l68 REPORTS ON INVESTIGATIONS AND PROJECTS. 



entire visible spectrum with the exception of a part of the violet. The 

 linear scale of the spectra at A 4300 is i mm. = 1.4 Angstrom. 



The results of an investigation of the spectra of Sirius, Procyon, and Arc- 

 turus are contained in a communication by Mr. Adams read at the Cam- 

 bridge meeting of the Astronomical and Astrophysical Society of America 

 in August 1910. The principal results found are as follows : 



(i) The enhanced lines in the spectra of Sirius and Procyon are displaced 

 toward the red relative to the arc lines. In the case of Sirius this displace- 

 ment amounts to 0.014 Angstrom, or 0.90 km. if measured as radial velocity. 

 In the case of Procyon, 0.009 Angstrom, or 0.58 km., as radial velocity. 

 The displacement of the enhanced lines in Arctunis is so small as to be 

 negligible. 



(2) If we may assume, as seems probable from the investigations of Mr. 

 Gale on the spectrum of the spark under pressure, that the enhanced lines 

 are shifted on an average 50 per cent more than the arc lines by pressure, 

 these results furnish us with the means of estimating the pressures in the 

 reversing layers of these two stars. For Sirius the pressure would be 12 

 atmospheres greater than in the sun's reversing layer, and in Procyon 7 

 atmospheres greater. These values are in harmony with our general knowl- 

 edge concerning the physical conditions of these stars. 



(3) In the case of Arcturus the lines of different elements are displaced 

 by different amounts, the lines of iron being displaced toward the red with 

 reference to the lines of all the other elements investigated. Hydrogen, 

 calcium, and magnesium show the largest differences from iron, and in gen- 

 eral the elements behave in almost exactly the same way that they do at the 

 sun's limb. 



(4) It seems probable from these results that Arcturus is a star with a 

 comparatively shallow reversing layer, and that within this layer the general 

 arrangement of the gases is similar to that in the sun. The high-level gases 

 accordingly would be subject to less pressure than those lying at a lower 

 level, and the lines of the latter would be displaced toward the red relative 

 to the lines of the higher elements. 



It has been necessary during the past year to use a 64° prism of compara- 

 tively small aperture with the 18-foot spectrograph. We have, however, 

 placed an order with European glass manufacturers for a block of glass 

 sufficiently large to provide a prism which will utilize the full beam of light 

 coming from the telescope. It will then be possible to investigate the spectra 

 of several additional stars with this powerful instrument. 



The low-dispersion spectrograph used at the principal focus of the 60- 

 inch reflector was planned originally for use with a multiple-slit device of 

 Mr. Hale's design with the object of obtaining the spectra of several stars 

 at the same time. A test of the short-focus lenses employed in the instru- 

 ment soon showed, however, that the curvature of the field was too great to 

 allow of this, and accordingly we have been using it during the past year as 

 an ordinary compound-slit spectrograph for photographing the spectra of 



