MOUNT WIIvSON SOLAR OBSERVATORY. 1 69 



Spectroscopic Observations oe the Rotation of the Sun. 

 In accordance with a plan adopted at the fourth conference of the Inter- 

 national Union for Cooperation in Solar Research, held on Mount Wilson in 

 September 1910, the various observers engaged in v^rorking upon the rotation 

 of the sun by spectrographic methods selected separate regions of the spec- 

 trum within which to carry on their investigation, at the same time uniting 

 upon one region common to all observers for the purpose of comparison of 

 results. The region selected in common extends from A 4200 to A 4300, and 

 the two special regions selected for work at Mount Wilson are from X 4500 

 to A 4700, and from A 5100 to A 5300. During the year Mr, Adams has ob- 

 tained a number of photographs in each of these regions, using the 60-foot 

 tower telescope and the 30-foot spectrograph. Two different gratings have 

 been employed with the latter instrument. The results of the measures of 

 these plates by Miss Lasby are almost identical with those obtained at Mount 

 Wilson in 1908 for the latitudes between 30° and the sun's pole. For the 

 equator and latitude 15° the average result from the three regions is about 

 I per cent less than that found in 1908. This is due in large measure to the 

 low value given by the least refrangible region of A 5100 to A 5300, and it 

 may perhaps be caused by the fact that the level of the lines measured in this 

 region is lower than that of the lines in the more refrangible regions and the 

 velocity given by them slightly less. The investigation will be continued with 

 the 1 50- foot tower telescope when the latter is available. 



Miscellaneous Investigations. 



The question of the existence of free oxygen in the solar atmosphere has 

 never received a definite answer, although a considerable amount of work 

 has been done upon it. Recently Mr. St. John took up the study of the 

 problem with the powerful 30-foot spectrograph and has been led to the 

 following results : 



(i) Of the two oxygen triplets in the principal series, upon whose pres- 

 ence or absence in the solar spectrum any definite conclusions must be based, 

 the violet triplet beginning at A 3947 does not appear to be present. 



(2) An infra-red triplet present in the solar spectrum, beginning at 

 A 7772, has been found by means of a comparison of the opposite limbs of 

 the sun to be of purely solar origin, the displacements shown being closely 

 those given by known solar lines ; but existing wave-length determinations of 

 the oxygen lines, however, are not sufficiently accurate to make certain their 

 identification with the solar lines. The wave-lengths of the oxygen lines will 

 be determined accurately by photographic methods as soon as the instru- 

 mental equipment for producing a sufficiently bright light source showing 

 the oxygen lines has been prepared. 



In the course of this work Mr. St. John has proven that the great a and B 

 and A oxygen bands present in the solar spectrum are of purely terrestrial 

 origin, and has made a determination of the wave-lengths of the lines present 

 in the a band to a high degree of precision on the international system. 



