MOUNT AVILSON SOLAR OBSERVATORY. 279 



factor in producing the pole-effect. In attempting to make a compari- 

 son of the magnitude of the pole-effect with that of the pressure dis- 

 placements for the same lines it was found necessary to obtain new 

 values of the pressure effect which should be free from pole-effect. 

 This was carried out by means of the vacuum arc and the arc at 

 normal pressure, using both the grating and the interferometer, and 

 taking hght only from the center of the arc. The results show clearly 

 that pressure alone is insufficient to account for the pole-effect, 

 although it may play some part in its production. 



Several parts of this investigation are still in progress, namely, 

 extension of our study of the pressure effect, energy distribution in 

 the arc for d Unes, examination of velocities of vapors in the arc, 

 attempts to develop a type of arc better suited for use as a standard 

 source, tests of spectra of other elements for pole-effect, etc. 



STANDARDS OF WAVE-LENGTH. 



Improvements in both the secondary and tertiary standards of wave- 

 length are being sought by Mr. St. John and Mr. Babcock with the aid 

 of the interferometer and the plane grating. A number of such spectra 

 have been taken both for iron and also for standardizing certain lines 

 in one of the ultra-violet carbon flutings. Reductions are in progress 

 upon this material. Means are now available for wave-length com- 

 parisons of high precision by two independent methods, namely, a 

 plane-grating spectrograph of great resolving power provided with 

 devices for simultaneous exposure to two sources, and quartz invar 

 interferometers of the highest rehability. 



THE ZEEMAN EFFECT. 



Mr. Babcock's work upon the Zeeman effect has consisted mainly in 

 the collection of additional data for iron, chromium, and vanadium. To 

 this end 25 new photographs have been taken, most of which have been 

 measured. 1,132 lines of iron, 1,089 of chromium, and 1,361 of vana- 

 dium have now been reduced to a standard field and tabulated, the 

 increase during the year being 363, 186, and 718 Unes for these metals 

 respectively. On all but the weakest Unes in these spectra two or more 

 values are now available, generally in excellent agreement. The 

 spectral ranges covered are X2325 to X6678 for iron, X2307 to X6978 

 for chromium, and X2977 to X6625 for vanadium, only a few small 

 gaps remaining which will require additional plates. 



REFLECTING POWER OF STELLITE. 



A comparison has been made by Mr. Babcock of the reflecting powers 

 of stellite and speculum metal over the region XX 3200-5800 by means 

 of the 1-meter radius concave grating. Photographic densities were 

 compared by means of the Hartmann microphotometer. The measures 

 show distinctly higher reflecting power for stellite over the whole 

 range of wave-length. 



