MOUNT WILSON SOLAR OBSERVATORY. 277 



determination of the conditions under which displacements occur that 

 are not due to pressure differences in the arc. There are indications 

 that the reversals of lines give somewhat different results, a subject 

 not yet fully worked out. The importance of determining any changes 

 of wave-length due to the presence of the vapors of other elements is 

 very great, because of the conditions of mixture existing in sun and 

 stars. It has been reported by Barus that when some elements are 

 present only in traces, the wave-lengths are shorter than when the given 

 element is the main constituent. An investigation of alloys in which 

 an element is present in varying proportions is being carried on. The 

 method of simultaneous exposures makes it possible to use a constant 

 light source as a standard in reference to which the effects of dilution 

 may be determined with high precision. Other elements than iron 

 show the pole-center effects, which probably are of influence in the 

 determination of the wave-lengths of the elements in international 

 units now in progress in various laboratories. A direct comparison of 

 pole and center offers the speediest way of detecting the questionable 

 lines, and such an examination will be made of elements in which the 

 work of the Observatory is particularly concerned. 



INVESTIGATION OF THE ZEEMAN EFFECT. 



The work upon the Zeeman effect accomplished by Mr. Babcock 

 includes the collection of measures for large numbers of lines for many 

 elements, and the determination of the absolute separations for certain 

 selected lines and the evaluation of ejm. 



Data from the measures on a total of 28 plates are available for 903 

 chromium lines between X 2394 and X 6479, measured on two or more 

 plates; 769 iron lines between X 3440 and X 6678, mostly measured on 

 more than one plate ; 643 vanadium lines between X 2978 and X 6625 ; 

 and a few lines belonging to titanium, manganese, zinc, cadmium, and 

 barium. The field-strength used has generally been near 30,000 

 gausses, but some plates have been taken with 4,000 gausses. The 

 results have been summarized, and their reduction to a conmion field- 

 strength and comparative studies are in progress. 



For the standardization of our values of the Zeeman separation, the 

 following method of measurement of field-strength has been adopted: 

 A minute coil of very fine wire, wound upon a spool of ivory or hard 

 rubber, is connected in series with the secondary of a standard mutual 

 inductance, a 10,000-ohm resistance, and a sensitive ballistic galva- 

 nometer of long period. The coil is inserted into the field, the galva- 

 nometer brought to rest, and the coil suddenly removed to a consid- 

 erable distance from the field. After reading the deflection, the coil is 

 inverted, and the process repeated, giving a deflection of opposite sign. 

 The double deflection obtained by combining the two readings is then 

 evaluated by reversing known currents in the primary of the mutual 

 inductance. 



