208 M. Arago on the Physical Constitution of the Sun. 



generalizations, will unveil phenomena, wliich, by their na- 

 ture, or the immensity of the numbers which measure them, 

 will throw the most firm minds into a sort of vertigo. 



But abandoning these speculations, however worthy they 

 may be of admiration, we shall come back to the chief question 

 which I have proposed to treat in this account, — to try, if 

 possible, to establish a connection between the physical nature 

 of the sun and of the stars. 



We have succeeded, by the help of the polarizing telescope, 

 to determine the nature of the substance which composes 

 the solar photosphere, because by reason of the great appa- 

 rent diameter of the orb we have been able to observe sepa- 

 rately the different points of its circumference. If the sun 

 were removed from us to a distance where its diameter would 

 appear as small to us as that of the stars, this method 

 would be inapplicable. The coloured rays proceeding from 

 the different points of the circumference would then be in- 

 timately mixed, and, we have said already, that their mixture 

 would be white. 



It appears, then, that we must not apply to stars of imper- 

 ceptible dimensions the process which so satisfactorily con- 

 ducted us to the result in regard to the sun. There are, 

 however, some of these stars which supply us with the means 

 of investigation. I allude to the changing stars. 



Astronomers have remarked some stars whose brilliancy 

 varies considerably ; there are even some which, in a very 

 few hours, pass^from the second to the fourth magnitude ; 

 and there are others amongst which the changes in intensity 

 are much more decided. These stars, quite visible at certain 

 epochs, totally disappear, to reappear in periods longer or 

 shorter, and subject to slight irregularities. 



Two explanations of these curious phenomena present 

 themselves to the mind ; the one consists in supposing that 

 the star is not equally luminous on all parts of its surface, 

 and that it experiences a rotatory movement upon itself; 

 thus it is brilliant when the luminous part is turned towards 

 us, and dark when the obscure portion arrives at the same 

 position. 



According to the other hypothesis, an opaque, and, in 



