210 D. Trowbridge on the Invisibility of Nebulous Matter. 
Art, XIX.—On the Invisibility of Nebulous Matter ; by 
D. TROWBRIDGE. 
Ir has generally been supposed that if nebulous matter, in the 
proper sense of the word, or cosmical vapor, exists in the heav- 
ens, and within reach of our telescopes, it will be visible to the 
eye, with suitable optical aid. It is proposed to show in this ar- 
ticle, with some plausibility, that this is an erroneous idea, except 
in some particular cases. ; 
Comets are the only celestial objects, whose physical constitu- 
tion is approximately understood, that afford us anything like 4 
distinct notion of what nebulous matter is. By far the greater 
proportion of these bodies are composed of materials so extreme 
ri rare that the solar rays can penetrate completely through the 
e 
regions of our atmosphere, must be- looked upon as dense and 
ive bodies in comparison with the almost spiritual texture of 
these light bodies. A cloud composed of materials so rare, and 
whose distance from us did not exceed fifteen or twenty miles, 
would searcely be visible. A comet, however, will be visible 
when its distance from us is many millions of miles. 
_ What conclusion can we draw from these facts? Do they not 
indicate that comets do not shine wholly by reflected light? re 
the 3d of July, 1819, Arago made an attempt to analyze the 
light of comets, by applying his polariscope to the great comet 
of 1819. The instrument showed unmistakable signs of polar 
ized light, and, therefore, of reflected sun-light. Similar observ 
ations on Halley’s comet, in 1835, more clearly indicated the ex: 
istence of polarized light. “These beautiful experiments still 
leave it undecided, whether, in addition to this reflected solar 
light, comets may not have light of their own. Even . 
case of the planets, as, for instance, in Venus, an evolution © 
independent light seems very probable.” ' 
“The variable intensity of light in comets cannot always be 
explained by the position of their orbits, and their distance from 
the sun.”* “After mentioning Arago’s observations, with his po” 
lariscope, on Halley’s comet, in 1835, Mr. Hind says, “ Still the 
variation in the intensity of light is not universally such a8 
Should follow if the comet merely reflected the sun’s rays —_ 
* Cosmos, vol. i, pp. 90, 91. Bohn’s edition. * Cosmos, vol. i, p. 9 
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