HERSCHEL THE DISCOVERER.
One result of Herschel’s discoveries among the stars and nebulæ is that
his studies of the Sun and planets, with the exception of the discovery
of Uranus, have been completely thrown into the shade. Nevertheless, his
work in solar and planetary astronomy alone would have gained for him a
higher position in astronomy than his contemporaries. The planets,
satellites, and comets were all attentively studied by the great
astronomer; indeed, the scientific investigation of the surfaces of Mars
and Saturn began with Herschel.
“His attention to the Sun,” Miss Clerke truly remarks, “might have been
exclusive, so diligent was his scrutiny of its shining surface.”
Sunspots were specially investigated by Herschel, who closely studied
their peculiarities, regarding them as depressions in the solar
atmosphere. He also paid much attention to the faculæ, but could not
observe them to the north and south of the Sun, thus proving their
connection with the spots which are confined to the regions north and
south of the equator. “There is all over the Sun a great unevenness,”
said Herschel, “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.”
Herschel’s solar observations were very valuable, and did much for our
knowledge of the orb of day. His theory of the Sun’s constitution—a
development of the hypothesis put forward by _Alexander Wilson_
(1714-1786), Professor of Astronomy in Glasgow—was, however, very far
from the truth. This was almost the only instance in which Herschel was
mistaken. He regarded the Sun as a cool, dark globe, “a very eminent,
large, and lucid planet, evidently the first, or, in strictness of
speaking, the only primary one of our system.” In his opinion an
extensive atmosphere surrounded the Sun, the upper stratum forming what
Schröter named the “photosphere.” This atmosphere, estimated as two or
three thousand miles in depth, was regarded as giving out light and
heat. Below this shining atmosphere there existed, Herschel believed, a
region of clouds protecting the globe of the Sun from the glowing
atmosphere, and reflecting much of the light intercepted by them. The
spots were believed to be openings in these atmospheres, caused by the
action of winds, the umbra or dark portion of the spot thus representing
the globe of the Sun, which Herschel believed to be “richly stored with
inhabitants.” This theory held its ground for many years. Newton, it is
true, believed the Sun to be gaseous, but he propounded no hypothesis of
its constitution. Herschel’s theory, on the other hand, was fully
developed, plausible, and attractive. It was held by eminent men of
science until 1860, when the revelations of the spectroscope showed it
to be quite untenable. The theory was supported for many years by Sir
John Herschel, who, however, abandoned it in 1864. Herschel made several
attempts to ascertain whether any connection existed between the state
of the Sun and the condition of the Earth. In 1801 he was inclined to
believe that “some temporary defect of vegetation” resulted from the
absence of sun-spots, which, he thought, “may lead us to expect a
copious emission of heat, and, therefore, mild seasons.” Herschel
believed, in fact, that food became dear at the times of spot-minima. It
may be remarked that Herschel never noted the spot-period of eleven
years, the discovery of which was afterwards made by Schwabe.
Herschel closely scrutinised the surfaces of the planets. Mercury alone
was neglected by him. From 1777 to 1793 he observed Venus, with the
object of determining the rotation period, but he was unable to observe
any markings on the surface of the planet. He did not place reliance on
Schröter’s value of the rotation period (about twenty-three hours).
Meanwhile, Schröter announced the existence on Venus of mountains which
rose to five or six times the height of Chimborazo. As to these, said
Herschel, “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.” Herschel demonstrated the existence of an extensive
atmosphere round Venus.
“The analogy between Mars and the Earth,” Herschel wrote in 1783, “is
perhaps by far the greatest in the whole Solar System.” In 1777 he
began, in his house at Bath, a series of observations on the red planet,
which yielded results of the utmost importance. Fixing his attention on
the white spots at the north and south poles,—discovered by Maraldi,
nephew of Cassini,—he soon ascertained the fact that they waxed and
waned in size, the north polar cap shrinking during the summer of the
northern hemisphere, increasing in winter, and _vice versa_ in the
southern hemisphere. He regarded the caps as masses of snow and ice
deposited from “a considerable, though moderate, atmosphere,” a theory
now generally accepted. Herschel gave an immense impetus to the study of
Mars. He carefully examined the planet’s surface, and the dark markings
were regarded by him as oceans.
