( i>()2 ) 



]Most likelv Hale's almormal spectnini lias sliowii ns a oase, where 

 lliese seeming: eiiiissio]i])aiHls acquired au uiicominoii extent. We may 

 thei-efore expect tiiat a systematical iiivesti<iatioii of solar spectra, 

 Ijhotógraplied at dilfereiit limes, uill atlord all kinds of intermediate 

 cases. 



It would he desirable, for the moments when the photographs 

 are taken. I o know form and position of the coronal streamers ex- 

 tending toward the Eaitli. At all events the actual jtliase ofthesun- 

 sj)ot period, with which the shape of the corona seems to be con- 

 nected, should he taken into consideration; and perhaps the simul- 

 taneous ol)servati(tn of the |)hotos|)heric reticulation, discovered by 

 Janssen, may jirocure some e\ idence concerning' the position of coi-o- 

 nal streamers, and thus conti-ibute to our know ledge of their iidluence 

 on the Fi'auidiofcr specti-uiu. 



Mineralogy. — "iht tlw n'/ntrtlve inde.r of rork-ijhtssos," by P. Tesch : 

 (Communicated by Prof. J. L. ('. S(hroedek van der Kolk). 



Of the group of the igneous I'ocks, the origin of which out of 

 tluid red-hot condition we accept, the volcanic rocks con.stitnte that 

 subdivision, which includes the rocks, that as lavas have broken 

 through the surface of the earth. 



The quick cooling at the atmosphere renders it possible that in 

 these rocks part of the magma congeals amorphously, so that next 

 to the minerals a rockglass appears, wiiich constitutes either an infe- 

 rior part or a prevailing one of the rocks. So in general this glass 



Let us consider a beam of light of an exactly defined wavelength belonging to 

 the shaded background of an absorption line. This beam leaves the deeper layers 

 of the Sun witli a certain divergence. As it passes along a "tube" of the corona, 

 its divergence will alternately diminish and increase, and on reaching the Earth 

 it shows in tlie spectrum an intensity, depending on the divergence (or perhaps 

 convergence) with which it has left tlie last traces of the corona. For a beam of 

 light whose wavelength is only slightly nearer to that of the absorption line, the 

 medium will have a considerably greater refraction constant, so that the rays of 

 this beam, on their way through the corona, may make part of a bend more than 

 the former ones. Tlie beam may therefore arrive with a quite different degree of 

 divergence and, consequently, of intensity. Thus, proceeding towards the absorption 

 Hne from eilfier side, we easily see that we must meet with a periodically changing 

 intensity. Rays, corresponding to the middle of one of the so formed fringes, will 

 have made one full bend more or less than the rays, belonging to the middle of 

 the next fringe. 



If this interpretation be correct, the width and the number of fringes visible 

 must prove to be variable. As far as I know, the observations made on this point 

 are not numerous. }\\x\ the proposed views serve to further the investigation of 

 this interesting phenomenon. 



