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clearly that, when sufficient observations of any particular
binary star had been accumulated, it would then be within the
power of the mathematician to elicit from those observations the
shape and the position in space of the path which each of the
revolving stars described around the other. Indeed, in some cases
he would be able to perform the astonishing feat of determining
from his calculations the weight of these distant suns, and thus
be enabled to compare them with the mass of our own sun.
[PLATE: NEBULA IN SOUTHERN HEMISPHERE, drawn by Sir John
Herschel.]
But this work must follow the observations, it could not precede
them. The first step was therefore to observe and to measure with
the utmost care the positions and distances of those particular
double stars which appear to offer the greatest promise in this
particular research. In 1821, Herschel and a friend of his, Mr.
James South, agreed to work together with this object. South was
a medical man with an ardent devotion to science, and possessed of
considerable wealth. He procured the best astronomical
instruments that money could obtain, and became a most
enthusiastic astronomer and a practical observer of tremendous
energy.
South and John Herschel worked together for two years in the
observation and measurement of the double stars discovered by Sir
William Herschel. In the course of this time their assiduity was
rewarded by the accumulation of so great a mass of careful
measurements that when published, they formed quite a volume in
the “Philosophical Transactions.” The value and accuracy of the
work, when estimated by standards which form proper criteria for
that period, is universally recognised. It greatly promoted the
progress of sidereal astronomy, and the authors were in
consequence awarded medals from the Royal Society, and the
Royal Astronomical Society, as well as similar testimonials from
various foreign institutions.
This work must, however, be regarded as merely introductory to the
main labours of John Herschel’s life. His father devoted the
greater part of his years as an observer to what he called his
“sweeps” of the heavens. The great reflecting telescope, twenty
feet long, was moved slowly up and down through an arc of about
two degrees towards and from the pole, while the celestial
panorama passed slowly in the course of the diurnal motion before
the keenly watching eye of the astronomer. Whenever a double star
traversed the field Herschel described it to his sister Caroline,
who, as we have already mentioned, was his invariable assistant in
his midnight watches. When a nebula appeared, then he estimated
its size and its brightness, he noticed whether it had a nucleus,
or whether it had stars disposed in any significant manner with
regard to it. He also dictated any other circumstance which he
deemed worthy of record. These observations were duly committed
to writing by the same faithful and indefatigable scribe, whose
business it also was to take a memorandum of the exact position of
the object as indicated by a dial placed in front of her desk, and
connected with the telescope.
John Herschel undertook the important task of re-observing the
various double stars and nebulae which had been discovered during
these memorable vigils. The son, however, lacked one inestimable
advantage which had been possessed by the father. John Herschel
had no assistant to discharge all those duties which Caroline had
so efficiently accomplished. He had, therefore, to modify the
system of sweeping previously adopted in order to enable all the
work both of observing and of recording to be done by himself.
This, in many ways, was a great drawback to the work of the
younger astronomer. The division of labour between the observer
and the scribe enables a greatly increased quantity of work to be
got through. It is also distinctly disadvantageous to an observer
to have to use his eye at the telescope directly after he has been
employing it for reading the graduations on a circle, by the light
of a lamp, or for entering memoranda in a note book. Nebulae,
especially, are often so excessively faint that they can only
be properly observed by an eye which is in that highly sensitive
condition which is obtained by long continuance in darkness. The
frequent withdrawal of the eye from the dark field of the
telescope, and the application of it to reading by artificial
light, is very prejudicial to its use for the more delicate
purpose. John Herschel, no doubt, availed himself of every
precaution to mitigate the ill effects of this inconvenience as
much as possible, but it must have told upon his labours as
compared with those of his father.
But nevertheless John Herschel did great work during his “sweeps.”
He was specially particular to note all the double stars which
presented themselves to his observation. Of course some little
discretion must be allowed in deciding as to what degree of
proximity in adjacent stars does actually bring them within the
category of “double stars.” Sir John set down all such objects as
seemed to him likely to be of interest, and the results of his
discoveries in this branch of astronomy amount to some thousands.
Six or seven great memoirs in the TRANSACTIONS of the Royal
Astronomical Society have been devoted to giving an account of his
labours in this department of astronomy.
[PLATE: THE CLUSTER IN THE CENTAUR, drawn by Sir John Herschel.]
