198 M. Arago on the Physical Constitution of the Sun, 



when unpolished, presents evident marks of polarization ; so 

 that in passing through the polarizing telescope it is decom- 

 posed into two coloured pencils. 



The light emanating from an inflamed gaseous substance, 

 such as that which now-a-days illuminates our streets and 

 our shops, on the contrary, is always in its natural state, 

 whatever may have been its angle of emission. 



The means used to decide whether the substance which 

 renders the sun visible is solid, liquid, or gaseous, will be 

 nothing more than a very simple application of the foregoing 

 observations, in spite of the difficulties which appeared to 

 flow from the immense distance of the orb. 



The rays which indicate the margin of the disc have evi- 

 dently issued from the incandescent surface under a very 

 small angle. The question here occurs : The margins of 

 the two images, which the polarizing telescope furnishes, 

 do they, when viewed directly, appear coloured ? — then, the 

 light of these margins proceeds from a liquid body ; for any 

 theory which would make the exterior of the sun a solid body 

 is definitively removed by the observation of the rapid chang- 

 ing of the form of the spots. Have the margins maintained 

 their natural whiteness in the glass ? then they are neces- 

 sarily gaseous.* 



* The incandescent bodies which have been studied by a polariscope, the 

 light being emitted under different angles, are the following : — Of solids, forged 

 iron and platina ; of liquids, fused iron and glass. From these experiments it 

 may be said, you have a right to affirm, that the sun is neither fused iron nor 

 glass ; but what authority have you further to generalize ? My response is this : 

 Following the two explanations which have been given of the abnormal polariza- 

 tion which presents rays emitted under acute angles, all ought to be the same, 

 ■with the exception of the quantity, whatever be the liquid, provided that the 

 surface of emergence has a sensible reflecting power. There would remain only 

 the case in which the incandescent body would, as to its density, be analogous 

 to a gas ; as, for example, the liquid of an almost ideal rarity, which many geo- 

 metricians have been led to place hypothetically at the extreme limit of our 

 atmosphere, where the phenomena of polarization and of colorization may per- 

 haps disappear. I am not ignorant that I should add a value to the experi- 

 ment reported in the text by discussing it in a photometrical point of view. 

 I possess the materials for such an examination, but this is not the place for 

 pursuing it. 



I shall, however, anticipate a difficulty which may suggest itself. It ought to 



