MOUNT WILSON SOLAR OBSERVATORY. 157 



(9) The measurement of several photographs of the spectra of the oppo- 

 site edges of the sun indicates that no appreciable change has taken place in 

 the sun's period of rotation since 1908. 



(10) Direct photographs of certain of the star-clusters and spiral nebulae 

 show an immense amount of detail hitherto unknown. In particular the 

 spiral nebulae are found to contain great numbers of nebulous stars enveloped 

 in the curved streamers. 



(11) Spectra of several of the brighter stars have been obtained with the 

 1 8- foot spectrograph and the 60-inch reflector. The measurement of the 

 photographs indicates that the enhanced lines in the spectra of Sirius and 

 Procyon are shifted toward longer wave-lengths relative to the arc-lines. An 

 investigation of the spectrum of Arctiirus shows a relationship among the 

 displacements of the lines due to different elements similar to that found at 

 the sun's edge. The effect is probably due to pressure. 



(12) A low-dispersion spectrograph containing one prism has been used 

 at the primary focus of the 60-inch reflector since September 1909 for the 

 determination of radial velocities of stars and the classification of spectra. 

 With this instrument the spectrum of a star of the solar type of 8.0 visual 

 magnitude may be photographed in 70 minutes. 



(13) The spectra of four spiral nebulas and of nine star clusters have been 

 photographed with a low-dispersion spectrograph. The spectrum of all of 

 the spiral nebulas investigated is approximately of the G type, and that of the 

 globular star clusters of the F type. 



(14) Experiments with diaphragms of varying aperture on the 60-inch 

 reflector have indicated that the effects of diffraction are essentially negli- 

 gible in their influence on the brightness of the star images. In view of the 

 results obtained it seems reasonably probable that by this simple means an 

 absolute scale of photographic stellar magnitudes may be obtained which will 

 be satisfactory within a wide range of brightness. 



(15) A statistical study of the motions of certain stars of the Orion type 

 of spectrum points to the existence of systematic motion on the part of two 

 large groups of stars situated in Scorpius and Perseus. The motions are in 

 opposite directions, and the groups probably form part of two main stellar 

 streams. 



(16) An extensive laboratory investigation of the Zeeman effect for iron, 

 chromium, and titanium, and comparison with pressure displacements, has 

 shown that no direct quantitative relationship appears to exist between the 

 amount of separation and the pressure-shift. 



(17) The Zeeman effect for vanadium and nickel has also been investigated 

 for purposes of comparison with sun-spot spectra, and the lines have been 

 classified according to their type of separation in the magnetic field. 



(18) Photographs of the spectrum of the electric spark under pressure 

 have shown that the enhanced lines remain bright under a pressure at which 

 the great majority of the remaining lines are reversed. An application has 

 been made of this result to the spectrum of the solar chromosphere. 



