MOUNT WILSON OBSERVATORY. 261 



The importance of work upon absorption lines of oxygen and 

 iodine will readily be recognized in connection with such problems as 

 the absolute determination of wave-lengths in the solar spectrum, 

 and, for example, in the search for indications of the Einstein effect. 

 Our aim is to supply four convergent lines of e\'idence bearing 

 upon this fundamental question, as follows : (1) simultaneous exposures 

 to the solar spectrum and that of the iron arc, using high-dispersion 

 plane-grating spectrographs; (2) direct deteraiinations of solar wave- 

 lengths with the interferometer, using the iron-arc standards; (3) di- 

 rect observations of solar wave-lengths in tenns of atmospheric lines 

 upon the same plates, using both grating and interferometer, no com- 

 parison spectrum being employed; (4) use of the iodine spectrum in 

 place of the atmospheric spectrum in (3). 



It is to be borne in mind that the accurate determination of solar 

 wave-lengths must be supplemented by detailed knowledge of the ter- 

 restrial values for the individual lines concerned before valid conclu- 

 sions can be reached regarding such a problem as the Einstein effect. 

 To this end a laboratory study of the spectra of certain elements which 

 are of special importance in the sun is in progress. The interferometer 

 is being used for measuring wave-lengths in these spectra in terms of 

 secondary standards from the iron spectrum. On account of the pos- 

 sibility of disturbing influences like the pole effect in the arc, it is neces- 

 sary to proceed with caution, appljdng tests for any possible variation 

 of the terrestrial wave-length which may be dependent upon the kind 

 or condition of the source employed. Up to the present about 25 lines 

 for barium and lanthanum have been measured on a number of inter- 

 ferometer plates, with 16 lines for strontium. A few observations have 

 been made on some of the lines of calcium and nickel in the red part of 

 the spectrum. 



PRESSURE EFFECT FOR ARC SPECTRA. 



An accurate knowledge of the effects of pressure on wave-lengths 

 of special lines will prove of great assistance in correctly interpreting 

 observations of the solar spectrum. Physicists and astronomers 

 who have studied the sun for evidence of the Einstein effect have 

 recognized this in selecting the cyanogen band at X3883, chosen because 

 it was supposed to have no pressure effect. A recent paper announces, 

 however, that this band has an appreciable negative pressure-displace- 

 ment, so that conclusions based upon its wave-length in the sun must 

 be suspended until its pressure effect is finally determined. 



In the case of iron, the pressure-displacements obtained by Gale and 

 Adams for Unes of gi*oups a and h of the Mount Wilson classification 

 have been satisfactorily confirmed, but a further examination of the 

 sensitive lines, in which extreme care is taken to avoid the introduction 

 of pole effect, is now required. The means and time are now at hand 

 for effectively attacking these problems in our laboratory, and the ob- 

 servations are already in progress. The interferometer is being used 



