HERSCHEL’S SPECIAL INVESTIGATIONS.
Double stars were, when Herschel began to pay attention to them,
regarded as mere chance productions. No suspicion was entertained
that a real, physical bond united their components. Only the Jesuit
astronomer, Christian Mayer, maintained that bright stars were often
attended by faint ones; and since his observations were not such as
to inspire much confidence, his assertions counted for very little.
“In my opinion,” Herschel wrote in 1782, “it is much too soon to form
any theories of small stars revolving round large ones.” He, indeed,
probably even then, suspected that close _equal_ stars formed genuine
couples; but he waited, if so, for evidence of the connection. The
chief subject of his experiments on parallax was Epsilon Boötis, an
exquisitely tinted, unequal pair. But he soon became aware that either
stellar parallax was elusively small, or that he was on the wrong track
for detecting it. And, since his favourite stars have proved to be a
binary combination, it was, of course, drawing water in a sieve to make
one the test of perspective shifting in the other.
The number of Herschel’s double stars alone showed them to be integral
parts of an express design. Such a crop of casualties was out of all
reasonable question. And it was actually pointed out in 1784 by John
Michell, a man of extraordinary sagacity, that the odds in favour of
their physical union were truly “beyond arithmetic.”
Herschel meantime kept them under watch and ward, and after the lapse
of a score of years found himself in a position to speak decisively.
On July 1, 1802, he informed the Royal Society that “casual situations
will not account for the multiplied phenomena of double stars,” adding,
“I shall soon communicate a series of observations proving that many
of them have already changed their situation in a progressive course,
denoting a periodical revolution round each other.” A year later he
amply fulfilled this pledge. Discussing in detail the displacements
brought to light by his patient measurements, he made it clear that
they could be accounted for only by supposing the six couples in
question to be “real binary combinations, intimately held together
by the bond of mutual attraction.” His conclusion was, in each case,
ratified by subsequent observation. The stars instanced by him--Castor,
Gamma Leonis, Epsilon Boötis, Delta Serpentis, Gamma Virginis, and
Zeta Herculis--are all noted binaries. Not satisfied with establishing
the fact, Herschel assigned the periods of their revolutions. But he
could only do so on the hypothesis of circular motion, while the real
orbits are highly elliptical. His estimates then went necessarily wide
of the mark. For one pair only, he was able to use an observation
anterior to his own. Bradley had roughly fixed, in 1759, the relative
position of the components of Castor, the finest double star in the
northern heavens; and the preservation of the record in Dr. Maskelyne’s
note-book extended by twenty years the basis of Herschel’s conclusions
regarding this system.
He continued, in 1803, his discussions of double stars; announced
a leisurely circulation of both the pairs composing the typical
“double-double star,” Epsilon Lyræ; and conjectured the union of the
two into one grand whole--a forecast verified by the evidence of
common proper motion. The Annus Magnus of the quadruple system cannot,
according to Flammarion, be less than a million of years.
The discovery of binary stars was, in Arago’s phrase, “one with a
future.” In itself an amazing revelation, it marked the beginning of
a series of investigations of immense variety and importance. By it,
a science of sidereal mechanics was shown to be possible; the sway of
gravitation received an unlimited extension; and the perception of
order, which is the precursor of knowledge, ranged at once over the
whole visible creation. Herschel, it is true, had not the means of
formally proving that stellar orbits are described in obedience to the
Newtonian law. His affirmative assertion rested only on the analogy of
the solar system. But the rightness of his judgment has never seriously
been called in question.
His research into the transport of the solar system through space
proved, as Bessel said, that the activity of his mind was independent
of the stimulus supplied by his own observations. It was one of his
most brilliant performances.
The detection of progressive star-movements was due to Halley. It
was announced in 1718. The bright objects spangling the sky are then
“fixed” only in name. “But if the proper motion of the stars be
admitted,” asked Herschel, “who can deny that of our sun?” The same
idea had occurred to several earlier astronomers, but only one, Tobias
Mayer, of Göttingen, had tried to test it practically; and he had
failed. “To discern the proper motion of the sun between so many other
motions of the stars,” Herschel might well designate “an arduous task.”
Yet it was not on that account to be neglected. The conditions of the
problem were perfectly clear to him. If the sun alone were in motion,
the stars should unanimously appear to drift backward from the “apex,”
or point on the sphere towards which his journey was directed. The
heavens would open out in front of his advance, and close up behind.
