Esq., possessed of a considerable jointure, and the union
proved a remarkable accession of domestic happiness. This lady
survived Sir William by several years. They had but this
son.--_Translator's Note_.
CHRONOLOGICAL TABLE
OF THE MEMOIRS OF WILLIAM HERSCHEL.[17]
1780. _Philosophical Transactions_, vol. lxx.--Astronomical
Observations on the Periodical Star in the Neck of the
Whale.--Astronomical Observations relative to the Lunar Mountains.
1781. _Phil. Trans._, vol. lxxi.--Astronomical Observations on the
Rotation of the Planets on their Axes, made with a View to decide
whether the Daily Rotation of the Earth be always the same.--On the
Comet of 1781, afterwards called the _Georgium Sidus_.
1782. _Phil. Trans._, vol. lxxii.--On the Parallax of the Fixed
Stars.--Catalogue of Double Stars.--Description of a Lamp
Micrometer, and the Method of using it.--Answers to the Doubts that
might be raised to the high magnifying Powers used by Herschel.
1783. _Phil. Trans._, vol. lxxiii.--Letter to Sir Joseph Banks on
the Name to be given to the new Planet.--On the Diameter of the
Georgium Sidus, followed by the Description of a Micrometer with
luminous or dark Disks.--On the proper Motion of the Solar System,
and the various Changes that have occurred among the Fixed Stars
since the Time of Flamsteed.
1784. _Phil. Trans._, vol. lxxiv.--On some remarkable Appearances
in the Polar Regions of Mars, the Inclination of its Axis, the
Position of its Poles, and its Spheroïdal Form.--Some Details on
the real Diameter of Mars, and on its Atmosphere.--Analysis of some
Observations on the Constitution of the Heavens.
1785. _Phil. Trans._, vol. lxxv.--Catalogue of Double Stars.--On
the Constitution of the Heavens.
1786. _Phil Trans._, vol., lxxvi.--Catalogue of a Thousand Nebulæ
and Clusters of Stars.--Researches on the Cause of a Defect of
Definition in Vision, which has been attributed to the Smallness of
the Optic Pencils.
1787. _Phil. Trans._, vol. lxxvii.--Remarks on the new
Comet.--Discovery of Two Satellites revolving round George's
Planet.--On Three Volcanoes in the Moon.
1788. _Phil. Trans._, vol. lxxviii.--On George's Planet (Uranus)
and its Satellites.
1789. _Phil. Trans._, vol. lxxix.--Observations on a Comet.
Catalogue of a Second Thousand new Nebulæ and Clusters of
Stars.--Some Preliminary Remarks on the Constitution of the
Heavens.
1790. _Phil. Trans._, vol. lxxx.--Discovery of Saturn's Sixth and
Seventh Satellites; with Remarks on the Constitution of the Ring,
on the Planet's Rotation round an Axis, on its Spheroïdal Form, and
on its Atmosphere.--On Saturn's Satellites, and the Rotation of the
Ring round an Axis.
1791. _Phil. Trans._, vol. lxxxi.--On the Nebulous Stars and the
Suitableness of this Epithet.
1792. _Phil. Trans._, vol. lxxxii.--On Saturn's Ring, and the
Rotation of the Planet's Fifth Satellite round an Axis.--Mixed
Observations.
1793. _Phil. Trans._, vol. lxxxiii.--Observations on the Planet
Venus.
1794. _Phil. Trans._, vol. lxxxiv.--Observations on a Quintuple
Band in Saturn.--On some Peculiarities observed during the last
Solar Eclipse.--On Saturn's Rotation round an Axis.
1795. _Phil. Trans._, vol. lxxxv.--On the Nature and Physical
Constitution of the Sun and Stars.--Description of a Reflecting
Telescope forty feet in length.
1796. _Phil. Trans._, vol. lxxxvi.--Method of observing the Changes
that happen to the Fixed Stars; Remarks on the Stability of our
Sun's Light.--Catalogue of Comparative Brightness, to determine the
Permanency of the Lustre of Stars.--On the Periodical Star _a_
Herculis, with Remarks tending to establish the Rotatory Motion of
the Stars on their Axes; to which is added a second Catalogue of
the Brightness of the Stars.
