The counterglow, first remarked by Pezénas in 1730,[112] soon fell into oblivion, and had to be rediscovered after six-score years by Brorsen, who bestowed upon it its current title of the 'Gegenschein.' Of late it has been pretty constantly observed, particularly by Professor Barnard, to whom it presented itself, owing to the scantiness of the available records, as a surprising novelty.[113] Surprising it certainly is. The appearance of the Gegenschein is that of a large elliptical patch of diffuse light, measuring about 12 by 9 degrees, and situated diametrically opposite to the sun.[114] Now and again, though somewhat rarely, it is perceived to be united to the cone by the 'zodiacal band,' a strip of evanescent luminosity nearly following the line of the ecliptic. We cannot, then, be mistaken in recognising the great pyramidal beam centred on the sun, with the counterglow and its linking band, as sections of a single formation, constituting in a manner the substratum of the solar system. A recent observation made by Professor Newcomb under unique conditions proves it to be much less exclusively 'zodiacal' than had been supposed. Looking north from the summit of the Rothhorn, at midnight, on July 29, 1905, he perceived a well-marked glow spreading 35 degrees from the sun's place.[115] It was the light in its thwartwise aspect, which had never before been seen, or even looked for; and we learn from it the remarkable fact that the sun is enclosed in a vast, dimly luminous sphere, with a girth not much smaller than the orbit of Venus, and indefinitely diffused along the equatorial plane.
Notwithstanding its dim indefiniteness, neither the spectroscope nor the camera is wholly ineffective for the scrutiny of this extraordinary appurtenance. We have learned positively that its radiance is of the continuous sort, the origin of which through the reflection of sunlight from small solid bodies seems more than probable. The whole structure must accordingly be of a pulverulent or meteoric nature; it consists of independently moving particles. But to the further question, Under what regimen do these particles circulate? no decisive answer is as yet forthcoming. M. Hansky[116] and others hold the light to be a true solar appendage, an extension of the corona, in which case it would have a formal, but no material permanence. It would represent the continually changing aggregate of multitudinous minute bodies issuing from or repelled by the sun, and in large proportion falling back towards his surface. Yet some difficulty is raised to this view by the vast dimensions of the problematical glow. That it extends far beyond the earth's orbit is rendered patent by the phenomena of the Gegenschein and the band. True, the scope of the sun's repulsive action cannot be limited; still, we might naturally expect its products to become too attenuated for recognition beyond a radius of perhaps fifty million miles.
Admitting, on the other hand, the residual character of the zodiacal light, we should attribute to it a constitution analogous to that of Saturn's rings. Each one of the cosmic atoms collected in it would revolve round the sun on its own account, scarcely disturbed by its neighbours. Nor need we despair of determining with reasonable certainty which way the truth lies in this matter. The rival hypotheses may be tried by a criterion the application of which is by no means remotely feasible. It is furnished by the geometrical relations of the zodiacal light. Evidently, if the sun can claim organic connection with it, its axis should coincide with the plane of the solar equator; while, if it represent wastage from the Kantian nebula, it should stretch along the principal plane of our system—the plane of maximum moment of momentum—the plane towards which the primitive agglomeration of revolving particles collapsed as it condensed. The question of planes is, then, crucial. Is the zodiacal effluence placed symmetrically as regards the solar equator, or does it appertain properly to the ecliptic, which deviates very slightly from the fundamental plane of the solar system? The evidence is, unfortunately, contradictory. Most observers have located the dim equinoctial cone right along the pathway of the sun; some, under exceptionally favourable circumstances, have perceived in it a marked departure from the track of the Signs.
M. Marchand's determinations from the Pic du Midi, for instance, indicated a probable coincidence between the solar equatorial plane and the axis of the light;[117] and Dr. Max Wolf succeeded, in 1889, in getting a photographic impression which, though partial and imperfect, tended to corroborate Marchand's inference.[118] Again, on November 16, 1904, when the cone showed a remarkable lustre, it was distinctly perceived at Königstuhl to sheer off and separate from the ecliptic as it mounted the sky. Now, however, that a beginning has been made in photographing this enigmatical tenant of the sphere (the feat has been performed at Flagstaff as well as at Heidelberg), we may confidently expect a speedy reconcilement of inconsistent statements regarding its whereabouts. Until then we cannot venture to assert that it is in actual reality what it appears to be, a nebulous survival.
FOOTNOTES:
[109] Ligondès, quoted by l'Abbé Moreux, Le Problème Solaire, p. 67.
[110] Moreux, Le Problème Solaire, p. 133; Ledger, Nineteenth Century, March, 1905.
[111] Humboldt, Cosmos, vol. iv., p. 563 (Otté's translation); Maunder, Journal of the British Astronomical Association, vol. viii., p. 174; Max Wolf, Königstuhl Report for 1904.
[112] Paris Memoirs, 1731, quoted by R. Wolf, Geschichte der Astronomie, p. 695.