During Herschel’s lifetime the four small planets, Ceres, Pallas, Juno,
and Vesta, were discovered by Piazzi, Olbers, and Harding. The great
astronomer was much interested in these small worlds. He commenced a
search through the Zodiacal constellations for new planets, but failed.
He was of opinion that many minor planets would be discovered. Accepting
Olbers’ theory of the disruption of a primitive planet, Herschel
calculated that Mercury might be broken up into 35,000 globes equal to
Pallas. Meanwhile Herschel named the four new planets “Asteroids,” owing
to their minute size. He estimated the diameter of Ceres at 162 miles
and Pallas at 147 miles, but Professor Barnard’s measures have shown
them to be larger.
In connection with the discovery of the Asteroids, Herschel showed a
very fine spirit. In ‘The Edinburgh Review’ Brougham declared that
Herschel had devised the word “asteroid,” so that the discoveries of
Piazzi and Olbers might be kept on a lower level than his own discovery
of Uranus. Many scientists would have been much offended at this
contemptible insult, but Herschel merely remarked that he had incurred
“the illiberal criticism of ‘The Edinburgh Review,’” and that the
discovery of the Asteroids “added more to the ornament of our system
than the discovery of another planet could have done.”
In Herschel’s time astronomers were acquainted with three of the outer
planets,—Jupiter, Saturn, and Uranus,—all of which were closely studied
by the great astronomer. The belts of Jupiter were supposed by him to be
analogous to the “trade-winds” in the atmosphere of the Earth; while the
drifting-spots on Jupiter’s disc and their irregular movements were
carefully noted. His observations on the four satellites of Jupiter led
him to believe that, like our Moon, they rotated on their axes in a
period equal to that of their revolution round their primary—an opinion
shared by Laplace, and by many modern astronomers.
Herschel’s researches regarding Saturn were, however, much more
important than those on Jupiter. The globe of the planet, the rings and
the satellites, were favourite objects of study at Bath and Slough. In
1794 he perceived a spot on the surface of Saturn, and made the first
determination of the rotation of the planet, which he fixed as 10 hours
16 minutes,—a result confirmed by modern astronomers. The rings were
subjected to the closest scrutiny. Herschel believed them to be solid,
and he also considered them to revolve round Saturn in about 10 hours.
It appears that he observed the famous “dusky ring,” but supposed it to
be a belt on the surface of the planet. He also studied Cassini’s
division in the ring, ascertaining its reality.
On completing his famous 40-foot reflector, Herschel, on August 28,
1789, turned it on Saturn and its five known satellites. Near the
planet, and in the plane of the ring, was seen another object, which
Herschel believed to be a sixth satellite. To settle the question, he
watched the planet for several hours to see if the object would partake
in the planet’s motion. Finding that it did, he announced it as a new
satellite, which he found to revolve round Saturn in 1 day 8 hours.
About three weeks later, on September 17, Herschel discovered another
satellite yet closer to Saturn, revolving round the planet in about 22
hours. These two satellites were not seen by any astronomers except
Herschel; and after his death they could not be observed. His son,
however, rediscovered them.
The eighth satellite, Japetus, was shown by Herschel to rotate on its
axis in a period equal to that of its revolution, and his observations
were confirmed by modern observers. “I cannot,” Herschel said, “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.” In April 1805 Herschel observed the globe
of Saturn to present not a spherical but a “square-shouldered” aspect.
It was for long believed that this was an optical illusion; but Proctor
and others have shown that it is quite possible for storms in Saturn’s
atmosphere to cause the planet’s apparent distortion in shape.
Herschel paid much attention to the planet Uranus, which he discovered
on March 13, 1781. The discovery of Uranus, which was mentioned in a
previous chapter, was in a sense the most striking of Herschel’s
achievements. Uranus was the first planet discovered within the memory
of man: besides, the discovery enlarged the diameter of the Solar System
from 886 to 1772 millions of miles. Throughout his lifetime Herschel
referred to the planet as the “Georgium Sidus,” out of gratitude to
George III. for appointing him King’s Astronomer; but the astronomers of
France and Germany, who, as Sir Robert Ball remarks, “saw no reason why
the King of England should be associated with Jupiter and Saturn,”
opposed this term. Lalande called the planet “Herschel,” but Herschel’s
countrymen, the Germans, named it Uranus, in keeping with the custom of
designating the planets from the Greek mythology. The name of Uranus
ultimately prevailed.