1797. _Phil. Trans._, vol. lxxxvii.--A Third Catalogue of the
comparative Brightness of the Stars; with an Introductory Account
of an Index to Mr. Flamsteed's Observations of the Fixed Stars,
contained in the Second Volume of the Historia Coelestis to which
are added several useful Results derived from that
Index.--Observations of the changeable Brightness of the Satellites
of Jupiter, and of the Variation in their apparent Magnitudes; with
a Determination of the Time of their rotary Motions on their Axes,
to which is added a Measure of the Diameter of the Second
Satellite, and an Estimate of the comparative Size of the Fourth.
1798. _Phil. Trans._, vol. lxxxviii.--On the Discovery of Four
additional Satellites of the Georgium Sidus. The retrograde Motion
of its old Satellites announced; and the Cause of their
Disappearance at certain Distances from the Planet explained.
1799. _Phil. Trans._, vol. lxxxix.--A Fourth Catalogue of the
comparative Brightness of the Stars.
1800. _Phil. Trans._, vol. xc.--On the Power of penetrating into
Space by Telescopes, with a comparative Determination of the Extent
of that Power in Natural Vision, and in Telescopes of various Sizes
and Constructions; illustrated by select
Observations.--Investigation of the Powers of the Prismatic Colours
to heat and illuminate Objects; with Remarks that prove the
different Refrangibility of radiant Heat; to which is added an
Inquiry into the Method of viewing the Sun advantageously with
Telescopes of large Apertures and high magnifying
Powers.--Experiments on the Refrangibility of the Invisible Rays of
the Sun.--Experiments on the Solar and on the Terrestrial Rays that
occasion Heat; with a comparative View of the Laws to which Light
and Heat, or rather the Rays which occasion them, are subject, in
order to determine whether they are the same or different.
1801. _Phil. Trans._, vol. xci.--Observations tending to
investigate the Nature of the Sun, in order to find the Causes or
Symptoms of its variable Emission of Light and Heat; with Remarks
on the Use that may possibly be drawn from Solar
Observations.--Additional Observations tending to investigate the
Symptoms of the variable Emission of the Light and Heat of the Sun;
with Trials to set aside darkening Glasses, by transmitting the
Solar Rays through Liquids, and a few Remarks to remove Objections
that might be made against some of the Arguments contained in the
former paper.
1802. _Phil. Trans._, vol. xcii.--Observations on the two lately
discovered celestial Bodies (Ceres and Pallas).--Catalogue of 500
new Nebulæ and Clusters of Stars, with Remarks on the Construction
of the Heavens.
1803. _Phil. Trans._, vol. xciii.--Observations of the Transit of
Mercury over the Disk of the Sun; to which is added an
Investigation of the Causes which often prevent the proper Action
of Mirrors.--Account of the Changes that have happened during the
last Twenty-five Years in the relative Situation of Double Stars;
with an Investigation of the Cause to which they are owing.
1804. _Phil. Trans._, vol. xciv.--Continuation of an Account of the
Changes that have happened in the relative Situation of Double
Stars.
1805. _Phil. Trans._, vol. xcv.--Experiments for ascertaining how
far Telescopes will enable us to determine very small Angles, and
to distinguish the real from the spurious Diameters of Celestial
and Terrestrial Objects: with an Application of the Result of these
Experiments to a Series of Observations on the Nature and Magnitude
of Mr. Harding's lately discovered Star.--On the Direction and
Velocity of the Motion of the Sun and Solar System.--Observation on
the singular Figure of the Planet Saturn.
1806. _Phil. Trans._, vol. xcvi.--On the Quantity and Velocity of
the Solar Motion.--Observations on the Figure, the Climate, and the
Atmosphere of Saturn and its Ring.
1807. _Phil. Trans._, vol. xcvii.--Experiments for investigating
the Cause of the Coloured Concentric Rings, discovered by Sir Isaac
Newton between two Object-glasses laid one upon
another.--Observations on the Nature of the new celestial Body
discovered by Dr. Olbers, and of the Comet which was expected to
appear last January in its Return from the Sun.
1808. _Phil. Trans._, vol. xcviii.--Observations of a Comet, made
with a view to investigate its Magnitude, and the Nature of its
Illumination. To which is added, an Account of a new Irregularity
lately perceived in the Apparent Figure of the Planet Saturn.