The effect was compared by Mayer to the widening prospect and narrowing
vista of trees to a man walking through a forest. On this supposition,
the perspective displacements of any two stars sufficiently far apart
in the sky would suffice to determine the solar apex. For it should
coincide with the intersection of the two great circles continuing the
directions of those displacements. But the question is far from being
of this elementary nature. The stars are all flitting about on their
own account, after--to our apprehension--a haphazard fashion. The sole
element of general congruity traceable among them is that “systematic,
or higher, parallax,” by which each of them is, according to a
determinate proportion, inevitably affected. If this can be elicited,
the line of the sun’s progress becomes at once known.
Herschel treated the subject in the simplest possible manner. Striking
a balance between the proper motions of only seven stars, he deduced,
in 1783, from simple geometrical considerations, an apex for the sun’s
way, marked by the star Lambda Herculis. But while he seemed to proceed
by rule, he was really led by the unerring instinct of genius. His
mode of conducting an investigation, small in compass, yet almost
inconceivably grand in import, distances praise. Its directness and
apparent artlessness strike us dumb with wonder. Eminently suited to
the materials at his command, it was summary, yet, within fairly narrow
limits, secure. And the result has stood the test of time. It ranks,
even now, as a valuable approximation to the truth. He himself regarded
his essay as nothing more than an experimental effort. In a letter to
Dr. Wilson, of Glasgow, he expressed his apprehensions lest his paper
on the sun’s motion “might be too much out of the way to deserve the
notice of astronomers.”
Provided with Maskelyne’s table of thirty-six proper motions, he
resumed the subject in 1805. He now employed a graphical method,
drawing great circles to represent the observed stellar movements, and
planting his apex impartially in the midst of their intersections. It
was, however, less happily located than that of 1783. The constellation
Hercules again just included it; but it lay certainly too far west,
and probably too far north. The memoir conveying the upshot of the
research is, none the less, a masterpiece. Philosophy and common-sense
have rarely been so fortunately blended as in this discussion. Without
any mathematical apparatus, the plan of attack upon a recondite problem
is expounded with the utmost generality and precision. The reasoning
is strong and sure; intelligible to the ignorant, instructive to the
learned.
In his earlier paper, Herschel, while venturing only to “offer a few
distant hints” as to the _rate_ of the sun’s travelling, expressed
the opinion that it could “certainly not be less than that which the
earth has in her annual orbit.” That is to say, his minimum estimate
was then nineteen miles per second. A direct inquiry, on the other
hand, convinced him, in 1806, that the solar motion, viewed at right
angles from the distance of Sirius, would cover yearly an arc of
1″. 112. This he called “its quantity;” the corresponding velocity
remained undetermined. We can, however, now, since the real distance
of his assumed station has been determined, translate this angular
value into a linear speed of about nine miles a second. The mean of
his two estimates, or fourteen miles a second, probably differs little
from the actual rate at which the solar system is being borne to its
unimaginable destination.
His conclusions regarding the solar translation obtained little notice,
and less acceptance from his contemporaries and immediate successors.
His son rejected them as untrustworthy; Bessel, the greatest authority
of his time in the science of “how the heavens move,” declared in
1818 that the sun’s apex might be situated in any other part of the
sky with as much probability as in the constellation Hercules. Not
until Argelander, by a strict treatment of multiplied and improved
data, arrived in 1837 at practically the same result, did Herschel’s
anticipatory efforts obtain the recognition they deserved. Scarcely in
any department has there been put on record so well-directed a leap
into the dark of coming discovery.
The systematic light-measurement of the stars began with the same
untiring investigator. He described in 1796 the method since named
that of “sequences,” and presented to the Royal Society the first of
six Photometric Catalogues embracing nearly all the 2,935 stars in
Flamsteed’s “British Catalogue.” They gave comparative brightnesses
estimated with the naked eye; classification by magnitudes was put
aside as vague and misleading. The “sequences” serving for their
construction were lists of stars arranged, by repeated trials, in
order of lustre, and rendered mutually comparable by the inclusion
in each of a few members of the preceding series. Their combination
into a catalogue was then easily effected. “Simple as my method is in
principle,” he remarked, “it is very laborious in its progress.” On a
restricted scale it is still employed for following the gradations of
change in variable stars.
