PORTION OF THE COSMOS. THE SUN. 283 



different causes to which they may by possibility be due. 

 Total eclipses of the Sun are excluded, were it only from the 

 recorded continuance of the obscurations for several hours, 

 whereas, according to Du Sejours' calculation, the longest 

 possible duration of a total solar eclipse is 7' 58" at the 

 equator, and in the latitude of Paris only 6' 10"). The three 

 causes to which I allude are : 1, disturbances in the process 

 by which light is evolved, or a less intensity in the photo- 

 sphere ; 2, impediments to the radiation of solar light and 

 heat arising in the external opaque vaporous veil or covering 

 surrounding the photosphere, by the formation in it of un- 

 usually large and dense clouds ; 3, extraneous admixtures 

 in our own atmosphere chiefly of an organic cha- 

 racter, as " trade wind dust/' " inky rain," or the Chinese 

 u sand-rain," described by Macgowan as lasting several 

 days. The causes mentioned under heads 2 and 3 require 

 no enfeeblement of the (perhaps) electro-magnetic, luminous 

 process in the Sun's atmosphere, (a perpetual Aurora or 

 polar light) ( 482 ) ; the third is open to the objection that 

 it is opposed to the visibility of stars in the middle of the 

 day, which is so often spoken of in the too scanty descrip- 

 tions given of the circumstances accompanying these mys- 

 terious phenomena. 



Arago's discovery of chromatic polarisation has tended 

 not only to strengthen the belief of a third and outermost 

 covering of the Sun, but also to confirm the conjectures 

 which have been formed respecting the physical constitution 

 of the central body of our planetary system. "A ray of 

 light arriving at our eyes from the remotest regions of space 

 tells us in the polariscope, as it were of itself, whether it is 



