From this account of what the zodiacal light appears to be, we proceed to consider what it is. Some inquirers—arguing from the 'nebular theory,' which assumes the formation of the several planets, one after another, from nebulous matter—have supposed the zodiacal light to be a remnant of that matter yet unconcentrated. In this view, it may be a nebula, brightest in the centre, as is the case with most, and fainter towards the margin. According to Humboldt, 'we may with great probability attribute the zodiacal light to the existence of an extremely oblate ring of nebulous matter, revolving freely in space between the orbits of Venus and Mars.' On several occasions he witnessed its fluctuations, night after night, from the plains of South America, shewing itself at times greatly collapsed or condensed, with intermittences of vividness and faintness, in the course of a few minutes, as is observed of the aurora. The light of the stars, of even the fifth or sixth magnitudes, can be seen through it: the same has been remarked of comets; and it is known also that the tails of comets undergo frequent flashings or pulsations, so that the two phenomena may be analogous in character. It is necessary, however, to distinguish the fluctuations from such effects as may be produced by movements in the lower strata of the atmosphere.
Mairan, who wrote in 1731, was of opinion that the zodiacal light consisted of particles thrown off from the sun by its rapid rotation, or a species of atmosphere peculiar to the central orb. Others have supposed the luminosity to be composed of 'revolving planetary particles,' shining by a direct or reflected light. But, according to Professor Olmsted, of Yale College, Massachusetts, it is something which has a motion of its own around the sun, notwithstanding that the general steadiness of its movements had warranted the notion that it was in some way attached to the body of the sun itself. Olmsted's conclusions are drawn from a diligent observation of the light during a period of six years, and are on this account, as well as from his scientific reputation, entitled to respect. He states the light to be, in constitution, colour, and density, similar to that of the tail of a comet, the portion nearest the sun being brightest, and both admitting of stars being seen through them. We may, therefore, infer it to be a nebulous ring surrounding the sun, in the same way that the magnificent rings of Saturn surround that planet. Of such nebulæ as this there are from 2000 to 3000 visible in the regions of space, compared with which the dimension of ours is insignificant: at the same distance, and sought for with the same instruments, it would be invisible.
In one point, Professor Olmsted's views are particularly interesting, as, out of one mysterious phenomenon, he endeavours to explain another, and inquires: 'Whether or not the zodiacal light is the origin of the meteoric showers of November and August, and especially those of November?' Many readers know that for some years past great numbers of falling-stars, or showers of meteors, have been observed periodically in November: the fall seen in the United States in 1834—when, as is estimated, more than 240,000 stars fell as thick as snow-flakes, in the space of nine hours—being the most remarkable hitherto known. The explanation is, that the zodiacal light is a nebulous body revolving round the sun, and arrives at its aphelion on the 13th November in that part of the earth's orbit which the earth then reaches, and, coming into contact with our atmosphere, portions of the nebulous matter are detached, and, taking fire as they pass through, appear to us as shooting-stars. This explanation of the phenomenon in question is one not hastily conceived; the reasoning on which it is founded is altogether satisfactory, as well with regard to the movement of the nebulous matter, as to that of the earth.
Professor Olmsted, in a communication addressed to the 'American Association for the Advancement of Science,' sums up his views as follows:
'1. The zodiacal light, as we have found, in our inquiry into its nature and constitution, is a nebulous body.
'2. It has a revolution round the sun.
'3. It reaches beyond and lies over the earth's orbit at the time of the November meteors, and makes but a small angle with the ecliptic.
'4. Like the nebulous body, its periodic time is commensurable with that of the earth, so as to perform a certain whole number of revolutions while the earth performs one, and thus to complete the cycle in one year, at the end of which the zodiacal light and the earth return to the same relative position in space. This necessarily follows from the fact, that at the same season of the year it occupies the same position one year with another, and the same now as when Cassini made his observations nearly 170 years ago.
'5. In the meteoric showers of November, the meteors are actually seen to come from the extreme portions of the zodiacal light, or rather a little beyond the visible portions.'
There is much that is suggestive in this summary, and, as we said at the commencement, the subject is one of a nature to stimulate inquiry and research, and to lead to further explanations of cosmical phenomena. M. Mathieson's observations, published in the Comptes Rendus of the Académie des Sciences for 1843, shew, that when tested with the thermo-multiplier, the zodiacal light was found to radiate heat as well as light—a fact which, if further verified, will support the evidence in favour of an independent luminous ring.