These researches lay, as Professor Holden expresses it, “directly on
the line of Herschel’s main work.” The separation of the stars into
light-ranks intimates at once something as to their distribution in
space; but the intimations may prove deceptive unless the divisions be
accurately established. Hence, stellar photometry is an indispensable
adjunct to the study of sidereal construction. Herschel prosecuted
the subject besides with a view to ascertaining the constancy of
stellar lustre. He had been struck with singular discordances between
magnitudes assigned at different dates. Not to mention stars obviously
variable, there were others which seemed to be affected by a slow,
secular waxing or waning. In some of the instances alleged by him,
the alteration was no doubt fictitious--a record of antique errors;
but there was a genuine residuum. Thus, the immemorially observed
constituents of the Plough preserve no fixed order of relative
brilliancy, now one, now another of the septett having, at sundry
epochs, assumed the primacy; while a small star in the same group,
Alcor, the “rider” of the second “horse,” has, in the course of a
millennium, plainly thrown off some part of its former obscurity. The
Arabs in the desert regarded it as a test of penetrating vision; and
they were accustomed to oppose “Suhel” to “Suha” (Canopus to Alcor) as
occupying respectively the highest and lowest posts in the celestial
hierarchy. So that _Vidit Alcor, at non lunam plenam_, came to be a
proverbial description of one keenly alive to trifles, but dull of
apprehension for broad facts. Now, however, Alcor is an easy naked-eye
object. One needs not be a “tailor of Breslau,” or a Siberian savage,
to see it. The little star is unmistakably more luminous than of old.
An inversion of brilliancy between Castor and Pollux, and between the
two leading stars in the Whale, is further generally admitted to have
taken place during the eighteenth century. The prevalence of such
vicissitudes was deeply impressive to Herschel, especially through
their bearing upon the past and future history of our own planet.
“If,” he said, “the similarity of stars with our sun be admitted, how
necessary will it be to take notice of the fate of our neighbouring
_suns_, in order to guess at that of our own. The _star_ which we have
dignified by the name of _Sun_ may to-morrow begin to undergo a gradual
decay of brightness, like Alpha Ceti, Alpha Draconis, Delta Ursæ
Majoris, and many other diminishing stars. It may suddenly increase
like the wonderful star in Cassiopeia, or gradually come on like
Pollux, Beta Ceti, etc. And, lastly, it may turn into a periodical one
of twenty-five days’ duration (the solar period of rotation), as Algol
is one of three days, Delta Cephei of five days, etc.” He found it,
accordingly, “perhaps the easiest way of accounting for past changes
in climate to surmise that our sun has been formerly sometimes more,
sometimes less, bright than it is at present.” Herschel attempted,
in 1798, to analyse star-colours by means of a prism applied to the
eye-glasses of his reflector. Nothing of moment could at that time come
of such experiments; but they deserve to be remembered as a sort of
premonition of future methods of research into the physical condition
of the stars.
His attention to the sun might have been exclusive, so diligent was
his scrutiny of its shining surface. Many of its peculiarities were
first described by him, and none escaped him, except the “deeper
deep,” or black nucleus of spots, detected by Dawes in 1852. The dusky
“pores” and brilliant “nodules,” the corrugations, indentations, and
ridges; the manifold aspects of spots, or “openings;” their “luminous
shelving sides,” known as penumbræ; were all noted in detail, ranged in
proper order, and studied in their mutual relations. Spots presented
themselves to him as evident depressions in the luminous disc; faculæ,
“so far from resembling torches,” appeared “like the shrivelled
elevations upon a dried apple, extended in length, and most of them
joined together, making waves, or waving lines.” Towards the north and
south, he went on, “I see no faculæ; there is all over the sun a great
unevenness, which has the appearance of a mixture of small points of
an unequal light; but they are evidently a roughness of high and low
parts.”
His theory of the solar constitution was a development of Wilson’s.
It was clearly conceived, firmly held, and boldly put forward. The
definite picturesqueness, moreover, of the language in which it was
clothed, at once laid hold of the public imagination, and gave it
a place in public favour from which it was dislodged only by the
irresistible assaults of spectrum analysis.
The sun was regarded by Herschel as a cool dark body surrounded by
an extensive atmosphere made up of various elastic fluids. Its upper
stratum--Schröter named it the “photosphere”--was of cloud-like
composition, and consisted of lucid matter precipitated from the
elastic medium by which it was sustained. Its depth was estimated at
two or three thousand miles, and the nature of its emissions suggested
a comparison with the densest coruscations of the aurora borealis.
