MOUNT WILSON SOLAR OBSERVATORY. 175 



bined with the powerful transformer spark, provides the means of getting 

 more definite data on the subject than has been possible with most other 

 apparatus. The experiments, so far as they have progressed, show that the 

 spark, with a proper adjustment of circuit conditions, gives in most cases 

 lines which would be measured as having a wave-length greater than the arc 

 lines, though the photographs also show to what extent unsymmetrical widen- 

 ing may explain the effect. The conditions requisite for this apparent differ- 

 ence in wave-length appear to be confined to the central part of the spark, 

 the outer portions giving lines coinciding with the arc lines. 



The instrumental equipment of the laboratory has received a number of 

 additions during the year, chief among which are the following: an 8-inch 

 plane grating by Michelson, which has proved very efficient when used in 

 the spectrograph of 30 feet focal-length ; a Rowland concave grating of 1 

 meter radius to be mounted for use in low-dispersion work; a Gaertner 

 comparator ; a D'Arsonval ballistic galvanometer ; standards of resistance, 

 electromotive force, capacity, and self-induction ; several voltmeters and am- 

 meters of different ranges ; a Gaede high-vacuum pump, and a Cox mercury 

 interrupter. 



A plate condenser having a capacity of about 0.06 microfarad and capable 

 of sustaining a i-inch spark has been built in the laboratory. 



The measurement of plates and the computations in connection with the 

 laboratory work have been carried on for the most part by Miss Griffin of 

 the Computing Division. 



THE COMPUTING DIVISION. 



The Computing Division has remained throughout the year under the 

 charge of Mr. Seares. 



Miss Burwell has been engaged in the measurement and reduction of sun- 

 spot spectra. The discussion of the material available for the catalogue of 

 sun-spot lines was practically finished at the time of the last report. During 

 the year she has devoted her attention to various phases of the Zeeman effect 

 in spot spectra. Some 60 photographs taken with and without a Nicol prism 

 have been examined for the purpose of analyzing the character of the lines 

 in the presence of the magnetic field, and such as are suitable for the purpose 

 have been measured for the determination of the strength of field. The 

 plates of various spots and spot groups have been examined for polarity. 

 Miss Burwell has also undertaken a detailed comparison of the lines of the 

 spectrum of Arcturus with those of the spot-spectrum, using the photographs 

 obtained with the 18-foot stellar spectrograph. 



Miss Griffin, since her appointment on February 1, has devoted the greater 

 part of her time to the measurement and reduction of laboratory spectrum 

 photographs, about 100 in all, relating to the Zeeman effect. She has also 

 made measures of brightness on several plates taken for photometric pur- 

 poses, and has determined the screw errors of three small comparators used 

 in the measurement of spectra. 