One of the achievements by which Sir John Herschel is best known
is his invention of a method by which the orbits of binary stars
could be determined. It will be observed that when one star
revolves around another in consequence of the law of gravitation,
the orbit described must be an ellipse. This ellipse, however,
generally speaking, appears to us more or less foreshortened, for
it is easily seen that only under highly exceptional circumstances
would the plane in which the stars move happen to be directly
square to the line of view. It therefore follows that what we
observe is not exactly the track of one star around the other; it
is rather the projection of that track as seen on the surface of
the sky. Now it is remarkable that this apparent path is still
an ellipse. Herschel contrived a very ingenious and simple method
by which he could discover from the observations the size and
position of the ellipse in which the revolution actually takes
place. He showed how, from the study of the apparent orbit of the
star, and from certain measurements which could easily be
effected upon it, the determination of the true ellipse in which
the movement is performed could be arrived at. In other words,
Herschel solved in a beautiful manner the problem of finding the
true orbits of double stars. The importance of this work may be
inferred from the fact that it has served as the basis on which
scores of other investigators have studied the fascinating subject
of the movement of binary stars.
The labours, both in the discovery and measurement of the double
stars, and in the discussion of the observations with the object
of finding the orbits of such stars as are in actual revolution,
received due recognition in yet another gold medal awarded by the
Royal Society. An address was delivered on the occasion by the
Duke of Sussex (30th November, 1833), in the course of which,
after stating that the medal had been conferred on Sir John
Herschel, he remarks:—
“It has been said that distance of place confers the same
privilege as distance of time, and I should gladly avail myself
of the privilege which is thus afforded me by Sir John Herschel’s
separation from his country and friends, to express my
admiration of his character in stronger terms than I should
otherwise venture to use; for the language of panegyric, however
sincerely it may flow from the heart, might be mistaken for that
of flattery, if it could not thus claim somewhat of an historical
character; but his great attainments in almost every department of
human knowledge, his fine powers as a philosophical writer, his
great services and his distinguished devotion to science, the high
principles which have regulated his conduct in every relation of
life, and, above all, his engaging modesty, which is the crown of
all his other virtues, presenting such a model of an accomplished
philosopher as can rarely be found beyond the regions of
fiction, demand abler pens than mine to describe them in adequate
terms, however much inclined I might feel to undertake the task.”
The first few lines of the eulogium just quoted allude to
Herschel’s absence from England. This was not merely an episode
of interest in the career of Herschel, it was the occasion of one
of the greatest scientific expeditions in the whole history of
astronomy.
Herschel had, as we have seen, undertaken a revision of his
father’s “sweeps” for new objects, in those skies which are
visible from our latitudes in the northern hemisphere. He had
well-nigh completed this task. Zone by zone the whole of the
heavens which could be observed from Windsor had passed under his
review. He had added hundreds to the list of nebulae discovered
by his father. He had announced thousands of double stars. At
last, however, the great survey was accomplished. The contents of
the northern hemisphere, so far at least as they could be
disclosed by his telescope of twenty feet focal length, had been
revealed.
[PLATE: SIR JOHN HERSCHEL’S OBSERVATORY AT FELDHAUSEN,
Cape of Good Hope.]
But Herschel felt that this mighty task had to be supplemented by
another of almost equal proportions, before it could be said that
the twenty-foot telescope had done its work. It was only the
northern half of the celestial sphere which had been fully
explored. The southern half was almost virgin territory, for no
other astronomer was possessed of a telescope of such power as
those which the Herschels had used. It is true, of course, that
as a certain margin of the southern hemisphere was visible from
these latitudes, it had been more or less scrutinized by observers
in northern skies. And the glimpses which had thus been obtained
of the celestial objects in the southern sky, were such as to make
an eager astronomer long for a closer acquaintance with the
celestial wonders of the south. The most glorious object in the
sidereal heavens, the Great Nebula in Orion, lies indeed in that
southern hemisphere to which the younger Herschel’s attention now
became directed. It fortunately happens, however, for votaries of
astronomy all the world over, that Nature has kindly placed her
most astounding object, the great Nebula in Orion, in such a
favoured position, near the equator, that from a considerable
range of latitudes, both north and south, the wonders of the
Nebula can be explored. There are grounds for thinking that the
southern heavens contain noteworthy objects which, on the whole,
are nearer to the solar system than are the noteworthy objects in
the northern skies. The nearest star whose distance is known,
Alpha Centauri, lies in the southern hemisphere, and so also does
the most splendid cluster of stars.
Influenced by the desire to examine these objects, Sir John
Herschel determined to take his great telescope to a station in
the southern hemisphere, and thus complete his survey of the
sidereal heavens. The latitude of the Cape of Good Hope is such
that a suitable site could be there found for his purpose. The
purity of the skies in South Africa promised to provide for the
astronomer those clear nights which his delicate task of surveying
the nebulae would require.
On November 13, 1833, Sir John Herschel, who had by this time
received the honour of knighthood from William IV., sailed from
Portsmouth for the Cape of Good Hope, taking with him his
gigantic
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