( 279 ) 



\\Iiicli does nol allrct llic ai'iiiiiiu'iils of our fonuci- coiicliisioiis. 

 When llie li,U"Iit (ra\'cls llii-oiiuii lliis stiicliii'c aliiiosi parallel to llic 

 siii-faccs of (liscoiiliiiiiilx , llie mosi sti-oiiulv rdVartcd ravs follow 

 undulaliii'^- paths (vide Pi-oc. Kov. Acad. Anisl. 1\'. p. 5i)() 597). 

 Tlipy are kept tog'ether, ai'e so to spc^k uuided al(Miu the lamellar 

 striR'tiire, and their iiileiisitv when reaehiiiu' the Ivu'th is greater I hail 

 that of rays which imderiio a lesser refraction. ('onse(pienlly in the 

 Fkalxhoi-'ER lines which show a broad background (bein.ü' jjrodiiced 

 by elements which are rather strongly represented in the coronal gases 

 and which tiierefore, even during the minima of s[)ot periods, gi\e rise to 

 marked anomalons dispersion) the parts nearest to the true ahsor[)tion 

 line will at periods of spot maxima a[)pear brighter. In fact, the 

 greater ray-curving, whicii marks these periods, eidiances the shadowy 

 background, but at the same time it restores the light to the central 

 parts of the band. Thence the im}»ression that the absor[)tion line 

 has been vveakejied. 



On the other hand, lines with a faint, narrow backgrouiid will 

 at times of maxima appear considerably strengthened, because their 

 brighter central parts, if present, are too narrow to be \isible. 



The explanations here given are su[)})orted by the residts of an 

 experimental investigation, a detailed account of which will appear 

 elsewhere. Our object was to ascertain the action of a system of 

 artiticial surfaces of discojitinnity on the absor|»tion specti'um of sodium 

 vaiHHir. In [)rinciple the a])|>aratus is similar to that described in my 

 paj)er : "On maxima and minima of intensity sometimes observed 

 within the shading of sli'ongly widened spectral lines')." A beam of 

 electric light, whicli had lii'sl been |»assed through a loiiu' sodium 

 flame \vas directed on the slit of a big grating s|)ectr()scope. Hut a 

 great improvement had been since made in the Iturner. The a|)ei'ture 

 is 75 c.M. lo]ig and 0.15 c.M. w ide : the supply of an adjustable 

 mixtui-e of gas and air is so regulated that the llame bin'iis e\-euly 

 over its whole length. A sj)ecial conlrixance within the bin-ner 

 allows of the feeding (»f the llame with sodium \a|toui' during the 

 experiment and regulating the (piantily as re(piired. 



l)y means of this iiislrument we tested the eU'ect on the alisorp- 

 tion spectrum of the \arialions in tlie angle of inrlinalion between 

 the planer of discontinuity ajid the beam of light ; of modidca- 

 tions in the (piantity of sodium vaponi-; of diaphi-agms on the 

 path of the rays, etc. .\11 the phenomena obserxed may b(> explained 



1) W. H. Julius, Ptoc. Roy. Acad. Amst. V, p. 11G5. 



19' 