1809. _Phil. Trans._, vol. xcix.--Continuation of Experiments for
investigating the Cause of Coloured Concentric Rings, and other
Appearances of a similar Nature.
1810. _Phil. Trans._, vol. c.--Supplement to the First and Second
Part of the Paper of Experiments for investigating the Cause of
Coloured Concentric Rings between Object-glasses, and other
Appearances of a similar Nature.
1811. _Phil. Trans._, vol. ci.--Astronomical Observations relating
to the Construction of the Heavens, arranged for the Purpose of a
critical Examination, the Result of which appears to throw some new
Light upon the Organization of the Celestial Bodies.
1812. _Phil. Trans._, vol. cii.--Observations of a Comet, with
Remarks on the Construction of its different Parts.--Observations
of a Second Comet, with Remarks on its Construction.
1814. _Phil. Trans._, vol. civ.--Astronomical Observations relating
to the Sidereal Part of the Heavens, and its Connection with the
Nebulous Part; arranged for the Purpose of a critical Examination.
1815. _Phil. Trans._, vol. cv.--A Series of Observations of the
Satellites of the Georgian Planet, including a Passage through the
Node of their Orbits; with an Introductory Account of the
Telescopic Apparatus that has been used on this Occasion, and a
final Exposition of some calculated Particulars deduced from the
Observations.
1817. _Phil. Trans._, vol. cvii.--Astronomical Observations and
Experiments tending to investigate the Local Arrangement of the
Celestial Bodies in Space, and to determine the Extent and
Condition of the Milky Way.
1818. _Phil. Trans._, vol. cviii.--Astronomical Observations and
Experiments selected for the Purpose of ascertaining the relative
Distances of Clusters of Stars, and of investigating how far the
Power of Telescopes may be expected to reach into Space, when
directed to ambiguous Celestial Objects.
1822. _Memoirs of the Astronomical Society of London._--On the
Positions of 145 new Double Stars.
The chronological and detailed analysis of so many labours would throw
us into numerous repetitions. A systematic order will be preferable; it
will more distinctly fix the eminent place that Herschel will never
cease to occupy in the small group of our contemporary men of genius,
whilst his name will reëcho to the most distant posterity. The variety
and splendour of Herschel's labours vie with their extent. The more we
study them, the more we must admire them. It is with great men, as it is
with great movements in the arts, we cannot understand them without
studying them under various points of view.
Let us here again make a general reflection. The memoirs of Herschel
are, for the greater part, pure and simple extracts from his
inexhaustible journals of observations at Slough, accompanied by a few
remarks. Such a table would not suit historical details. In these
respects the author has left almost every thing to his biographers to do
for him. And they must impose on themselves the task of assigning to the
great astronomer's predecessors the portion that legitimately belongs to
them, out of the mass of discoveries, which the public (we must say) has
got into an erroneous habit of referring too exclusively to Herschel.
At one time I thought of adding a note to the analysis of each of the
illustrious observer's memoirs, containing a detailed indication of the
improvements or corrections that the progressive march of science has
brought on. But in order to avoid an exorbitant length in this
biography, I have been obliged to give up my project. In general I shall
content myself with pointing out what belongs to Herschel, referring to
my _Treatise on Popular Astronomy_ for the historical details. The life
of Herschel had the rare advantage of forming an epoch in an extensive
branch of astronomy; it would require us almost to write a special
treatise on astronomy, to show thoroughly the importance of all the
researches that are due to him.
FOOTNOTE:
[17] These titles are copied direct from the Philosophical
Transactions, instead of being retranslated.--_Translator's
Note_.
IMPROVEMENTS IN THE MEANS OF OBSERVATION.
The improvements that Herschel made in the construction and management
of telescopes have contributed so directly to the discoveries with which
that observer enriched astronomy, that we cannot hesitate to bring them
forward at once.
I read the following passage in a Memoir by Lalande, printed in 1783,
and forming part of the preface to vol. viii. of the _Ephemerides of the
Celestial Motions_.
"Each time that Herschel undertakes to polish a mirror (of a telescope),
he condemns himself to ten, or twelve, or even fourteen hours' constant
work. He does not quit his workshop for a minute, not even to eat, but
receives from the hands of his sister that nourishment without which one
could not undergo such prolonged fatigue. Nothing in the world would
induce Herschel to abandon his work; for, according to him, it would be
to spoil it."
