( 280 ) 



fro 11 the diifereiit measure in wliifh llie anomalously disj)ersed rays 

 are curved; and the varying peculiarities of the sun-spot spectrum are 

 easily reproduced in this ex[)erinient. We make particular meutiou 

 of the fact that, when the tlame is parallel to the beam of light, a 

 small quantity of sodium \apour produced very dark enhanced D 

 lines, e. g. 0.5 or 1 Angstr. units wide; by adding more of this 

 vapour the lines extended into \ cry wide bands in which the central 

 portions became gradually l)righter, leaving only a narrow, well- 

 defined central absorption line. 



III. Tlll'^ PEIUODK AT, VAKl.VTIONS IN THE APPEARANCE OF THE SUN. 



J. St(NsiKi/s and factiliic. 



In a conuniniicalion dalrd l'\'itr. 11)00^) I started the hypothesis 

 that sun-spots are the residts of refraction, nu)re especially of ano- 

 malous dis|iersiin. Siiu'c then Krekt") has |)nblislied an e.\[)erimont 

 in which, through the dispersion of" the light of an arc lamp in a 

 thvme of bm'niiig sochum, effects were j)rodnced, closely resend>liiig 

 the i)henomena obserxed in sun-spots, sncii as their spectral p(H-uliai'- 

 ities, revei'sals, displacenuMits, ramification of the lines, etc. 



1 have recently re|)eal(Ml this experiment, but instead of using 

 pieces of burning sodium, I entployed the long flame, which afforded 

 greater facility for controlling the operation and noting the phenomena. 

 They were substantially the same as those observed by Ebeht. 



Moreover, the use of the long tlame enabled us to make some 

 observations with respect to the optical effects of almost flat surfaces 

 of discontinuity. Similar surfaces being important factors in our 

 tlieorv, a short description of our experiment may be found useful. 



The light of an arc lamp was concentrated on a diaphragm 15 

 m.M. in diameter, which was placed almost in the focus of a second 

 lens. The emergent rays were slightly (Hvei-geut; within this beam, 

 at a distance of '20 M., a telescope was placed and focussed on the lens. 

 By pushing out the eye-piece an enlarged image of the lens was 

 projected on a screen: this represented the Sun. Between the lens 

 and telescope, l)ut close to the f(ti'mer, was placed the long fiame. 

 When the mouth of the burner was so adjusted as to lie exactly 

 parallel to the axis of the beam, so that the ])rolongations of the 

 surfaces of discontinuitv intersected the objective of the telescope, 



ij Proc. Roy. Acad. Amst. 11, p. 585—587. 



~) H. Ebert, Die anomale Dispersion gliihender Melalldampfe und ihr Einfluss 

 auf Pliiinomene der Sonnenoberflaclie. Astron. Nachr. 155, S. 170 — 18"2. 



