DEPARTMENT OF MERIDIAN ASTROMETRY. 221 



It is evident from table 5 that there is a distinct diurnal term. 

 The refraction, which is well represented by the Pulkova tables in the 

 afternoon, decreases rapidly about sunset, continues to decrease during 

 the night, and about sunrise it begins to increase again. About an 

 hour after sunrise the increase becomes rapid. The observations 

 furnish no data as to the refraction at noon. 



ANOMALOUS DISPERSION IN THE SUN. 



The investigation described in this abstract constitutes an extension 

 of a piece of work completed by Dr. Albrecht in the preceding year, 

 for which the results were published in the Astrophysical Journal (vol. 

 41, p. 333, 1915). In the first article it was shown that iron lines with 

 close companions in the solar spectrum (Rowland's Preliminary Table 

 of Solar Spectrum Wave-Lengths) are displaced relatively to their posi- 

 tions in the arc spectrum; v/hen the companion is to the violet the 

 displacement is toward the red, and when the companion is to the red 

 the displacement is toward the violet; in the former case the displace- 

 ment is only two-thirds as great as in the latter, and in both cases it 

 diminishes progressively with increasing separation of the two lines. 

 These observed facts are strikingly in accord Vv'ith the requirements of 

 the anomalous-dispersion theory of Julius. Personahty in the measure- 

 ment of close pairs of lines was also suggested as a possible cause for the 

 observed facts. However, it would be difficult to explain, on this ground, 

 the observed inequality of the displacements for the two components. 



In the present investigation a marked distinction was found to exist 

 between pairs of lines in which both components are due to iron and 

 those in which one of the com.ponents is due to some other element. 

 For lines with companion to the red the mean deviation is +.01151 

 (S wt. 27.5) in the cases where the companion is not due to iron, and 

 only -f .0062 A (S wt. 18) when the companion is due to iron. For 

 companion to the violet the corresponding quantities are respectively 

 -.0077 a (S wt. 53) and -.00401 (S wt. 27). That is, for com- 

 panion to the red, as well as for companion to the violet, the displace- 

 ment is only one-half as great when both components of a close pair 

 are due to iron as when one of the components is due to some other 

 element. As above, so also in each of these subdivisions, the displace- 

 ment for companion to the violet is only two-thirds as great as for 

 companion to the red. On the anomalous-dispersion theory the 

 observed smaller displacement for pairs of lines in which both com- 

 ponents are due to iron is explained on the basis that the components 

 of these pairs represent only in part physical connection in the mole- 

 cule and in part entirely independent vibrations. These observed facts 

 are also in accord with the anomalous-dispersion theory as modified 

 by a recent suggestion of Sir Joseph Larmor. 



For the fines with iron companions an attempt was made to deter- 

 mine a relation between displacement and difference in solar level 



