Padre Secchi, at Rome, believed he saw in the comet "carbon, or an oxide of carbon, as the source of the bright luminous bands," and the Abbé Moigno asks whether this comet may not be, after all, "_un gigantesque diamant volatilisé_." "Whatever may be the answer hereafter given to that question, the verdict of the spectroscope is clearly to the effect that the comet is made up of a _commingling of thin vapor and of denser particles_, either compressed into the _condition of solidification_, or into some physical state approaching to that condition, and is therefore entirely in accordance with the notion formed on other grounds that the nucleus of the comet is a _cluster of solid nodules or granules_, and that the luminous coma and tail are jets and jackets of vapor, associated with the more dense ingredients, and _swaying and streaming about them as heat and gravity, acting antagonistic ways, determine_."[1] [1. "Edinburgh Review," October, 1874, p. 210.] {p. 67} If the comet shines by reflected light, it is pretty good evidence that there must be some material substance there to reflect the light. "A considerable portion of the light of the comet is, nevertheless, borrowed from the sun, for it has one property belonging to it that only reflected light can manifest. It is capable of being polarized by prisms of double-refracting spar. Polarization of this character is _only possible_ when the light that is operated upon has already been reflected _from an imperfectly transparent medium_."[1] There is considerable difference of opinion as to whether the bead of the comet is solid matter or inflammable gas. "There is nearly always a point of superior brilliancy perceptible in the comet's head, which is termed its nucleus, and it is necessarily a matter of pressing interest to determine what this bright nucleus is; whether it is really a kernel of hard, solid substance, or merely a whiff of somewhat more condensed vapor. Newton, from the first, maintained that the comet is _made partly of solid substance_, and _partly of an investment of thin, elastic vapors_. If this is the case, it is manifest that the central nodule of dense substance should be capable of intercepting light when it passes in front of a more distant luminary, such as a fixed star. Comets, on this account, have been watched very narrowly whenever they have been making such a passage. On August 18, 1774, the astronomer Messier believed that he saw a second bright star _burst into sight from behind the nucleus of a comet which had concealed it the instant before_. Another observer, Wartmann, in the year 1828, noticed that the light of an eighth-magnitude star was _temporarily quenched as the nucleus of Encke's comet passed over it_."[2] Others, again, have held that stars have been seen through the comet's nucleus. [1. "Edinburgh Review," October, 1874, p. 207. 2. Ibid., p. 206.] {p. 68} Amédée Guillemin says: