MOUNT WILSON SOLAR OBSERVATORY. 227 



of this last series which have been measured, the Hnes of the negative 

 pole showing shifts to red and violet according to their classification 

 under pressure by Gale and Adams, though the magnitudes of the 

 displacements plainly indicate the effect of some cause other than 

 pressure alone. Many lines, which are sharp and narrow in the center 

 of the arc, show great widening and lack of symmetry at the negative 

 pole, and nearly all lines show an increase in intensity there. In the 

 majority of cases this increase is probably about fourfold. This 

 last series includes at present 77 photographs, covering the spectrum 

 of iron from X 3500 to X 6500. It is to be extended to include com- 

 parisons between the positive pole and the center of the arc, together 

 with other features which may be found of interest. The series of 

 monochromatic images includes about 100 photographs covering the 

 region X 4500 to X 5500. The spectroheliograph was tried for this 

 work, but the arc could not be held steady enough to get good results. 

 During the year Mr. Babcock has also made a number of photo- 

 graphs of secondary standards in the iron arc for Mr. St. John, and 

 some comparison photographs of the silicon spark and the iron arc, 

 using for the latter the concave-grating spectrograph of 1 m. focal 

 length belonging to the laboratory. 



Zeeman Effect. 



Mr. Babcock has taken 153 photographs of spectra for the study 

 of the Zeeman effect during the year. About 25 of these were made 

 with short exposures for a determination of the magnetization of the 

 large magnet, and about 60 were obtained for the study of the three 

 strong chromium lines X 4254, X 4275, and X 4290. The remainder 

 of the plates were divided between iron, chromium, vanadium, and 

 nickel, which are now being measured. 



The wave-lengths of the three chromium lines in the arc, the spark, 

 and the spark in magnetic fields have been measured with results 

 shown herewith. The wave-lengths were , 



4254.340 

 4274.801 ! 

 4289.722 ' 



Mean of all 

 components 

 in magnetic 

 field of 28,500 



taken from impurity lines of chromium occur- 

 ring in the iron arc. An examination of | Arc. spark 

 these fines in a very intense magnetic field 

 led to the discovery that X 4254 has 21 

 components — apparently the largest number 

 hitherto found in the study of the Zeeman 

 effect. Under certain conditions three of 

 these components reverse, giving an appearance of 24 components. 

 In connection with the Director's investigation of the general 

 magnetic field of the sun much time has been devoted to observa- 

 tions, both visual and photographic, on certain weak lines found to 

 show displacements on the sun. The visual observations were made 

 with a concave grating of 1 m. focal length, mounted in the grating 

 stellar spectrograph of the 60-inch reflector. The most interesting 



