“From its changes” he “concluded that by far the largest part of its surface reflects much less light than the rest; and that neither the darkest nor the brightest side of the satellite is turned towards the planet, but partly the one and partly the other.”
Guessing at once that our moon and Japetus did not present the only examples of equality in the times of rotation and revolution, he continued: “I cannot help reflecting with some pleasure on the discovery of an analogy which shows that a certain uniform plan is carried on among the secondaries of our solar system; and we may conjecture that probably most of the satellites are governed by the same law, especially if it be founded upon such a construction of their figure as makes them more ponderous towards their primary planet.” This very explanation was long afterwards adopted by Hansen. The peculiarity in question may without hesitation be set down as an effect of primordial tides.
In 1797 Herschel brought forward detailed evidence to shew that his generalisation applied to the Jovian system; but recent observations at Lick and Arequipa demand a suspension of judgment on the point.
The Uranian train of attendants was left by Herschel in an unsettled condition. Two of them, as we have seen, he discovered in 1787; and he subsequently caught glimpses of what he took to be four others. But only Oberon and Titania have maintained their status; the four companions assigned to them are non-existent. An unmistakable interior pair—Ariel and Umbriel—was, however, discovered by Mr. Lassell, at Malta, in 1851; and they may possibly have combined with deceptive star-points to produce Herschel’s dubious quartette. He described in 1798 the exceptional arrangement of the Uranian system. Its circulation is retrograde. The bodies composing it move from east to west, but in orbits so tilted as to deviate but slightly from perpendicularity to the plane of the ecliptic.
No trifling sensation was created in 1783, and again in 1787, by the news that Herschel had seen three lunar volcanoes in violent eruption. “The appearance of the actual fire” in one of them was compared by him to “a small piece of burning charcoal when it is covered with a very thin coating of white ashes. All the adjacent parts of the volcanic mountain seemed to be faintly illuminated by the eruption, and were gradually more obscure as they lay at a greater distance from the crater.” He eventually became aware that his senses had imposed upon him; but the illusion was very complete and has since occasionally been repeated. What was really seen was probably the vivid reflection of earth-shine from some unusually white lunar summits.
He never knowingly discovered a comet, although some few such bodies possibly ensconced themselves, under false pretences, in his lists of nebulæ. But he made valuable observations upon the chief of those visible in his time, and introduced the useful terms, corresponding to instructive distinctions, “head,” “nucleus,” and “coma.” He inferred from the partial phases of the comet of 1807, that it was in a measure self-luminous; and from their total absence in the great comet of 1811, that its light was almost wholly original. The head of this object, which shone with an even, planetary radiance, he determined to be 127,000, the star-like nucleus within, 428 miles across. The tail he described as “a hollow, inverted cone,” one hundred millions of miles long, and fifteen millions broad. This prodigious appurtenance was, in grade of luminosity, an exact match for the Milky Way. That comets wear out by the waste of their substance at perihelion, he thought very probable; the extent of their gleaming appendages thus serving as a criterion of their antiquity. They might, indeed, arrive in the solar system already shorn of much of their splendour by passages round other suns than ours; but their “age” could, in any case, be estimated according to the progress made in their decline from a purely nebulous to an almost “planetary” state. He went so far as to throw out the conjecture that “comets may become asteroids;” although the converse proposition that “asteroids may become comets,” of which something has been heard lately, would scarcely have been entertained by him.
Enough has been said to show how greatly knowledge of the solar system in all its parts was furthered by Herschel’s observational resources, fertility of invention, and indomitable energy. He was, so to speak, ubiquitous. He had taken all the heavens for his province. Nothing that they included, from the faintest nebula to the sun, and from the sun to a telescopic shooting-star, evaded his consideration. A whole cycle of discoveries and successful investigations began and ended with him.
His fame as an astronomer has cast into the shade his merits as a physicist. He made pioneering experiments on the infra-red heat-rays,[D] and anticipated, by an admirable intuition, the fact ascertained with the aid of Professor Langley’s “bolometer,” that the invisible surpass in extent the visible portions of the solar spectrum.[E] A search for darkening glasses suitable to solar observations, led him to the inquiry. Finding that some coloured media transmitted much heat and little light, while others stopped heat and let through most of the light, he surmised that a different heating power might belong to each spectral tint. His own maxim that “it is sometimes of great use in natural philosophy to doubt of things that are commonly taken for granted,” here came in appropriately. With a free mind he set about determining the luminous and thermal powers of successive spectral regions. They seemed to vary quite disconnectedly. A thermometer exposed to red rays during a given interval, rose three and a half times as much as when exposed to violet rays; and he showed further, by tracing the heat- and light-curves of the prismatic spectrum, that its heat-maximum lay out of reach of the eye in the infra-red, while luminous intensity culminated in the yellow. He even threw out the sagacious conjecture that “the chemical properties of the prismatic colours” might be “as different as those which relate to light and heat;” adding that “we cannot too minutely enter into an analysis of light, which is the most subtle of all the active principles that are concerned in the operations of nature.”
[D] Phil. Trans. 1800, p. 255.
[E] Ibid., p. 291.