NATURAL PHILOSOPHY. 135 



the sun have their maximum and minimum at the same time as the 

 variations of the needle. It follows from this, that the cause of these 

 two changes on the sun and on the earth must be the same, and con- 

 sequently, from this disco very, it will be possible to solve several 

 important problems, whose solution has hitherto never been attempted. 



ARAGO OX THE PHYSICAL CONSTITUTION OF THE SUX. 



FROM a paper recently submitted to the French Academy by M. 

 Arago, we copy the following extracts. They embody many of the 

 author's investigations and results, and are of the most interesting 

 character. 



After briefly reviewing the phenomena of the solar spots, and the 

 peculiar radiance, less luminous than the rest of the orb, with which 

 they are surrounded, the penumbra, M. Arago says : This 

 penumbra, first noticed by Gallileo, and carefully observed by his 

 astronomical successors in all the changes which it undergoes, has led 

 to a supposition, concerning the physical constitution of the sun, which 

 at first must appear altogether astonishing. According to this view 

 the orb would be regarded as a dark body, surrounded at a certain 

 distance by an atmosphere, which might be compared to that envelop- 

 ing the earth, when composed of a continuous bed of opaque and 

 reflecting clouds. To this first atmosphere would succeed a second, 

 luminous in itself, and which has been called the photosphere. This 

 photosphere, more or less removed from the interior cloudy atmos- 

 phere, would determine by its circumference the visible limits of the 

 orb. According to this hypothesis, spots upon the sun would appear 

 as often as there were found in the concentric atmospheres corres- 

 ponding vacant portions, which would permit us to see exposed the 

 dark central body. Those who have studied with powerful instru- 

 ments, professional astronomers, and competent judges, acknowledge 

 that this hypothesis concerning the physical constitution of the sun, 

 supplies a very satisfactory account of the facts. Nevertheless, it is 

 not generally adopted ; recent authoritative works describe the spots 

 as scorite floating on the liquid surface of the orb, and issuing from 

 solar volcanoes, of which terrestial volcanoes are but a feeble type. 



It was desirable then, to determine, by direct observation, the nature 

 of the incandescent matter of the sun, but when we consider that a 

 distance of 95,000,000 of miles separates us from this orb and that 

 the only means of communication with its visible surface are luminous 

 rays issuing therefrom, even to propose this problem seems an act of 

 unjustifiable temerity. The recent progress in the science of optics, 

 has, however, furnished the means for completely solving the problem. 



None are now ignorant that natural philosophers have succeeded in 

 distinguishing two kinds of light, viz., natural and polarized. A ray 

 of the former of these lights exhibits, on all points of its surface, the 

 same properties ; whilst, with regard to the polarized light, the prop- 

 erties exhibited on the different sides of its rays are different. These 

 discrepancies, manifest themselves in a multitude of phenomena which 