In January 1787 Herschel discovered two satellites to Uranus, with the
aid of his 20-foot telescope. These satellites he believed to revolve
round Uranus in 8 days and 13 days respectively, and accordingly he made
a drawing of what their positions should be on February 10. On that day
he found them in their predicted places. In 1797 he announced that the
satellites revolved round Uranus in orbits at right angles to the
ecliptic, and in a retrograde direction. In subsequent years Herschel
believed that he had discovered other four satellites to Uranus, but he
was unable to confirm his belief. As Mr Gore says, some of the
satellites “must, therefore, have been either optical ‘ghosts’ or else
small fixed stars which happened to be near the planet’s path at the
time of observation. Herschel also suspected that he could see traces of
rings round Uranus like those round Saturn, but his observation was
never confirmed, either by himself or other observers.”
Although Herschel made several important observations on the Moon, and
measured the heights of the lunar mountains, he was not a devoted
student of our satellite. Caroline Herschel remarks in her memoirs that
if it had not been for clouds or moonlight, neither her brother nor
herself would have got any sleep; adding that Herschel on the moonlight
nights prepared his papers or made visits to London. However, he did
make some investigations, and in 1783 and 1787 believed himself to have
witnessed the eruption of three lunar volcanoes. He afterwards
concluded, however, that what he believed to be eruptions was really the
reflexion of earth-shine from the white peaks of the lunar mountains.
Herschel never discovered a comet, leaving that branch of astronomy to
his sister, who discovered eight of these objects. He was, however, much
interested in comets, and attentively studied them, introducing the
terms of “head,” “nucleus,” and “coma.” Herschel anticipated the view
that comets are not lasting, but are partly disintegrated at their
perihelion passages. He was of opinion that they travelled from star to
star. The extent of their tails and appendages he thought to be a test
of their age.
We have now completed our sketch of Herschel’s important labours
regarding our Solar System. As Miss Clerke says, “A whole cycle of
discoveries and successful investigations began and ended with him.” But
through observing the stars he made a further discovery in connection
with the Solar System; indeed, one of the greatest discoveries in the
history of astronomy—the movement through space of the Sun, carrying
with it planets and comets.
“If the proper motion of the stars be admitted,” said Herschel, “who can
deny that of our Sun?” Of course it was plain that the motion of the Sun
could only be detected through the resulting apparent motion of the
stars. Thus, if the Sun is moving in a certain direction, the stars in
front will appear to open out, while those behind will close up. But the
problem is by no means so easy as this. The stars are also in motion,
and, before the solar motion can be discovered, the proper motions of
the stars—themselves very minute—have to be decomposed into two parts,
the real motion of the star, and the apparent motion, resulting from the
movement of the Solar System. To any astronomer but Herschel the problem
would have been insoluble. Only sixty years had elapsed since Halley had
announced the proper motions of the brighter stars which had been
previously supposed to be immovable—hence the name of “fixed stars.”
Herschel did not deal with the motions of many stars. Only a few proper
motions were known with accuracy when he attacked the problem in 1783.
Making use of the proper motions of seven stars, and separating the real
from the apparent motion, he found that the Solar System was moving
towards a point in the constellation Hercules, the “apex” being marked
by the star λ Herculis. The rate of the solar motion, Herschel thought,
was “certainly not less than that which the Earth has in her annual
orbit.” This extraordinary discovery was one of Herschel’s greatest
works. “Its directness and apparent artlessness,” Miss Clerke remarks,
“strike us dumb with wonder.” In 1805 Herschel again attacked the
subject, utilising the proper motions of thirty-six stars. His second
inquiry, on the whole, confirmed his previous result, the “apex” being
again situated in Hercules; but the determination of 1783 was probably
the more accurate of the two.
