90 COSMOS. 



encing any deflection during its passage.* If such an absence 

 of refracting power must be ascribed to the nucleus of a comet, 

 we can scarcely regard the matter composing comets as a 

 gaseous fluid. The question here arises, whether this absence 

 of refracting power may not be owing to the extreme tenuity 

 of the fluid ? or does the comet consist of separated particles, 

 constituting a cosmical stratum of clouds, which, like the 

 clouds of our atmosphere, that exercise no influence on the 

 zenith distance of the stars, does not affect the ray of light 

 passing through it ? In the passage of a comet over a star, a 

 more or less considerable diminution of light has often been 

 observed : but this has been justly ascribed to the brightness 

 of the ground from which the star seems to stand forth during 

 the passage of the comet. 



The most important and decisive observations that we 

 possess on the nature and the light of comets, are due to 

 Arago's polarization experiments. His polariscope instructs 

 us regarding the physical constitution of the Sun and comets, 

 indicating whether a ray that reaches us from a distance of 

 many millions of miles, transmits light directly, or by reflec- 

 tion ; and if the former, whether the source of light is a 

 solid, a liquid, or a gaseous body. His apparatus was used 

 at the Paris Observatory, in examining the light of Capella, 

 and that of the great comet of 1819. The latter showed 

 polarized, and therefore reflected light, whilst the fixed star, 

 as was to be expected, appeared to be a self-luminous sun.f 



* Bessel, in the Astron. Nadir., 1836, No. 301, s. 204, 206. Struve, 

 in Recueil des Mem. de I' A cad. de St. Petersb., 1836, p. 140, 143, and 

 Astr. Nadir., 1836, No. 303, s. 238, writes as follows : " At Dorpat the 

 star was in conjunction only 2 "'2 from the brightest point of the comet. 

 The star remained continually visible, and its light Avas not perceptibly 

 diminished whilst the nucleus of the comet seemed to be almost ex- 

 tinguished before the radiance of the small star of the ninth or tenth 

 magnitude." 



t On the 3d of July, 1819, Arago made the first attempt to analyse 

 the light of comets by polarization, on the evening of the sudden ap- 

 pearance of the great comet. I was present at the Paris Observatory, 

 and was fully convinced, as were also Matthieu and the late Bouvard, 

 of the dissimilarity in the intensity of the light seen in the polariscope, 

 when the instrument received cometary light. When it received light 

 from Capella, which was near the comet, and at an equal altitude, the 

 images were of equal intensity. On the reappearance of Halley's comet, 

 in 1835, the instrument was altered, so as to give, according to Arago's 



