MOUNT WILSON SOLAR OBSERVATORY. 229 



devised which has worked well, and during the year some 100 or more 

 spectrograms of iron, chromium, nickel, vanadium, titanium, and man- 

 ganese have been secured. A general survey of the electric effect for 

 chromium, nickel, and iron has been made with the following results: 



1. The majority of the arc and spark lines shows no appreciable 

 electric displacement or decomposition. 



2. A few hazy arc lines of Cr and Ni are affected by the electric field, 

 showing either displacement or decomposition. 



3. The affected lines are relatively very intense in the field, especially 

 after the cathode has been used for a few hours without coming in con- 

 tact with air at pressures above a few millimeters. 



4. In all, 74 affected lines ascribed to Cr, 8 to Fe, and about 12 to 

 Xi have been recorded. 



5. The relative intensity of all the lines in the electric field differs 

 from that of the arc, spark, or furnace, and will no doubt be an interest- 

 ing subject for future work. 



6. A few of the affected Cr lines appear in the sun, and their study 

 will make possible a determination of the upper limit of intensity of 

 possible electric fields existing at this level in the solar atmosphere. 

 Prehminary measures of these lines in sun-spots show no appreciable 

 electric effect. 



DEVELOPMENT OF THE PFUND ARC FOR STANDARD PURPOSES. 



An extensive investigation by Mr. Babcock and Mr. St. John has 

 had for its purpose the development of an improved source of secondary 

 standards of wave-length, free from the disturbing influences of pole 

 effect. The interference apparatus recently installed has proved of 

 great service in this investigation. It possesses the important advan- 

 tages found in complete achromatism of the mirror system and in 

 comparative freedom from temperature disturbances resulting from 

 the use of f used-quartz plates and separators in the etalons. By the 

 use of large-scale interference rings and increased auxiliary dispersion 

 an anticipated gain in accuracy has been reached. The evidence col- 

 lected with this apparatus and with the 30-foot plane-grating spectro- 

 graph, confirmed by independent observations of sun-arc displace- 

 ments with the instruments on Mount Wilson, has estabhshed the 

 presence of pole effect in the international arc, the source adopted for 

 the production of secondary standards of wave-length. Even in the 

 center of this arc, displacements amounting on the average to 0.006 a 

 are found for unstable lines. It has also been shown that the center 

 of the 6 mm. 6-ampere Pfund arc, formerly thought to be free from 

 pole effect, is somewhat affected. Modified specifications have been 

 developed, however, by means of which the uncertainties due to pole 

 effect are eliminated. As a result, the unstable lines of iron may now 

 safety be employed in astrophysical investigations far more widely 