The advantages that Herschel found in 1783, 1784, and 1785, in
employing telescopes of twenty feet and with large apertures, made him
wish to construct much larger still. The expense would be considerable;
King George III. provided for it. The work, begun about the close of
1785, was finished in August, 1789. This instrument had an iron
cylindrical tube, thirty-nine feet four inches English in length, and
four feet ten inches in diameter. Such dimensions are enormous compared
with those of telescopes made till then. They will appear but small,
however, to persons who have heard the report of a pretended ball given
in the Slough telescope. The propagators of this popular rumour had
confounded the astronomer Herschel with the brewer Meux, and a cylinder
in which a man of the smallest stature could scarcely stand upright,
with certain wooden vats, as large as a house, in which beer is made and
kept in London.
Herschel's telescope, forty English feet[18] in length, allowed of the
realization of an idea, the advantages of which would not be
sufficiently appreciated if I did not here recall to mind some facts.
In any telescope, whether refracting or reflecting, there are two
principal parts: the part that forms the aërial images of the distant
objects, and the small lens by the aid of which these images are
enlarged just as if they consisted of radiating matter. When the image
is produced by means of a lenticular glass, the place it occupies will
be found in the prolongation of the line that extends from the object to
the centre of the lens. The astronomer, furnished with an eye-piece, and
wishing to examine that image, must necessarily place himself _beyond_
the point where the rays that form it have crossed each other; _beyond_,
let us carefully remark, means _farther off_ from the object-glass. The
observer's head, his body, cannot then injure the formation or the
brightness of the image, however small may be the distance from which we
have to study it. But it is no longer thus with the image formed by
means of reflection. For the image is now placed between the object and
the reflecting mirror; and when the astronomer approaches in order to
examine it, he inevitably intercepts, if not the totality, at least a
very considerable portion of the luminous rays, which would otherwise
have contributed to give it great splendour. It will now be understood,
why in optical instruments where the images of distant objects are
formed by the reflection of light, it has been necessary to carry the
images, by the aid of a second reflection, out of the tube that contains
and sustains the principal mirror. When the small mirror, on the surface
of which the second reflection is effected, is plane, and inclined at an
angle of 45° to the axis of the telescope; when the image is reflected
laterally, through an opening made near the edge of the tube and
furnished with an eye-piece; when, in a word, the astronomer looks
definitively in a direction perpendicular to the line described by the
luminous rays coming from the object and falling on the centre of the
great mirror, then the telescope is called _Newtonian_. But in the
_Gregorian_ telescope, the image formed by the principal mirror falls on
a second mirror, which is very small, slightly curved, and parallel to
the first. The small mirror reflects the first image and throws it
beyond the large mirror, through an opening made in the middle of that
principal mirror.
Both in the one and in the other of these two telescopes, the small
mirror interposed between the object and the great mirror forms relative
to the latter a sort of screen which prevents its entire surface from
contributing towards forming the image. The small mirror, also, in
regard to intensity, gives some trouble.
Let us suppose, in order to clear up our ideas, that the material of
which the two mirrors are made, reflects only half of the incident
light. In the course of the first reflection, the immense quantity of
rays that the aperture of the telescope had received, may be considered
as reduced to half. Nor is the diminution less on the small mirror. Now,
half of half is a quarter. Therefore the instrument will send to the eye
of the observer only a quarter of the incident light that its aperture
had received. These two causes of diminished light not existing in a
refracting telescope, it would give, under parity of dimensions, four
times more[19] light than a Newtonian or Gregorian telescope gives.
Herschel did away with the small mirror in his large telescope. The
large mirror is not mathematically centred in the large tube that
contains it, but is placed rather obliquely in it. This slight obliquity
causes the images to be formed not in the axis of the tube, but very
near its circumference, or outer mouth, we may call it. The observer may
therefore look at them there direct, merely by means of an eye-piece. A
small portion of the astronomer's head, it is true, then encroaches on
the tube; it forms a screen, and interrupts some incident rays. Still,
in a large telescope, the loss does not amount to half by a great deal;
which it would inevitably do if the small mirror were there.