Herschel was far in advance of his time regarding the solar motion. The
two greatest astronomers of the next generation, Bessel and Sir John
Herschel, rejected the results reached by Sir William Herschel. But in
1837 Argelander, after a profound mathematical discussion, confirmed
Herschel’s views, and proved the solar motion to be a reality. Since
that date the problem has been attacked by various methods by Otto
Struve, Gauss, Mädler, Airy, Dunkin, Ludwig Struve, Newcomb, Kapteyn,
Campbell, and others, with the result that the reality of the solar
motion and of the direction fixed by Herschel has been proved beyond a
doubt. As Sir Robert Ball well remarks, mathematicians have exhausted
every refinement, “but only to confirm the truth of that splendid theory
which seems to have been one of the flashes of Herschel’s genius.”
In his volume ‘Herschel and his Work,’ Mr James Sime writes: “To
Herschel belongs the credit not merely of having suspected the
revolution of sun around sun in the far-distant realms of space, but
also of actually detecting that this was going on among the stars.”
Throughout his career double stars were favourite objects of
observation. The study of double stars was commenced by Herschel while a
musician in Bath. Before his day, of course, double stars had been
discovered and studied, but it was believed that the proximity of two
stars was merely an optical accident, the brighter star being much
nearer to us than the other. Herschel, at first sharing the general
view, observed double stars in the hope of measuring their relative
parallaxes; assuming one star to be much farther away from the Solar
System than another, he attempted to measure the parallactic
displacement of the brighter star relatively to the position of the
fainter. “This,” he afterwards wrote, “introduced a new series of
observations. I resolved to examine every star in the heavens with the
utmost attention, that I might fix my observations upon those that would
best answer my end. I took some pains to find out what double stars had
been recorded by astronomers; but my situation permitted me not to
consult extensive libraries, nor, indeed, was it very material; for as I
intended to view the heavens myself, Nature, that great volume, appeared
to me to contain the best catalogue.”
Herschel, on January 10, 1782, submitted to the Royal Society a
catalogue of 269 double stars: of these he himself discovered 227. In
December 1784 he forwarded another catalogue, containing 434 stars. He
soon found that he was unable to measure stellar parallax, and the idea
dawned on him that the double stars were physically connected by the law
of gravitation, though he made no announcement to that effect for many
years. On July 1, 1802, Herschel informed the Royal Society that “casual
situations will not account for the multiplied phenomena of double
stars.... 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.” In 1803 he
showed that many stars were revolving round their centres of gravity,
proving them, in his own words, to be “intimately held together by the
bond of mutual attraction.” In other words, Herschel discovered that the
law of gravitation prevailed in the Stellar Universe, as well as in our
Solar System—that the law which Newton ascertained to prevail in the
Solar System extended throughout the depth of space.
Herschel did not merely prove the revolution of the binary stars; he
assigned periods to those which he had particularly studied. He believed
the period of Castor to be 342 years; γ Leonis 1200 years; δ Serpentis
375 years; and ε Böotis 1681 years. Herschel did not compute the orbits
mathematically. This was not done for nearly thirty years, when the
calculation of binary star-orbits was commenced by Savary, Sir John
Herschel, and Encke.
In 1782 the French astronomer, _Charles Messier_ (1730-1817), published
a list of 103 nebulæ. In the following year Herschel commenced his
famous sweeps of the heavens with his large reflectors, and during these
he made many remarkable discoveries. In 1786 he published in the
‘Philosophical Transactions’ of the Royal Society a catalogue of a
thousand new nebulæ and star-clusters, in which he gave the position of
each object with a short description of its appearance, written by
Caroline Herschel while her brother actually had the object before his
eyes. In 1786 Herschel published a catalogue of another thousand
clusters and nebulæ, followed in 1802 by a list of 500; making a total
of 2500 clusters and nebulæ discovered by the great astronomer. This
alone would have gained a great name for William Herschel in this branch
of astronomy. In the space of only twenty years 2500 nebulæ and clusters
had been discovered. The various nebulæ and clusters were divided into
eight classes, as follows: the first class being “bright nebulæ,” the
second “faint nebulæ,” the third “very faint nebulæ,” the fourth
“planetary nebulæ,” so named by Herschel from their resemblance to
planetary discs, the fifth class contained “very large nebulæ,” the
sixth “very compressed and rich clusters of stars,” the seventh “pretty
much compressed clusters of large or small stars,” and the eighth
“coarsely scattered clusters of stars.”