Below lay a region of “planetary,” or protective clouds. Dense, opaque,
and highly reflective, “they must add,” he said, “a most capital
support to the splendour of the sun by throwing back so great a share
of the brightness coming to them.” Their movements betrayed the action
of vehement winds; and indeed the continual “luminous decompositions”
producing the radiating shell, with the consequent regeneration of
atmospheric gases beneath, “must unavoidably be attended with great
agitations, such as with us might even be called hurricanes.” The
formation and ascent of “empyreal gas” would cause, when moderate in
quantity, pores, or small openings in the brilliant layers. But should
it happen to be generated in uncommon quantities, “it will burst
through the planetary regions of clouds, and thus will produce great
openings; then, spreading itself above them, it will occasion large
shallows, and, mixing afterwards gradually with other superior gases,
it will promote the increase, and assist in the maintenance of the
general luminous phenomena.”
The solid globe thus girt round with cloud and fire was depicted as
a highly eligible place of residence. An equable climate, romantic
scenery, luxuriant vegetation, smiling landscapes, were to be found
there. It might, accordingly, be admitted without hesitation that “the
sun was richly stored with inhabitants.” For the lucid shell visible
from the exterior possessed, according to this theory, none of the
all-consuming ardour now attributed to it. Its blaze was a superficial
display; beneath, “the immense curtain of the planetary clouds was
everywhere closely drawn” round a world perfectly accommodated to vital
needs.
In order to reconcile this supposed state of things with the observed
order of nature, it was suggested that traces of it subsist in the
planets, “all of which, we have pretty good reason to believe, emit
light in some degree.” The night-side illumination of Venus, the
sinister glare of the eclipsed moon, the auroral glimmerings of the
earth, were adduced as evidence to this effect. The contrast between
the central body and its dependants was softened down to the utmost.
“The sun, viewed in this light,” Herschel wrote in 1794, “appears to be
nothing else than a very eminent, large, and lucid planet, evidently
the first, or, in strictness of speaking, the only primary one of our
system; all others being truly secondary to it. Its similarity to the
other globes of the solar system with regard to its solidity, its
atmosphere, and its diversified surface; the rotation upon its axis,
and the fall of heavy bodies, lead us on to suppose that it is also
most probably inhabited, like the rest of the planets, by beings whose
organs are adapted to the peculiar circumstances of that vast globe.”
To us, nearing the grey dawn of the twentieth century, the idea seems
extravagant; it was, in the eighteenth, plausible and alluring. The
philosophers of that age regarded the multiplicity of inhabited
worlds as of axiomatic certainty. The widest possible diffusion of
life followed, they held, as a corollary from the beneficence of the
Creator; while their sense of economy rendered them intolerant of
_wasted_ globes. Herschel was then reluctant to attribute to the sun
a purely _altruistic_ existence. Only from the point of view of our
small terrestrial egotism could so glorious a body figure as solely
an attractive centre, and a focus of warmth and illumination to a
group of planets. Besides, looking abroad through the universe, we see
multitudes of stars which can exercise no ministerial functions. Those
united to form compressed clusters, or simply joined in pairs, are
unlikely, it was argued, to carry a train of satellites with them in
their complex circlings. Unless, then, “we would make them mere useless
brilliant points,” they must “exist for themselves,” and claim primary
parts in the great cosmical life-drama.
Herschel’s sun is to us moderns a wholly fabulous body. Still, there
is a fantastic magnificence about the conception so strongly realised
by his powerful imagination. Moreover, its scientific value was by
no means inconsiderable. It represented the first serious effort to
co-ordinate solar phenomena; it implied the spontaneous action of some
sort of machinery for the production of light and heat. Spots were
associated with a circulatory process; the photosphere was portrayed
under its true aspect. The persistence of its hollows and heights, its
pores and rugosities, convinced Herschel that the lustrous substance
composing it was “neither a liquid nor an elastic fluid,” which should
at once subside into an unbroken level. “It exists, therefore,” he
inferred, “in the manner of lucid clouds swimming in the transparent
atmosphere of the sun.”
“The influence of this eminent body on the globe we inhabit,” he wrote,
continuing the subject in 1801, “is so great, and so widely diffused,
that it becomes almost a duty to study the operations which are carried
on upon the solar surface.” This duty he fulfilled to perfection. His
telescopic readings from the changeful solar disc were of extraordinary
precision and comprehensiveness. They show his powers as an observer
perhaps at their best. And, since reasoning was with him inseparable
from seeing, the appearances he noted took, as if of their own accord,
their proper places. The history of spots was completely traced. He
recorded their birth by the enlargement of pores; their development
and sub-division; established their connexion with faculous matter,
piled up beside them like mountain-ranges round an Alpine lake, or
flung across their cavities like blazing suspension-bridges; and
watched finally their closing-up and effacement, not even omitting the
post-mortem examination of the disturbances they left behind.
