( 597 ) 



F\s. 1. 



Ill Fir>-. I I he opiical deiisily of the 

 matter iiiav l»e iv|)r('S(Mile(l hy the eom- 

 paetness ol' tlie sireakiiii;-. A ray tor 

 which the iiKMliiim has a lar,ü,e positive 

 retVaelioiu'oiislaiit woiiM for instance 

 foHow the |»alh .1.1', curving- ronnd tlie 

 denser |)arts of the structure ; a ray y>'/>', 

 for which the medium possesses a large 

 negative refract ionconstant, would move 

 ill a similar way through the more rare- 

 tied regions. On the other liaiid, the light 

 CO for \N hicli the constant exactly equals 

 zero is not influenced hy the (luctuations 

 of the density ; and if for some kind of 

 light the refract ioncoiistaiit is very nearly 

 zero, the ray would have to travel a long 

 way almost jiaraliel to the structure before 

 its curving would be perceptible. 

 Now the corona sometimes siiows exceedingly long, pointed strea- 

 mers. We only ha\e to suppose that the Earth ^^' as e x a c t 1 y 

 in the direction o f s u c h a s t r e a m e rat the m o m e n t 

 the a I) n o r m a I s p c c t r u m w a s p h o t o g r a p h e d ; then 

 all the irregularities observed in this spectrum become clear. Light, 

 under normal cii-cumslances absent from the solar spectrum tiirough 

 strcnig dispersion, has been collected by the coronal streamer ; hence 

 the weakening of the Fraunhofer lines, especially also of those 

 in the s])ectrum of the spot. As the al (normalities were caused by a 

 peculiar (bstribulion of niattei' in the \ast regions of the corona, lying 

 between the source of light and the Earth (and not l)y disturbances 

 in a relatively thin "reversing layer") they couhl aj)j)eai' in the 

 same way over a great j)art of tlie SuiTs disk. The i- a i' i t y of the 

 phenomenon is the result of the slight chance \\ c liaxe to lake a 

 photograph al the vory niomenl on \vhich an uncommonly long coronal 

 streamer is projected exactly on the |»arl of Suns disk illuminating 

 the slit ; the s ii o i' t d u rati o n linally is a conseciuence of the 

 diflercnce between the angular velocity of the corona and that of the 

 Earth in ils orbit. 



As we have mentioned before, iw chromosphere lines correspond, 

 in general, to those lines showing extraordinarily ,s7/7^;?y in the abnor- 

 mal spectrum. Ho\v are we to account for llic strengthening of 

 these lines? 



