244 CARNEGIE INSTITUTION OF WASHINGTON. 



limb-center shifts, which they formerly referred to aa increase in pres- 

 sure of 7 atmospheres in passing from the center to the limb and which, 

 if taken into account, would give displacements in excess of the Ein- 

 stein requirement. P6rot applies a large correction, 0.027a, for centri- 

 petal motion, and finds the difference between the wave-length at the 

 limh and the ate in vacuum to agree with the Einstein effect. Fabry 

 and Buisson apply no correction for radial movement of the solar 

 vapors and find the differences between the wave-lengths at the 

 center and the arc in vacuum to agree with the Einstein requirejnent, 

 but not the differences between the wave-lengths at the limb and 

 the arc. 



Owing to the different and eveti inconsistent corrections applied to 

 the observed sun-arc displacements, the resulting approximate agree- 

 ment with the deductions from the Einstein theory fails to carry con- 

 viction. In view of the situation in which this important question 

 now stands, it appears necessary, in order to reach a definitive 

 conclusion, to carry out an extensive program on sun-arc displace- 

 ments, including observations at center and limb and covering the 

 widest possible range in wave-length and line-intensity; to obtain, 

 in short, a reliable body of data as a basis for statistical discussions. 

 The problem must be envisaged as a whole and not in detached portions 

 and a consistent and probable role found for the gravitational effect 

 if the theory of relativity is to find confirmation in the displacement of 

 Fraunhofer lines. 



In addition to data for disentangling the causes involved in the dis- 

 placement of the solar lines, the progra^m includes a study of the rela- 

 tive consistency of the solar wave-lengths at the center a^nd limb, the 

 determination of a series of solar standards in the international sys- 

 tem, and observations on a limited spectral region in conmion with the 

 Kodaikanal Observatory. 



WAVE-LENGTHS OF LINES AT THE CENTER OF THE SUN. 



The determination of wave-lengths at the center of the sun in terms 

 of iron-arc standards has been continued by Messrs. St. John and 

 Babcock with the interferometer in conjunction with the Snow tele- 

 scope. Further experience in the use of this equipment has resulted 

 in such minor improvements and changes of technique as the limita- 

 tions of the instruments have required, and has confirmed previous 

 judgment of its usefulness. Observations covering the range X4000 

 to X6500 have been made at intervals throughout the year. The 

 reduction of the plates, though incomplete, confirms as heretofore the 

 values of the solar wave-lengths found with grating spectrographs. 

 A list of lines has been carefully selected for measurement upon these 

 photographs, special reference being given to (1) elements of paramount 

 interest in the sun; (2) range of atomic weight; (3) individual line 

 intensity; (4) probable vertical distribution in the solar atmosphere; 