At first Herschel believed all nebulæ to be clusters of stars, the
irresolvable nebulæ being supposed to be farther from our system than
the resolvable nebulæ. As many of the nebulæ which Messier could not
resolve had yielded to Herschel’s instruments, Herschel believed that
increase of telescopic power would resolve the hazy spots of light which
remained nebulous. In the paper of 1785, in which Herschel dealt with
the construction of the heavens, he stated his belief that many of the
nebulæ were external galaxies—universes beyond the Milky Way; and in
1786 he remarked that he had discovered fifteen hundred universes!
Arago, Mitchel, Nichol, Chambers, and other writers quite misinterpreted
Herschel’s views on the nebulæ when they said that he believed them to
be all external galaxies. In 1785 Herschel believed many to be connected
with the sidereal system; considering that in some parts of the Galaxy
“the stars are now drawing towards various secondary centres, and will
in time separate into different clusters.” He was coming to the view
that the star-clusters were secondary aggregations within the Galaxy,
probably the true theory. He pointed out that in Scorpio, the cluster
Messier 80 is bounded by a black chasm, four degrees wide, from which he
believed the stars had been drawn in the course of time to form the
cluster. His sister records that one night, after a “long, awful
silence,” he exclaimed on coming on this chasm—“Hier ist wahrhaftig ein
Loch im Himmel!” (Here, truly, is a hole in the heavens.)
Herschel was now gradually giving up his theory of external galaxies and
his “disc-theory” of the Universe; but he still believed even the
nebulous objects to be irresolvable only through immensity of distance.
In 1791, however, he drew attention to a remarkable star in Taurus,
surrounded by a nebulous atmosphere, regarding which he wrote, “View,
for instance, the nineteenth cluster of my sixth class, and afterwards
cast your eye on this cloudy star. Our judgment, I will venture to say,
will be that _the nebulosity about the star is not of a starry_
_nature_. We therefore either have a central body which is not a star,
or have a star which is involved in a shining fluid, of a nature totally
unknown to us.” And with caution he added that “the envelope of a cloudy
star is more fit to produce a star by its condensation than to depend
upon the star for its existence.”
This was written in 1791, five years before Laplace propounded his
nebular theory. Meanwhile Herschel, believing that “these nebulous stars
may serve as a clue to unravel other mysterious phenomena,” found that
the theory of a “shining fluid” would suit the appearance of the
irresolvable planetary nebulæ and the great nebula in Orion much better
than the extravagant idea of “external universes.” Herschel now
considered the Orion nebula to be much nearer to the Solar System than
he formerly did, and ceased to regard it as external to the Galaxy. For
twenty years Herschel patiently observed the nebulæ, and it was not
until 1811 that he propounded his nebular hypothesis of the evolution of
the Sun and stars. He found the gaseous matter in all stages of
condensation, from the diffused cloudy nebulæ like that in Orion,
through the planetary nebula and the regular nebula, to the perfect
stars, like Sirius and the Sun. Herschel’s nebular theory was a grand
conception, and a magnificent attack on the secrets of nature.
Sir Robert Ball says: “Not from abstract speculation like Kant, nor from
mathematical suggestion like Laplace, but from accurate and laborious
study of the heavens, was the great William Herschel led to the
conception of the nebular theory of evolution.” Herschel’s nebular
theory was wider and less rigorous than that of Laplace. Laplace reached
his theory by reasoning backwards; Herschel by observing the nebulæ in
process of condensation. Consequently, while Laplace’s theory has
required modification, Herschel’s, from its width, is universally
accepted, because there is nothing mathematically rigorous in it. The
great German did not go into details like his French contemporary. He
sketched the evolution of the stars in a wider sense.
The astronomer’s “1500 universes,” Miss Clerke remarks, “had now
logically ceased to exist.” Herschel had gathered much evidence about
nebular distribution which shattered his belief in external universes,
although he still thought in 1818 that some galaxies were included among
the non-gaseous nebulæ. In 1784 Herschel pointed out that the clusters
and nebulæ “are arranged to run in strata”; and some time later he found
that the nebulæ were aggregated near the galactic poles; in other words,
where nebulæ are numerous, stars are scarce, and _vice versa_. So
rigorously did this rule hold, that when dictating his observations to
his sister Caroline, he would, on noting a paucity of stars, warn her to
“prepare for nebulæ.”
