228 CARNEGIE INSTITUTION OF WASHINGTON. 



and low altitude plates were seciu'ed with an average difference in 

 altitude of 13 ?4. Of these, 10 showed shorter wave-lengths for the 

 lower altitude, 3 gave no difference, and 5 gave longer wave-lengths. 

 The mean difference in wave-length was 0.0014 ±0.0008 a. Since 

 there was no change in the angle Venus-sun-earth between these 

 plates there must be an effect, in some way due to the low altitude 

 of the planet, which shortens the observed wave-lengths for Venus. 

 As the angle Venus-sun-earth changed, Venus was observed at differ- 

 ent altitudes, so that the variation in wave-length which was found 

 may be correlated with changes in either of these angles, but the low- 

 altitude effect mentioned above indicates that the altitude of the 

 planet is the vital factor in the observations. 



SOLAR WAVE-LENGTHS. 



It has long been recognized that the wave-lengths of Rowland's 

 Preliminary Table of Solar Spectrum Wave-Lengths, owing to an error 

 in his primary standard, do not represent absolute values in the C. G. S. 

 system and that the errors in the relative wave-lengths due to the 

 method of coincidence used in passing from his primary standard are 

 roughly periodic. It was the opinion of the solar physicists at the 

 Brussels meeting of the International Astronomical Union in 1919 

 that the time had arrived when consideration should be given to the 

 preparation of a table of solar wave-lengths based upon the inter- 

 national system. It was with that end in view that the chairmanship 

 of the International Commission of Standards of Wave-Lengths was 

 assigned to Mr. St. John, of the Mount Wilson Observatory. 



The first step is the determination of the wave-lengths of a series of 

 solar standards in the international system. For some time this work 

 has been under way at Mount Wilson through comparison of simulta- 

 neous spectrograms of the sun and the iron arc. Some preliminary 

 work on the application of the interferometer to solar observations was 

 carried on by Mr. Babcock last summer. The results showed that the 

 designs of the instrumental equipment were suitable to the purpose 

 and that the accuracy obtainable was of the same order as in the grating 

 method. 



A program whose immediate purpose is the precision measurement 

 of the wave-lengths of a series of solar standards and of lines whose be- 

 havior in the solar atmosphere is important in other observations is 

 in progress. The comparison of the double series, by grating and in- 

 terferometer, will furnish, it is hoped, a criterion of the reliability of the 

 Mount Wilson determination. The program includes, further, an in- 

 vestigation of the degree of stabihty of the wave-lengths of the solar 

 lines at the sun's center and other related regions, also a comparative 

 study of the wave-lengths, at the elevations of Pasadena (800 feet) 

 and of Mount Wilson (5,650 feet), of the atmospheric lines due re- 



