MOUNT WILSON SOLAR OBSERVATORY. I99 



The principal objects hitherto photographed are : 



(i) The North Polar Sequence to the fifteenth magnitude. 



(2) The two regions right-ascension 13*^ 50"^, declination +42°, and 

 right-ascension 21^ 15*", declination +41°, where stars down to the thir- 

 teenth magnitude, with well-determined proper-motions, are available. 



(3) The cluster x and h Persei. 



(4) Faint stars having large parallaxes. 



(5) Regions in the Milky Way showing great differences in star density. 

 Most of the plates have been taken with a i-meter diaphragm, to increase 



the diameter of the measurable field. The size of the field can only be de- 

 termined from actual measures of the effective wave-lengths, but it appears 

 to be about i ° when the aperture is reduced to i meter. 



For stars of magnitude 13.5 to 15.0 the mean error of the effective wave- 

 length from a single pair of spectra is 29 A. From the mean of two plates 

 it is 20 A., while the difference in effective wave-length between stars of 

 types A and K ( Vega and Arcturus) is about 200 A. 



Special attention will be paid to regions in the Milky Way where the star 

 density changes suddenly, with a view to detecting any differences in color 

 such as might be caused by the absorption of light in space. 



PHYSICAL LABORATORY. 



Investigations made in the physical laboratory in Pasadena during the 

 year have been on electric-furnace spectra by Mr. King, and on special 

 problems, dealing for the most part with magnetic phenomena connected 

 with the solar work, conducted by Mr. Babcock. 



The effect of pressure upon electric-furnace spectra, partially summar- 

 ized in last year's report, was observed by Mr. King for groups of lines 

 favorable for measurement at pressures ranging from i to 24 atmospheres, 

 a collection of over loo plates being obtained. The proportionality of dis- 

 placement to pressure appeared to hold closely. The displacements given 

 by the furnace, when compared with such measurements for the pressure 

 arc as were available, gave consistently larger values for equal pressures. 

 The difference for some groups of lines appeared greater than for others in 

 the same spectrum, but as a rule the different character of the lines in fur- 

 nace and arc made this relative difference uncertain as to its reality. Varia- 

 tions in the furnace conditions by the use of different temperatures, large 

 and small quantities of radiating vapor, the presence of foreign vapors, 

 compressed air instead of carbon dioxide, long and short columns of vapor, 

 failed to show any distinct effect on the displacements. In the publication 

 of the results it was pointed out that a fundamental difference exists in the 

 structure of arc and furnace lines in general, with which their different 

 susceptibility to pressure displacement may be connected, though the nature 

 of this connection is obscure. The results of the investigation leave little 

 room for doubt that a source of light such as the furnace gives displace- 



