MOUNT WILSON OBSERVATORY. 243 



assumed downward movement in the solar atmosphere and for a nega- 

 tive pressure shitt of the cyanogen-band lines, the latter approxi- 

 mately equal in magnitude to the shift required by relativity, finds 

 that the sun-arc displacement is that predicted by the theory. On the 

 other hand. Dr. Birge, after applying a correction for an upward 

 movement and assuming no pressure shift for the band lines, finds, 

 from the published data for 2 lines in the X3883 band, lines which he 

 considers free from superposed lines of other series, that the sun-arc 

 displacement for these lines is approximately that required by the 

 relativity hypothesis. Though from the series \iew these lines are 

 suitable, their character in the solar spectrum is such that the measures 

 are unreliable. Grebe and Bachem assumed no radial movement of 

 the solar vapors and no pressure shift for the band lines, but applied a 

 correction for a supposed asymmetry of the arc-lines, and found 

 approximately the Einstein effect (Phys. Gesell. Verh. 21, 1919). 



Investigations on the lines of the cj^anogen band made in our labora- 

 tory by Mr. Babcock do not confirm the displacement to the red in 

 passing from atmospheric pressure to vacuum, as found by P^rot, 

 but indicate a normal displacement with pressure of perhaps 0.0001 a 

 per atmosphere. Mr. King finds that the lines of the cyanogen band 

 belonging to different series vary in relative intensity with change in 

 furnace temperature. In a recent article (Science, Apr. 15, 1921), 

 Dr. Birge calls attention to the extensive overlapping of lines of the 

 different series. In view of this superposition of lines, of the changes 

 in relative intensity with temperature, and of the line-density in the 

 solar spectrum, it appears that the cyanogen band is not well adapted 

 for a definitive test of the theory. 



Other Elements. 



From the magnesium liae X5172, P^rot (C. R., Apr. 25, 1921), 

 after correcting his observed shift between the arc and the sun's 

 center for a centripetal movement in the solar atmosphere of 1.57 km. 

 per second and reducing the arc wave-lengths to zero pressure, finds 

 a difference between sun and arc in approximate agreement with the 

 Einstein requirement. The lines of the mag lesium triplet in the green 

 are, however, subject to a marked pole-effect and three independent 

 investigations at Mount Wilson by Messrs. Adams, Joy, and St. 

 John, fail to show the centripetal motion of the solar vapors used in 

 Perot's reduction of his observations. Fabry and Buisson find (C. R., 

 Apr. 25, 1921) that the differences between the arc wave-lengths of 

 iron reduced to vacuum and the wave-lengths at the sun's center are 

 of the order of the Einstein effect. They conclude that the differences, 

 sun minus arc, are perfectly interpreted by assuming the Einstein 

 effect to be the sole cause of the displacement of the Fraunhofer lines. 

 They assume zero pressure in the solar atmosphere, but disregard the 



