( 140 ) 



times, when passing nearly parallel to the system of the levels of 

 equal density (in the manner described on a former occasion ^)) and 

 will therefore have a greater chance of reaching the slit S„ than 

 rays which are less strongly curved. The relative intensity with 

 which the waves, belonging to those central parts of the dispersion 

 bands, appear in the spectrum increases with the distance over which 

 the light has travelled along such a lamellar or tubular structure. 

 Should the true absorption line happen to be exceedingly narrow, 

 the dispersion band may give the impression of a double absorption 

 band, which need not be symmetrical "). 



We hold that the dispersion bands play an important part in many 

 of the well known spectral phenomena, such as the widening, shifting, 

 reversal and doubling of lines. In a subsequent communication I 

 purpose to examine from this premise various phenomena observed 

 in the spectra of variable stars and other celestial bodies. 



Physics. — " Spectrolieliographic results explained by anomalous 

 dispersion.'" By Prof. W, H. Julius. 



It is not surprising that the scientific world should be highly interested 

 in the beautiful results, obtained by Hale and Ellerman with the 

 spectroheliograpli ^). The brilliant method elaborated and applied by 

 these investigators enables us to see at a glance as well as to 

 study in minute details how tlie light of any selected wavelength 

 was distributed on the total solar disk at any given moment. W. S. 

 LocKYER, in giving an abstract from the paper here alluded to in 

 Nature N°. 1800, rightly entitles it : "A now epoch in solar physics." 

 Indeed, the spectroheliograpli proves capable of providing us with an 

 abundance of new information, which other existing methods could 

 never give and the value of which will remain, whatever may 

 be the ideas on the Sun's constitution derived from it. 



But, nevertheless, even the most splendid collection of new facts 

 is useless so long as we have no theoretical ideas connecting them 

 with achieved knowledge. Hale and Ellerman, accordingly, in 



1) Proc. Roy. Acad. Amst. IV, p. 596. 



2) In Fig. 4 on the plate is given an enlargement of one of the photographs 

 obtained by an almost symmetrical position of the flame, it has been somewhat 

 spoiled in the reproduction. The original is less blotchy and the transition of the 

 dispersion bands to the bright background of the spectrum is there much more 

 gradual. 



3) G, E. Hale and F. Ellerman, "Tiie Rumford Spectrolieliograph oftheYerkes 

 Observatory," Publications of the Yerkes Observatory, Vol. III. Part. I, (1903). 