“A knowledge of the construction of the heavens has always been the
ultimate object of my observations.” So Herschel wrote in 1811. All his
investigations were secondary to the problem which was constantly before
his mind—the extent and structure of the Universe. He aspired to be the
Copernicus of the Sidereal System. Although Bruno, Kepler, Wright, Kant,
and Lambert had speculated regarding the construction of the heavens,
they had not the slightest evidence on which to base their ideas. There
was no science of sidereal astronomy. The stars were observed only to
assist navigation, and the primary object of star-catalogues was to
further knowledge of the motions of the planets. In Herschel’s day,
also, the distances of the stars had not been measured, and he had to
base his views on the distribution of the stars. In 1784, therefore, he
commenced a survey of the heavens, in order to ascertain the number of
stars in various parts of the sky. This method, which he named
“star-gauging,” consisted in counting the number of stars in the
telescopic field. Totally he secured 3400 gauges. His studies showed
that in the region of the Galaxy the stars were much more numerous than
near the galactic poles. Sometimes he saw as many as 588 stars in a
telescopic field, at other times only 2. He remarked that he had “often
known more than 50,000 pass before his sight within an hour.” Assuming
that the stars were, on the average, of about the same size, and
scattered through space with some approach to uniformity, Herschel was
able to compute the extent to which his telescope penetrated into space;
and, assuming that the Universe was finite and that his
“gauging-telescope” was sufficiently powerful to completely resolve the
Milky Way, he was enabled to sketch the shape and extent of the
Universe.
Thus Herschel concluded that the Universe extended in the direction of
the Galaxy to 850 times the mean distance of stars of the first
magnitude. In the direction of the galactic poles the thickness was only
155 times the distance of stars of the same magnitude. Herschel was thus
enabled to sketch the probable form of the Universe, which he regarded
as cloven at one of its extremities, the cleft being represented by the
famous gap in the Milky Way. The Universe was, in fact, supposed to be a
cloven disc, and the Milky Way was merely a vastly extended portion of
it and not a region of actual clustering. On this theory the clusters
and nebulæ were supposed to be galaxies external to the Universe. Even
in 1785, however, Herschel believed that there were regions in the Milky
Way where the stars were more closely clustered than others. “It would
not be difficult,” he wrote in 1785, “to point out two or three hundred
gathering clusters in our system.”
Strange to say, Herschel’s original ideas regarding the Universe were
accepted for many years by astronomical writers. Arago accepted
Herschel’s original theory, unaware that he had in reality abandoned it,
and he was followed by a host of French and English writers who did not
take the trouble to read each of Herschel’s papers, merely quoting that
of 1785, and believing that it represented his final ideas on the
subject. Even Sir John Herschel seems to have been unaware that his
father gave up the disc theory of the Universe. The famous German
astronomer, Wilhelm Struve, after an exhaustive study of Herschel’s
papers, was enabled to prove in 1847 that the theory had been abandoned
by Herschel; and in England the late R. A. Proctor independently
demonstrated the same thing. Meanwhile, supposing Herschel had not given
up his theory, it would be quite untenable. After considering the fact
that the brighter stars, down to the ninth magnitude, aggregate on the
Milky Way, Mr Gore says: “As the stars are by hypothesis supposed to be
uniformly distributed throughout every part of the disc, and as the
limiting circles for stars to the eighth and ninth magnitudes fall well
within the thickness of the disc, there is no reason why stars of these
magnitudes should not be quite as numerous in the direction of the
galactic poles as in that of the Milky Way itself. We see, therefore,
that the disc theory fails to represent the observed facts, and that
Struve and Proctor were amply justified in their opinion that the theory
is wholly untenable, and should be abandoned.”
