MOUNT WILSON SOLAR OBSERVATORY. 231 



on account of the greater number of lines suitable for measurement, 

 but also because of the additional plates which must be taken. Many 

 of the poorest lines in the iron spectrum can be measured with high 

 precision when free from pole effect. The methods of reduction have 

 been simplified as far as is consistent with accuracy. Aside from the 

 adjustment of the etalon, no observations are required except the 

 measurement of two or three diameters for each line. 



A study of the relative values of the wave-lengths of the stable 

 secondary standards has led to the use of these instead of the red cad- 

 mium line. There is no doubt that the published wave-lengths of the 

 stable lines are accurately corrected for phase change and nothing is 

 to be gained by redetermining them, while the saving of labor is very 

 great. We are not precluded from introducing our own values of the 

 relative wave-lengths when these differ by small amounts from those 

 published. 



In the measurement and reduction of the numerous photographs 

 required for the interferometer investigations, much assistance has been 

 given by Miss Margherita Burns, who has devoted all her time to the 

 work, and, during the latter part of the year, by Miss Braj'ton. 



WAVE-LENGTHS OF IODINE ABSORPTION LINES. 



In addition to the determination of wave-lengths of iron lines by 

 the interference method, 20 absorption lines of iodine have also been 

 measured by Mr. Babcock with high orders of interference in terms of 

 selected iron standards. The iodine Unes are among those which lie 

 superposed upon iron lines and are valuable as reference marks in the 

 spectrum. 



WAVE-LENGTHS OF SPARK LINES. 



A few interferometer plates have been reduced by Mr. Babcock for 

 the puipose of comparing wave-lengths of iron lines in arc and spark. 

 Lines of groups a and b are found to have the same wave-length in 

 both sources, while those of group c^ have slightly greater wave-lengths 

 in the spark. Groups Cr, and d are shifted to the red about 0.007 A in 

 the spark and group e to the ^dolet about 0.008 A. 



PRESSURE EFFECT FOR IRON. 



A redetermination of the pressure effect for iron has been undertaken 

 by Mr. Babcock, in view of the discovery that pole effect has played a 

 part in most previous work on this subject. Observations are made 

 in vacuo, at atmospheric pressure, and at a pressure of two atmospheres, 

 with precautions to avoid the introduction of pole effect. The inter- 

 ference apparatus, admirably adapted for this work, will be 

 mented by the 30-foot spectrograph. Although the data at hand are 

 meager, it may be said that the values for the stable lines of iron are 

 in good agreement with former determinations, but for the sensitive 

 Unes the displacements appear to be somewhat reduced. 



