PHYSICAL CONSTITUTION OF THE SUN. 



[1853 



On the Physical Constitution of the Sim. 



Abstract of a paper submitted to the French Academy, by 

 M. Arago. 



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 under- 

 goes has led to a supposition, concerning the physical constitu- 

 tion 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 enveloping 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 atmosphere, would de- 

 termine 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 cor- 

 responding vacant portions, which would permit us to see exposed 

 the dark central body. Those who have studied with powerful 

 instruments, professional astronomers, and competent judges, 

 acknowledge that this hypothesis concerning the physical consti- 

 tution of the sun, supplies a very satisfactory account of the facts. 

 Nevertheless, it is not generally adopted; recent authoritative 

 works describe the spots as scoria: 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 con- 

 sider 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 pro- 

 pose 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 suc- 

 ceeded 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 properties exhibited on the different sides 

 of its rays are different. These discrepancies, manifest themselves 

 in a multitude of phenomena which need not here be noticed. 

 Before going farther, let us remark, that the>-e is something won- 

 derful in the experiments which have led natural philosphers le- 

 gitimately to talk of the different sides of a ray of light. The 

 word " wonderful" which I have just u>ed, will certainly appear 

 natural to those who arc aware that millions and millions of these 

 rays can simultaneously pass through the eye of a needle, without 

 interfering one with the other. Polarized light has enabled 

 astronomers to augment the means of investigation by the aid of 

 some curious instruments, from which great benefit has accrued 

 already-7-among others, the paralizing telescope, or polariscope, 

 merits attention. In looking directly at the sun with, one of 

 these telescopes, two whito images of the same intensity, and the 

 same shade will be seen. Let us suppose the reflected image of 

 this orb to be seen in water, or a glass mirror; In the act of re- 

 flection the ravs become polarized, the lens no longer presenting 

 two white and similar images ; on the contrary they are tinged 

 with brilliant colore, their shape having experienced no alteration. 

 If the one be red, the other will be green ; if the former be yel- 

 low, the latter will present a violet shade, and so on ; the two 

 colors being always what are called complementary, or susceptible 



by their mixture, of forming white. By whatever means this 

 polarized light has been produced, the colors will displays them- 

 selves in the two images of the polarizing telescope, as when the 

 rays have been reflected by water or glass. The polarizing 

 telescope, thus furnishes a very simple means of distinguishing 

 natural from polarized light. 



It has been long believed, that light emanating from incan- 

 descent bodies, reaches the eye in the state of natural light, when 

 it has not been partially reflected, or strongly refracted, in its 

 passage. The exactitude of this proposition failed, however, in 

 certain points. A member of the Academy lias discovered that 

 light emanating under a sufficiently small angle, from the surface 

 of a solid or liquid incandescent body, even when unpolished, 

 presents evident marks of polarization ; so that in passing through 

 the polarizing telescope it is decomposed into two colored pencils. 

 The light emanating from an inflamed gaseous substance, such as 

 is used in street illumination, 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 arise from the immense dis- 

 tance of the orb. . 



The rays which indicate the margin of the disc, have evidently 

 issued from the incandescent surface under a very small angle. 

 The question here occurs, — The margin of the two images, which 

 the polarizing telescop3 furnishes, do they, when viewed directly, 

 appear colored? — then the light of these margins proceeds from 

 a liquid body ; for any supposition which would make the exte- 

 rior of the sun a solid body is definitely removed by the observa- 

 tions of the rapid changing of the form of the spots. Have the 

 margins maintained their natural whiteness in the glass? then 

 they are necessarily gaseous. The incandescent bodies which 

 have been studied by a polariscope, the light being emitted under 

 angles, are the following : — of solids, forged iron and platinum ; 

 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 general- 

 ize ? My response is this ; following the two explanations that 

 have been given of the abnormal polarization 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, Jje analogous to a gas; as for example, the 

 liquid of an almost ideal rarity, which many geometricians have 

 been led to place hypothetical]}', at the extreme limit of our at- 

 mosphere where the phenomena of polarization and colorization 

 may perhaps disappear. I shall however, anticipate a difficulty 

 which may suggest itself. It ought to be observed, that the 

 lights proceeding from two liquid substances, may, according 

 to the special nature of these substances, not be identical in refer- 

 ence to the number and position of the black bands of Frauen- 

 hofer, and which these prismatic hues offer to the eye of the 

 philosopher. These discrepances are of a nature to be considera- 

 bly augmented by the differently constituted atmospheres through 

 which the rays have to travel before reaching the observer. 



Observations made any day of the year, looking directly at 

 the sun, with the aid of powerfully polarizing telescopes, exhibit 

 no trace of colorization. The inflamed substances then, which 

 defines the circumference of the sun, is gaseons. We can gen- 

 eralize this conclusion, since, through the agency of rotation, the 

 different points of the surface of the sun come in succession to 

 form the circumference. This experiment removes out of the 

 domain of simple hypothesis the theory we have previously in- 



