256 CARNEGIE INSTITUTION OF WASHINGTON. 



effect, is being undertaken both with the interferometer and with the 

 grating. In fact, the value of the interferometer for studying small 

 variations of wave-length in general is appreciably increased by this 

 method of observation, inasmuch as the small uncertainties inherent 

 in the procedure involving alternate exposures are eliminated. 



NATURE OF POLE EFFECT. 



A beginning has been made upon the minute examination of the 

 structure of iron-arc lines which exhibit pole effect. For this purpose 

 visual observations are being made by Mr. Babcock with the 33-plate 

 Hilger echelon spectroscope upon two or three strong lines in the 

 yellow and green, which have been found to show pole effects of oppo- 

 site sign. When observed close to the negative pole of a Pfund arc 

 carrying about 6 amperes, the lines X5615 and X5383, groups d and 

 e, respectively, show evidence of structure. These observations are 

 being extended by photographs of very high dispersion, using gratings 

 of high resolving power, in the hope of deciding whether the structure 

 is due to incipient reversal or to true complexity of the spectral lines. 

 In this way additional information may be obtained as to the real 

 nature of pole effect. 



Mr. Merrill, who joined the Observatory staff in January, has made a 

 study of the pole effect in several metallic arcs, especially that of nickel. 

 Special attention was paid to the pole shift of lines known to be greatly 

 affected in an electric field. These results were used by Mr. Takamine 

 in his discussion of the Stark effect for nickel. 



OBSERVATIONS OF ARC, TUBE-ARC, AND SPARK SPECTRA. 



Observations of the relative intensities of lines in different portions 

 of the arc have been made by Mr. Merrill. Numerous photographs 

 were taken with the arc projected in such a way that its axis coincided 

 with the slit. The strengthening at the positive pole was the leading 

 feature observed. Grouping the lines according to the degree of 

 strengthening at the positive pole gave a classification which, with 

 certain interesting exceptions, runs parallel to the temperature classi- 

 fication developed during previous work at the Observatory. The 

 appearance of enhanced lines at the positive pole of the arc corresponds 

 with the furnace evidence that they indicate high temperature con- 

 ditions. These observations of the arc spectrum have resulted in the 

 classification of 541 lines of iron and 227 of cobalt according to the 

 degree of strengthening at the positive pole. Some material for the 

 nickel arc was also obtained. 



Observations of the infra-red spectrum of the tube-arc have been 

 made by Mr. King and Mr. Merrill. The carbon-spark lines are 

 strong in this source and much sharper than the spark, thus permitting 

 high accuracy of measurement; several new carbon-spark lines have 



