 



217 



CONSTANCY OF WAVE-LENGTH OF ATMOSPHERIC LINES. 



Reference has been made to the importance of the use of atmospheric Hnes 

 as standards for wave-length measurements and as criteria for instrumental 

 adjustments. Since the work of P^rot at Meudon has indicated a variation 

 of wave-length with altitude which differs greatly on different days, it has 

 become necessary to investigate the question as thoroughly as possible. The 

 observations of Evershed at Kodaikanal and those at Mount Wilson are in 

 agreement in showing that their wave-lengths are independent of altitude and 

 constant from year to year. The results of Mr. St. John and Mr. Babcock 

 for the fractional part of the mean wave-length of 8 lines of the a band for a 

 period of 11 years are given below: 



1911 

 0.214 



1912 

 0.213 



1913 

 0.214 



1914 

 0.216 



1915 

 0.214 



1916 

 0.212 



1917 

 0.213 



1918 

 0.212 



1919 

 0.214 



1920 

 0.214 



1921 

 0.215 



SOLAR ROTATION. 



The observations by Mr, St. John and Miss Ware with the 150-foot tower 

 telescope now extend over the period of 8 years from 1914 to 1921. They 

 give no evidence of a progressive change from year to year. The plan of 

 work contemplates the continuation of the observations through an 11-year 

 period, increased emphasis being given to observations at high latitudes, 

 because the data at present indicate that the formula of Faye may require 

 modification in these regions. To determine accm'ately the period of rotation 

 at high latitudes, a greatly increased number of observations will be required, 

 since the effects due to local disturbances are often larger than the quantities 

 to be observed. In this investigation the spectra of center and limb are 

 taken simultaneously and the observations are distributed symmetrically in 

 sets of four, the spectrograph being rotated and the reflecting prisms thus placed 

 successively at corresponding points in each quadrant. This method of ob- 

 servation makes it possible to study each hemisphere separately and to 

 measure the limb-center displacement at each point of the limb. 



Through the use of spectral regions where atmospheric lines occur, it 

 becomes possible to study also the local disturbances at both center and hmb. 

 Since the amount and frequency of these disturbances may be greater the 

 larger the image, similar and simultaneous observations will be undertaken 

 with the Snow telescope, which gives an image but two-fifths as large as that 

 of the 150-foot tower telescope. The use of duphcate instruments will also 

 separate the influences of personal equation from those which depend upon 

 the instruments. As a further check, observations for rotation will be carried 

 on with the interferometer equipment of the Snow telescope. 



