260 CARNEGIE INSTITUTION OF WASHINGTON. 



will be supplemented by other plates as rapidly as possible. It is not 

 planned at present, however, to extend our system beyond X8500, on 

 account of the extremely long exposures required beyond that point 

 with the present photographic appliances. 



In addition to the interferometer measurements, the grating has 

 been used direct for the determination of standards of wave-length by 

 Mr. St. John, the regions covered being the same. Owing to the higher 

 dispersion in the grating method, the number of lines obtained with it is 

 much larger. The agreement is in general so satisfactory that obvious 

 errors may be eliminated. Both series of measures are being prepared 

 for publication. 



OTHER OBSERVATIONS OF WAVE-LENGTH. 



In addition to the work noted above on standards of wave-length, 

 considerable attention has been given by Messrs. St. John and Babcock 

 to the determination of certain other wave-lengths for special purposes. 

 As noted in last year's report, a study had already been made of the 

 supposed variability of the absorption lines of oxygen present in the 

 earth's atmosphere, with entirely negative results. During the year 

 the wave-lengths of many of the lines in the a group have been measured 

 in terms of the secondary standards of wave-length, both with gratings 

 and with the interferometer. The observ^ations are being made in 

 Pasadena as well as on Mount Wilson, using both types of instrument 

 at each place. It is hoped that when this work is completed we shall 

 have available a group of lines whose wave-lengths can be relied upon 

 with certamty to the thousandth of an angstrom unit. They will 

 prove of extreme value in many observations on the solar spectrum, 

 because they will obviate the necessity of securing a reliable compari- 

 son spectrum. The work is being extended to include high-precision 

 measurements upon the B-group of oxygen lines and also upon certain 

 lines due to v/ater-vapor. 



On account of their extreme sharpness, absorption lines in the spec- 

 trum of iodine-vapor are of special importance in precision spectro- 

 scopy. The question of transferring the primary standard from the 

 red line of cadmium to one of these iodine lines is receiving serious 

 consideration at present, and furthermore, selected lines of accurately 

 known wave-length may serve as highly valuable fiducial points in the 

 spectnim, since they may be photographed simultaneously with another 

 spectrum. The study of the iodine spectrum has accordingly been 

 continued at intervals during the year, special attention being given to 

 lines which coincide with metallic arc hues. Thirty-one of these pairs, 

 which are as nearly monochromatic as any known spectral lines, are 

 under observation. For more than half of them it is thought that the 

 incomplete observations now at hand establish their absolute wave- 

 lengths with an uncertainty of 1 part in 5,000,000, and it is hoped that 

 when the program is finished this accuracy will be exceeded for most of 

 the lines. 