Those telescopes, in which the observer, placed at the anterior
extremity of the tube, looks direct into the tube and turns his back to
the objects, were called by Herschel _front view telescopes_. In vol.
lxxvi. of the _Philosophical Transactions_ he says, that the idea of
this construction occurred to him in 1776, and that he then applied it
unsuccessfully to a ten-foot telescope; that during the year 1784, he
again made a fruitless trial of it in a twenty-foot telescope. Yet I
find that on the 7th of September 1784, he recurred to a _front view_ in
observing some nebulæ and groups of stars. However discordant these
dates may be, we cannot without injustice neglect to remark, that a
front view telescope was already described in 1732, in volume vi. of the
collection entitled _Machines and Inventions approved by the Academy of
Sciences_. The author of this innovation is Jaques Lemaire, who has been
unduly confounded with the English Jesuit, Christopher Maire, assistant
to Boscovitch, in measuring the meridian comprised between Rome and
Rimini. Jaques Lemaire having only telescopes of moderate dimensions in
view, was obliged, in order not to sacrifice any of the light, to place
the great mirror so obliquely, that the image formed by its surface
should fall entirely outside the tube of the instrument. So great a
degree of inclination would certainly deform the objects. The _front
view_ construction is admissible only in very large telescopes.
I find in the _Transactions_ for 1803, that in solar observations,
Herschel sometimes employed telescopes, the great mirror of which was
made of glass. It was a telescope of this sort that he used for
observing the transit of Mercury on the 9th of November, 1802. It was
seven English feet long, and six inches and three tenths in diameter.
Practical astronomers know how much the mounting of a telescope
contributes to produce correct observations. The difficulty of a solid
yet very movable mounting, increases rapidly with the dimensions and
weight of an instrument. We may then conceive that Herschel had to
surmount many obstacles, to mount a telescope suitably, of which the
mirror alone weighed upwards of 1000 kilogrammes (_a ton_). But he
solved this problem to his entire satisfaction by the aid of a
combination of spars, of pulleys, and of ropes, of all which a correct
idea may be formed by referring to the woodcut we have given in our
_Treatise on Popular Astronomy_ (vol. i.). This great apparatus, and the
entirely different stands that Herschel imagined for telescopes of
smaller dimensions, assign to that illustrious observer a distinguished
place amongst the most ingenious mechanics of our age.
Persons in general, I may even say the greater part of astronomers, know
not what was the effect that the great forty-foot telescope had in the
labours and discoveries of Herschel. Still, we are not less mistaken
when we fancy that the observer of Slough always used this telescope,
than in maintaining with Baron von Zach (see _Monatliche Correspondenz_,
January, 1802), that the colossal instrument was of no use at all, that
it did not contribute to any one discovery, that it must be considered
as a mere object of curiosity. These assertions are distinctly
contradicted by Herschel's own words. In the volume of _Philosophical
Transactions_ for the year 1795 (p. 350), I read for example: "On the
28th of August 1789, having directed my telescope (of forty feet) to the
heavens, I discovered the sixth satellite of Saturn, and I perceived the
spots on that planet, better than I had been able to do before." (See
also, relative to this sixth satellite, the _Philosophical
Transactions_ for 1790, p. 10.) In that same volume of 1790, p. 11, I
find: "The great light of my forty-foot telescope was then so useful,
that on the 17th of September 1789, I remarked the seventh satellite,
then situated at its greatest western elongation."
The 10th of October, 1791, Herschel saw the ring of Saturn and the
fourth satellite, looking in at the mirror of his forty-foot telescope,
with his naked eye, without any sort of eye-piece.
Let us acknowledge the true motives that prevented Herschel from oftener
using his telescope of forty feet. Notwithstanding the excellence of the
mechanism, the manoeuvring of that instrument required the constant
aid of two labourers, and that of another person charged with noting the
time at the clock. During some nights when the variation of temperature
was considerable, this telescope, on account of its great mass, was
always behindhand with the atmosphere in thermometric changes, which was
very injurious to the distinctness of the images.
Herschel found that in England, there are not above a hundred hours in a
year during which the heavens can be advantageously observed with a
telescope of forty feet, furnished with a magnifying power of a
thousand. This remark led the celebrated astronomer to the conclusion,
that, to take a complete survey of the heavens with his large
instrument, though each successive field should remain only for an
instant under inspection, would not require less than eight hundred
years.
Herschel explains in a very natural way the rare occurrence of the
circumstances in which it is possible to make good use of a telescope of
forty feet, and of very large aperture.