MOUNT WILSON OBSERVATORY. 209 



STUDIES OF THE ZEEMAN EFFECT. 

 Extensive measurements of the Zeeman effect for iron, chromium, and 

 vanadium, which were begun some years ago by Babcock, primarily for com- 

 parison with observations on sun-spots, but which are for the most part still 

 unpublished, have assumed new importance since the recent visits of Professor 

 Sommerfeld and Professor Russell. The value of this material in the difficult 

 problem of finding series relationships in complicated spectra is proving so 

 great as to require its rapid extension to certain other elements. An examina- 

 tion of the data on iron, chromium, and vanadium from this new point of view 

 has been completed, with numerous valuable results. Measurements are now 

 in progress on titanium. The data have already been placed at the disposal 

 of Sommerfeld and Russell, under whose direction much is being accomplished 

 in reducing these spectra to rational systems based on the quantum theory. 



THE RATIO elm. 



A considerable body of observational data on the Zeeman effect, which has 

 been collected during previous years, has recently been discussed by Babcock 

 for a new determination of this important ratio. The spectroscopic material 

 is of greater variety and extent than any hitherto used in this way, and the 

 absolute magnetic measurements are of ample accuracy for the purpose. An 

 analysis of the influence of the various factors involved in this method of 

 determining e/w shows that further increase of accuracy will require higher 

 spectroscopic resolving power and magnetic fields of greater intensity. 



Forty-nine separate values of the ratio have been derived, which yield a mean 

 result of e/m = 1.761X10^ with an estimated uncertainty of 2 or 3 parts in 

 1,800. Dr. Birge, using this value of e/m, has calculated the value of Planck's 

 constant /i, and finds a result in excellent agreement with the most probable 

 value of U based on a variety of methods. 



FAINT ARC-LINES OF IRON. 



Lines easily measurable in the solar spectrum are often so faint in ordinary 

 laboratory sources as to require special conditions for their observation. 

 About 150 such iron arc-lines have been specially studied by Babcock with the 

 15-foot concave grating. Their wave-lengths have been measured on some 

 photographs with long exposures taken by Smith for another purpose, and 

 the results furnish data for comparison with their positions in the solar spec- 

 trum. For many of the lines no other modern measurements are available. 



VACUUM THERMO-COUPLES. 



Pettit and Nicholson have continued their laboratory studies of thermo- 

 couples and methods for improving their sensitiveness. By reducing the 

 mass of the thermo-couple to 0.01 mg., a gain of one magnitude over previ- 

 ous results has been made in stellar observations. The two junctions are 

 connected in such a way that they may be used either singly or together as a 

 compensated thermo-couple. Cells have been constructed for various pur- 

 poses with windows of microscope cover-glass, quartz, fluorite, and rock salt. 



In preparation for the reduction of the stellar observations, several investi- 

 gations have been undertaken to determine the constants of the apparatus 

 employed. For this purpose a rock-salt spectrograph was constructed with a 

 10-junction vacuum thermopile as the sensitive element. The apparatus was 

 connected to the registering device of the microphotometer and energy 