One of Herschel’s curiously original enterprises was his attempt to
ascertain a possible connexion between solar and terrestrial physics.
“I am now much inclined to believe,” he stated in 1801, “that openings
with great shallows, ridges, nodules, and corrugations, may lead us to
expect a copious emission of heat, and, therefore, mild seasons. And
that, on the contrary, pores, small indentations, and a poor appearance
of the luminous clouds, the absence of ridges and nodules, and of
large openings and shallows, will denote a spare emission of heat,
and may induce us to expect severe seasons. A constant observation
of the sun with this view, and a proper information respecting the
general mildness or severity of the seasons in all parts of the world,
may bring this theory to perfection, or refute it, if it be not well
founded.”
But the available data regarding weather-changes turning out to be
exceedingly defective, he had recourse to the celebrated expedient of
comparing the state of the sun in past years with the recorded prices
of corn. Fully admitting the inadequacy of the criterion, he still
thought that the sun being “the ultimate fountain of fertility, the
subject may deserve a short investigation, especially as no other
method is left for our choice.” He obtained, as the upshot, partial
confirmation of the surmise that “some temporary defect of vegetation”
ensued upon the subsidence of solar agitation. In plainer language,
food-stuffs tended to become dear when sun-spots were few and small. No
signs of cyclical change could, however, be made out. The discovery of
the “sun-spot period” was left to Schwabe. This admirable preliminary
effort to elicit the earth’s response to solar vicissitudes was
denounced by Brougham as a “grand absurdity;” and the readers of the
second number of the _Edinburgh Review_ were assured that “since the
publication of ‘Gulliver’s Travels,’ nothing so ridiculous had ever
been offered to the world!”
Herschel did not neglect the planets. His observations of Venus
extended from 1777 to 1793. Their principal object was to ascertain the
circumstances of the planet’s rotation; but they eluded him; which,
considering that they are still quite uncertain, is not surprising.
He would probably have communicated nothing on the subject had he not
been piqued into premature publication by Schröter’s statement that
the mountains of Venus rose to “four, five, or even six times the
perpendicular elevation of Chimborazo.” Herschel did not believe in
them, and expressed his incredulity in somewhat sarcastic terms. “As to
the mountains in Venus,” he wrote, “I may venture to say that no eye
which is not considerably better than mine, or assisted by much better
instruments, will ever get a sight of them.” He rightly inferred,
however, the presence of an extensive atmosphere from the bending of
the sun’s rays so as to form much more than a semicircular rim of light
to the dark disc of the planet when near inferior conjunction--that is,
when approximately in a right line between us and the sun. He fully
ascertained, too, the unreality of the Cytherean phantom-satellite.
The irritability visible in this paper made a solitary exception
to Herschel’s customary geniality. It might have led to a heated
controversy but for the excellent temper of Schröter’s reply.
Although we may not be prepared to gainsay Herschel’s dictum that “the
analogy between Mars and the earth is perhaps by far the greatest in
the whole solar system,” we can hardly hold it to be so probable as he
did that “its inhabitants enjoy a situation in many respects similar
to ours.” Yet the modern epoch in the physical study of Mars began
with his announcement in 1784 that its white polar caps spread and
shrank as winter and summer alternated in their respective hemispheres.
His conclusion of their being produced by snowy depositions from
“a considerable, though moderate, atmosphere,” is not likely to be
overthrown. He established, besides, the general permanence of the dark
markings, notwithstanding minor alterations due, he supposed, to the
variable distribution of clouds and vapours on the planet’s surface.
This vigilant “watcher of the skies” laid before the Royal Society,
May 6th, 1802, his “Observations of the two lately discovered Bodies.”
These were Ceres and Pallas, which, with Juno and Vesta, picked up
shortly afterwards, constituted the vanguard of the planetoid army.