The observations made by Herschel himself eventually proved fatal to the
disc theory—a hypothesis which he had all along held very lightly. His
ideas about subordinate clusters within the Milky Way were soon
confirmed, and though in 1799 he still adhered to the disc theory, he
wrote in 1802, “I am now convinced, by a long inspection and continued
examination of it, that the Milky Way itself consists of stars very
differently scattered from those which are immediately about us. This
immense starry aggregation is by no means uniform. The stars of which it
is composed are very unequally scattered”—a conclusion quite opposed to
the disc theory, where the Milky Way was supposed to be merely an
extended portion of the Universe.
In 1811 Herschel wrote as follows: “I must freely confess that by
continuing my sweeps of the heavens, my opinion of the arrangement of
the stars, and their magnitudes, and some other particulars, has
undergone a gradual change; and, indeed, when the novelty of the subject
is considered we cannot be surprised that many things formerly taken for
granted should on examination prove to be different from what they were
generally but incautiously supposed to be. For instance, an equal
scattering of the stars may be admitted in certain calculations; but
when we examine the Milky Way, or the closely compressed clusters of
stars, of which my catalogues have recorded so many instances, this
supposed equality of scattering must be given up.”
This was the virtual abandonment of the disc theory. Six years later
Herschel announced that in six cases he had failed to resolve the Milky
Way, stating that his telescopes could not fathom it. This was the
abandonment of his second assumption—namely, that his telescope was
sufficiently powerful to penetrate to the limits of the Universe. Yet he
still thought that some of the star-clusters might be external galaxies,
although he could not even dogmatically assert our Universe to be
limited. In an error of translation, Struve left the impression that
Herschel believed our Universe to be unfathomable or infinite, and was
obliged to devise a most artificial theory of the extinction of light to
account for the fact that the sky did not shine with the brilliance of
the Sun, which it would do were the stars infinite in number. Of course,
Herschel did not actually believe the Universe to be infinite, and, had
he lived, he would probably have shown that all the star-clusters which
we see are included within the bounds of our finite Galaxy.
In 1814 Herschel was “still engaged in a series of observations for
ascertaining a scale whereby the extent of the Universe, as far as it is
possible for us to penetrate into space, may be fathomed.” In 1817 he
described another method of star-gauging, which Arago and other writers
have confused with that which he devised in 1785. The two methods,
however, were quite distinct from each other. In the first system, one
telescope was used on different regions of the heavens; whereas in the
second method, various telescopes were used on identical regions. The
principle was that the telescopic power necessary to resolve groups of
stars indicates the distance at which the stars of the groups lie. This,
however, also assumed an equal distribution of stars, and as the late Mr
Proctor says, “I conceive that no question can exist that the principle
is unsound, and that Herschel would himself have abandoned it had he
tested it earlier in his observing career.... In applying it, Sir W.
Herschel found regions of the heavens very limited in extent, where the
brighter stars (clustered like the fainter) were easily resolved with
low powers, but where his largest telescopes could not resolve the
faintest. These regions, if the principle were true, must be long,
spike-shaped star groups, whose length is directed exactly towards the
astronomer on Earth,—an utterly incredible arrangement.”
Herschel, at the time of his death, left unsolved the problem of the
construction of the heavens. It is still unsolved, and will doubtless
remain so until astronomers know more about the distances and motions of
the stars. His last observation of the Galaxy showed that even with his
40-foot reflector he could not fathom it. Consequently, as we have
mentioned, Struve and his successors regarded the Universe as infinite—a
theory which has now received its death-blow. Herschel was undoubtedly
correct when he stated his belief in a limited Universe.
Herschel’s star-gauges, and those of his son, still remain of immense
value to astronomers in any discussion of the construction of the
heavens. Thus, although they failed to reveal to Herschel the structure
of the Universe, they have been of much use to his successors.
Herschel’s discussion of the supreme problem—the ultimate object of his
observations—constitutes one of the most interesting chapters in the
history of science, and marks a new era in human thought. In the words
of Miss Clerke: “One cannot reflect without amazement that the special
life-task set himself by this struggling musician—originally a penniless
deserter from the Hanoverian Guard—was nothing less than to search out
the ‘construction of the heavens.’ He did not accomplish it, for that
was impossible; but he never relinquished, and, in grappling with it,
laid deep and sure the foundations of sidereal science.”