SOLAR OBSERVATORY. 143 



The more important conclusions derived from the investigation may be 

 summarized as follows : 



( 1 ) The period of rotation agrees closely with that of Duner as far as 



latitude 50°. In higher latitudes it is considerably shorter. It 

 does not agree with either of the determinations of Halm, but 

 coincides well with their mean. 



(2) Different lines give slightly different rotation periods. Of the 



elements investigated carbon and lanthanum give the longest 

 periods. At the equator this difference amounts to about 0.1° in 

 the daily angular motion. 



(3) No appreciable change in the rotation period has been found during 



the 14 months covered by the observations. 



(4) The photographic method gives results for individual lines of 



slightly higher accuracy than the visual, if we may judge from a 

 comparison of the probable errors. The greatest advantage it 

 possesses, however, is the opportunity it affords for the inclusion 

 of many more lines than would be possible with visual measures. 



The work will be continued with the more powerful apparatus of the tower 

 telescope, and it is expected that a higher degree of accuracy can be attained. 



Laboratory Investigations. — The electric-furnace investigations mentioned 

 in the last report were carried as far as the available means permitted. It 

 was found possible to obtain the radiation spectra of iron, manganese, and 

 other metals having comparatively low melting-points, with results in perfect 

 harmony with those previously derived from the study of the flame of the 

 arc. Titanium and vanadium, however, could not be volatilized within the 

 carbon tube of the furnace, nor could the temperature be raised sufficiently 

 to produce an iron spectrum corresponding with that of the core of the arc. 

 In the light of our previous experience, and with the further information 

 now available, it is expected that a much more efficient furnace can be 

 readily constructed. This is being designed for the quantitative investiga- 

 tion of the anomalous dispersion phenomena of the more refractory metals 

 and for experiments with various gases at high and low pressures. 



A large number of photographs of metkllic spectra, corresponding to the 

 core and the flame of the arc, were made by Mr. Gale and Mr. Olmsted. 

 These have been used by Mr. Adams in a study of the relative intensities of 

 the lines for comparison with sun-spot results. 



Mr. Olmsted has devoted much of his time to a photographic investiga- 

 tion of the relation between the nature of the discharge of an electric spark 

 and the relative intensities of its spectral lines. The work was done in the 

 Pasadena laboratory, where a 5-kilowatt transformer, giving as high as 66,000 

 volts, is supplied with a iio-volt alternating current from the Edison 

 Company's system. Mirrors were so mounted that beams from the spark 