Herschel foresaw its arrival. He adopted unhesitatingly Olbers’s theory
of their disruptive origin, and calculated that Mercury, the least
of the true planets, might be broken up into 35,000 masses no larger
than Pallas. An indefinite number of such fragments (about 420 are
now known) were accordingly inferred to circulate between the orbits
of Mars and Jupiter. He distinguished their peculiarities, and, since
they could with propriety be designated neither planets nor comets, he
proposed for them the name of “asteroids.” But here again he incurred,
to use his own mild phrase, “the illiberal criticism of the _Edinburgh
Review_.” “Dr. Herschel’s passion for coining words and idioms,”
Brougham declared, “has often struck us as a weakness wholly unworthy
of him. The invention of a name is but a poor achievement in him who
has discovered whole worlds.” The reviewer forgot, however, that new
things will not always fit into the framework of old terminology. He
added the contemptible insinuation that Herschel had devised the word
“asteroid” for the express purpose of keeping Piazzi’s and Olbers’s
discoveries on a lower level than his own of Uranus.
Herschel made no direct reply to the attack; only pointing out, in
December, 1804, how aptly the detection of Juno had come to verify his
forecasts. “The specific differences,” he said, “between planets and
asteroids appear now, by the addition of a third individual of the
latter species, to be more fully established; and that circumstance,
in my opinion, has added more to the ornament of our system than the
discovery of another planet could have done.”
His endeavours to determine the diameters of these small bodies were
ineffectual. Although he at first estimated those of Ceres and Pallas
at 162 and 147 miles, he admitted later his inability to decide as to
the reality of the minute discs shown by them; and they were first
genuinely measured by Professor Barnard with the great Lick refractor
in 1894.
The “trade-wind theory” of Jupiter’s belts originated with Herschel;
and he took note of the irregular drifting movements of the spots on
his surface, and their consequent uselessness for determining the
period of his rotation. That of Saturn’s he fixed quite accurately
at ten hours sixteen minutes, with a marginal uncertainty of two
minutes, the period now accepted being of ten hours fourteen minutes.
The possession by this planet of a profound atmosphere was inferred
from the changes in its belts, as well as from some curious phenomena
attending the disappearance of its satellites. They were commonly
seen to “hang on the limb”--that is, to pause during an appreciable
interval on the brink of occultation. Mimas, on one occasion, remained
thus poised during twenty minutes! For so long it was geometrically
concealed, although visible by the effect of refraction. Saturn was
an object of constant solicitude at Slough; and it was only with the
surpassing instruments mounted there that much could be learned about
Galileo’s _altissimo pianeta_. Herschel supposed, with Laplace, the
rings to be solid structures; and he added that the interval of 2,500
miles separating them “must be of considerable service to the planet
in reducing the space that is eclipsed by the shadow of the ring.” The
“crape ring” was _seen_, but not recognised. In one of his drawings it
figures as a dusky belt crossing the body of the planet.
His satellite discoveries proved exceedingly difficult to verify. The
Saturnian pair were lost, after he left them, until his son once more
drew them from obscurity. Regarding the outermost member of the system,
Japetus, discovered by Cassini in 1671, Herschel noticed, in 1792, a
singular circumstance. It was already known to vary in brightness; we
receive from it, in fact, four and a-half times more light at certain
epochs than at others. The novelty consisted in showing that this
variation depended upon the satellite’s situation in its orbit in such
a manner as to leave no doubt that, like our moon, it keeps the same
face always directed inwards towards its primary. So that Japetus was
inferred to turn on its axis in the period of its revolution round
Saturn, that is, in seventy-nine and one-third days.
“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.
The ardour with which he pursued the inquiry betrays itself in the
rapid succession of four masterly essays communicated to the Royal
Society in 1800. They contained the first exposition worth mentioning
of the properties of radiant heat. They gave the details of experiments
demonstrating its obedience to the same laws of reflection, refraction,
and dispersion as light; and showing the varieties in the absorptive
action upon it of different substances. In the third memoir of the
series, Professor Holden finds himself at a loss “which to admire
most--the marvellous skill evinced in acquiring such accurate data
with such inadequate means, and in varying and testing such a number
of questions as were suggested in the course of the investigation--or
the intellectual power shown in marshalling and reducing to a system
such intricate, and apparently self-contradictory phenomena.” There
is, indeed, scarcely one of Herschel’s researches in which his
initiative vigour and insight are more brilliantly displayed than in
this _parergon_--this task executed, as it were, out of hours. It is
only a pity that he felt compelled, by the incompatibility of their
distribution in the spectrum, to abandon his original opinion in favour
of the essential identity of light and radiant heat. The erroneous
impression left on the public mind by his recantation has hardly yet
been altogether effaced.