MOUNT WILSON OBSERVATORY. 223 



range X2600 to X6600. The spectra show great wealth of detail and 

 indicate that all of the absorption lines are displaced toward the violet 

 because of the motion of the absorbing vapor toward the observer. 

 This displacement is, in general, greater for high-temperature than for 

 low-temperature lines, and for this and other reasons it is evident that 

 the special form of condenser needed for the development and exten- 

 sion of this work should be provided as soon as possible (p. 291). The 

 results of the work of Messrs. Anderson and Smith on the discharge 

 of the present large condenser through vacuum-tubes are also very 

 promising (p. 291). 



The publication of the wave-lengths of 1,026 iron-arc lines by Messrs. 

 St. John and Babcock marks an important advance in high-precision 

 spectroscopy. Instrumental errors have been largely eliminated by 

 the comparison of photographs taken wdth five different gratings and 

 with four pairs of interferometer plates, and the agreement indicates 

 that for most of the lines the mean wave-length is accurate to 0.001 a. 

 The results show the serious displacement produced in the standard 

 arc by pole effect, and the consequent necessity of taking exceptional 

 precautions in the comparison of solar and terrestrial wave-lengths 

 (p. 289). 



How essential such considerations are is well illustrated by the re- 

 cent history of the attempts of various spectroscopists to detect the 

 shift of solar lines predicted by Einstein. Several investigators, after 

 making arbitrary assumptions and applying various corrections for 

 pressure shift and motion in the solar atmosphere, have each con- 

 firmed Einstein's value. As a matter of fact, no final conclusion 

 seems to be warranted that is not based upon an exhaustive study 

 of pressure shifts, variations with temperature, pole effect, motions 

 in the solar atmosphere, and other phenomena under investigation 

 by Messrs. St. John, Babcock, King, and other members of the 

 Observatory staff (p. 242). Of prime importance in this work is the 

 series of interferometer observations of the wave-lengths of lines at 

 the center of the sun, made by Messrs. Babcock and St. John with 

 the Snow telescope. The lines selected for measurement cover a 

 range of 2,500 a, and are chosen with reference to elements of 

 chief interest in the sun, atomic weight, line intensity, probable level 

 in the solar atmosphere, absence of close companions, etc. (p. 245). 

 In conjunction with this investigation Mr. Babcock's interferometer 

 study of the pressure displacements of selected iron lines under a small 

 range of pressure is of great importance. A differential method of 

 observation and the precautions taken to eliminate pole effect have led 

 to results of the highest precision (p. 292). 



Several of the most important additions to the equipment of our 

 Pasadena laboratory have already been mentioned. Messrs. Nicholson 

 and Pettit have also made an extended investigation of various forms 

 of thermo-couples for laboratory and astrophysical uses. The most sue- 



