M. Arago on the Physical Constitution of the Sun. 197 



manifest themselves in a multitude of phenomena which need 

 not here be noticed. 



Before going further, let us remark that there is some- 

 thing wonderful in the experiments which have led natural 

 philosophers legitimately to talk of the different sides of a 

 ray of light. The word " wonderful" which I have just used, 

 will certainly appear natural to those who are aware that mil- 

 lions of millions of these rays can simultaneously pass through 

 the eye of a needle without interfering the one with the other. 



Polarized light has enabled astronomers to augment the 

 means of investigation by the aid of some curious instru- 

 ments, from which great benefit has accrued already, — 

 amongst others, the polarizing telescope, or polariscope, 

 merits attention. 



In looking directly at the sun with one of these telescopes, 

 two white 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 in a glass mirror. In the act of re- 

 flection the rays become polarized, the lens no longer pre- 

 senting two white and similar images ; on the contrary, they 

 are tinged with brilliant colours, their shape having expe- 

 rienced no alteration. If the one be red, the other will be 

 green ; if the former be yellow, the latter will present a 

 violet shade, and so on ; the two colours always being what 

 are called complementary, or susceptible, by their mixture, 

 of forming white. By whatever means this polarized light 

 has been produced, the colours will display themselves in the 

 tvyo images of the polarizing telescope, as when the rays have 

 been reflected by water or by glass. 



The polarizing telescope, then, furnished 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, nor strongly refract- 

 ed, in its passage. 



The exactitude of this proposition failed, however, in certain 

 points. A member of the Academy has succeeded in discover- 

 ing that the light emanating under a sufficiently small angle, 

 from the surface of a solid or liquid incandescent body, even 



