200 M. Arago on the Fhysical Constitution of the Sun. 



centrating to a single point the rays emitted from all parts 

 of the sun's limb, even after their decomposition in the 

 polarizing telescope, the mixture will be white. 



The constitution of the sun, as I have just established it, 

 may equally well serve to explain how, on the surface of the orb, 

 there exist some spots not black but luminous. These have 

 been called faculse. Galileo was the first to observe them. 

 The others, of much smaller dimensions, and generally round, 

 were discovered by Scheiner, and by him named lucules. 

 These latter cause the surface of the sun to appear spotted. 

 It is a singular fact ; but I might trace the origin of the 

 discovery of one of the principal causes of the faculce and 

 lucules to an administrative visit to a shop of novelties situ- 

 ated on our boulevards. 



" I have to complain," said the master of the establish- 

 ment," of the Gas Company ; it ought to direct on my goods 

 the broad side of the bat-wing burner, whilst, by the care- 

 lessness of their servants, it is often the edge which is 

 directed on them." '* Are you certain," said one of the assist- 

 ants, " that in that position the flame gives less light than 

 in the other V The idea appearing ill-founded, and, I would 

 even say, absurd, it was submitted to accurate experiment ; 

 and it was determined that a flame shed upon any object as 

 much light when it illuminates by its edge as when its broad 

 surface was presented to it. 



Thence resulted the conclusion, that a gaseous incande- 

 scent surface of a determined extent is more luminous when 

 seen obliquely than under the perpendicular incidence. Con- 

 sequently, if, like our atmosphere, when dappled with clouds, 

 the solar surface presents undulations, the parts of these 

 undulations which are presented perpendicularly to the ob- 

 server must appear comparatively dim, and the inclined por- 

 tions must appear more brilliant ; and hence every conic 

 cavity must appear a lucule. It is no longer necessary in 

 accounting for these appearances, to suppose that there exist 

 on the sun millions of fires more incandescent than the rest 

 of the disc, or millions of points distinguishing themselves 

 from the neighbouring regions by a greater accumulation of 

 luminous matter. 



