THE SUn's SPOTS. 77 



As, according to Du Sejour's calculation, the longest possi- 

 ble duration of a total eclipse of the Sun can not be more than 

 7m. 58s. at the equator, nor more than 6m. 10s. for the lati- 

 tude of Paris, the decrease of daylight which is recorded by 

 the annalists may, on account of its duration for many hours, 

 possibly be referred to one or other of the three following and 

 very different causes : 1 . A disturbance in the process of the 

 evolution of light, as it were a diminution of intensity in the 

 photosphere ; 2. Obstructions (such as a greater and denser 

 formation of clouds) in the outermost opaque vaporous en- 

 velope investing the photosphere, by which the radiation of 

 solar light and heat is impeded ; 3. The impure condition of 

 our atmosphere, arising, for instance, from the obscuring (most- 

 ly organic) meteoric dust, rain, or sand-rain, such as is de- 

 scribed by Macgowan to have continued for several days to- 

 gether in China. The second and third of these causes do 

 not require the occurrence of a diminution of the electro-mag- 

 netic light process, perhaps (of the perpetual polar light*), in 

 the solar atmosphere, but the last-named cause excludes the 

 visibility of stars at noon, of which such frequent mention is 

 made in these mysterious and vaguely-described obscurations. 

 Arago's discovery of chromatic 'polarization has not only 

 confirmed the existence of the third and outermost envelope 



1547, before the battle between Charles V. and the Duke of Saxony, 

 the Sun appeared for three days as if it were suffused by blood, while 

 at the same time many stars were visible at noon." " Refert Gemma, 

 pater et filius, anno 1547, ante conflictum Caroli V. cum Saxon ia; 

 Duce, Solem per tres dies ceu sanguine perfusum comparuisse, ut 

 etiam Stellas plereque in meridie conspicerentur." Kepler (in Stella 

 Nova in Serpentario, p. 113) further expresses his uncertainty as to 

 the cause of the phenomenon ; he asks whether the diminution of the 

 Sun's light be owing to some celestial causes: " Solis lumen ob can- 



i • • 



sas quasdam sublimes hebetari " whether it be owing to tht> 



wide diffusion of some cometary substance, " materia cometica latius 

 sparsa," for the cause can not have originated in our atmosphere, 

 since the stars were visible at noon. Schnurrer (Chronik der Seu- 

 chen, th. ii., p. 93) thinks, notwithstanding the visibility of the stars, 

 that the phenomenon must have been the same as the so-called 

 " Hohenrauch," for Charles V. complained before the battle " that 

 the Sun was always obscured when he was about to engage with the 

 enemy." "Semper se nebulae densitate infestari, quo ties sibi cum 

 hoste pugnanduui sit." (Lambert, Hortens. de hello German., lib. 

 vi., p. 182.) 



* Horrebow {Basis Astronomic, 1735, § 226) makes use of the same 

 expression. Solar light, according to him, is "a perpetual Northern 

 light within the Sun's atmosphere, produced by the agency of powerful 

 magnetic forces." (See Hanow, in Joh. Dan. Titius's Gemoinnutzige 

 Abhandlungcn uber natilrliche Dinge, 17G8, p. 102.) 



