_Newton, occupied in grinding Hyperbolical Lenses—His first
Experiments with the Prism made in 1666—He discovers the Composition
of White Light, and the different Refrangibility of the Rays which
compose it—Abandons his Attempts to improve Refracting Telescopes
and resolves to attempt the Construction of Reflecting ones—He
quits Cambridge on account of the Plague—Constructs two Reflecting
Telescopes in 1668, the first ever executed—One of them examined by
the Royal Society, and shown to the King—He constructs a Telescope
with Glass Specula—Recent History of the Reflecting Telescope—
Mr. Airy’s Glass Specula—Hadley’s Reflecting Telescopes—Short’s—
Herschel’s—Ramage’s—Lord Oxmantown’s._
The appointment of Newton to the Lucasian chair at Cambridge seems to
have been coeval with his grandest discoveries. The first of these
of which the date is well authenticated is that of the different
refrangibility of the rays of light, which he established in 1666. The
germ of the doctrine of universal gravitation seems to have presented
itself to him in the same year, or at least in 1667; and “in the year
1666 or before”[9] he was in possession of his method of fluxions, and
he had brought it to such a state in the beginning of 1669, that he
permitted Dr. Barrow to communicate it to Mr. Collins on the 20th of
June in that year.
Although we have already mentioned, on the authority of a written
memorandum of Newton himself, that he purchased a prism at Cambridge
in 1664, yet he does not appear to have made any use of it, as he
informs us that it was in 1666 that he “procured a triangular glass
prism to try therewith the celebrated phenomena of colours.”[10] During
that year he had applied himself to the grinding of “optic glasses,
of other figures than spherical,” and having, no doubt, experienced
the impracticability of executing such lenses, the idea of examining
the phenomena of colour was one of those sagacious and fortunate
impulses which more than once led him to discovery. Descartes in his
_Dioptrice_, published in 1629, and more recently James Gregory in
his _Optica Promota_ published in 1663, had shown that parallel and
diverging rays could be reflected or refracted, with mathematical
accuracy, to a point or focus, by giving the surface a parabolic, an
elliptical, or a hyperbolic form, or some other form not spherical.
Descartes had even invented and described machines by which lenses of
these shapes could be ground and polished, and the perfection of the
refracting telescope was supposed to depend on the degree of accuracy
with which they could be executed.
In attempting to grind glasses that were not spherical, Newton seems
to have conjectured that the defects of lenses, and consequently of
refracting telescopes, might arise from some other cause than the
imperfect convergency of rays to a single point, and this conjecture
was happily realized in those fine discoveries of which we shall now
endeavour to give some account.
When Newton began this inquiry, philosophers of the highest genius
were directing all the energies of their mind to the subject of light,
and to the improvement of the refracting telescope. James Gregory of
Aberdeen had invented his reflecting telescope. Descartes had explained
the theory and exerted himself in perfecting the construction of
the common refracting telescope, and Huygens had not only executed
the magnificent instruments by which he discovered the ring and
the satellites of Saturn, but had begun those splendid researches
respecting the nature of light, and the phenomena of double refraction,
which have led his successors to such brilliant discoveries. Newton,
therefore, arose when the science of light was ready for some great
accession, and at the precise time when he was required to propagate
the impulse which it had received from his illustrious predecessors.
The ignorance which then prevailed respecting the nature and origin of
colours is sufficiently apparent from the account we have already given
of Dr. Barrow’s speculations on this subject. It was always supposed
that light of every colour was equally refracted or bent out of its
direction when it passed through any lens or prism, or other refracting
medium; and though the exhibition of colours by the prism had been
often made previous to the time of Newton, yet no philosopher seems to
have attempted to analyze the phenomena.
[Illustration: _Fig. 1._]
When he had procured his triangular glass prism, a section of
which is shown at ABC, (_fig. 1_,) he made a hole H in one of his
window-shutters, SHT, and having darkened his chamber, he let in a
convenient quantity of the sun’s light RR, which, passing through
the prism ABC, was so refracted as to exhibit all the different
colours on the wall at MN, forming an image about five times as long
as it was broad. “It was at first,” says our author, “a very pleasing
divertisement to view the vivid and intense colours produced thereby,”
but this pleasure was immediately succeeded by surprise at various
circumstances which he had not expected. According to the received
laws of refraction, he expected the image MN to be circular, like the
white image at W, which the sunbeam RR had formed on the wall previous
to the interposition of the prism; but when he found it to be no less
than five times larger than its breadth, it “excited in him a more
than ordinary curiosity to examine from whence it might proceed. He
could scarcely think that the various thickness of the glass, or the
termination with shadow or darkness, could have any influence on light
to produce such an effect: yet he thought it not amiss first to examine
those circumstances, and so find what would happen by transmitting
light through parts of the glass of divers thicknesses, or through
holes in the window of divers bignesses, or by setting the prism
without (on the other side of ST), so that the light might pass through
it and be refracted before it was terminated by the hole; but he found
none of these circumstances material. The fashion of the colours was in
all those cases the same.”
Newton next suspected that some unevenness in the glass, or other
accidental irregularity, might cause the dilatation of the colours. In
order to try this, he took another prism BCB′, and placed it in such a
manner that the light RRW passing through them both might be refracted
contrary ways, and thus returned by BCB′ into that course RRW, from
which the prism ABC had diverted it, for by this means he thought
the regular effects of the prism ABC would be destroyed by the prism
BCB′, and the irregular ones more augmented by the multiplicity of
refractions. The result was, that the light which was diffused by the
first prism ABC into an oblong form, was reduced by the second prism
BCB′ into a circular one W, with as much regularity as when it did not
pass through them at all; so that whatever was the cause of the length
of the image MN, it did not arise from any irregularity in the prism.
Our author next proceeded to examine more critically what might be
effected by the difference of the incidence of the rays proceeding from
different parts of the sun’s disk: but by taking accurate measures of
the lines and angles, he found that the angle of the emergent rays
should be 31 minutes equal to the sun’s diameter, whereas the real
angle subtended by MN at the hole H was 2° 49′. But as this computation
was founded on the hypothesis, that the sine of the angle of incidence
was proportional to the sine of the angle of refraction, which from
his own experience he could not imagine to be so erroneous as to make
that angle but 31′, which was in reality 2° 49′, yet “his curiosity
caused him again to take up his prism” ABC, and having turned it round
in both directions, so as to make the rays RR fall both with greater
and with less obliquity upon the face AC, he found that the colours on
the wall did not sensibly change their place; and hence he obtained a
decided proof that they could not be occasioned by a difference in the
incidence of the light radiating from different parts of the sun’s disk.
Newton then began to suspect that the rays, after passing through the
prism, might move in curve lines, and, in proportion to the different
degrees of curvature, might tend to different parts of the wall; and
this suspicion was strengthened by the recollection that he had often
seen a tennis-ball struck with an oblique racket describe such a curve
line. In this case a circular and a progressive motion is communicated
to the ball by the stroke, and in consequence of this, the direction
of its motion was curvilineal, so that if the rays of light were
globular bodies, they might acquire a circulating motion by their
oblique passage out of one medium into another, and thus move like the
tennis-ball in a curve line. Notwithstanding, however, “this plausible
ground of suspicion,” he could discover no such curvature in their
direction, and, what was enough for his purpose, he observed that the
difference between the length MN of the image, and the diameter of the
hole H, was proportional to their distance HM, which could not have
happened had the rays moved in curvilineal paths.
These different hypotheses, or suspicions, as Newton calls them, being
thus gradually removed, he was at length led to an experiment which
determined beyond a doubt the true cause of the elongation of the
coloured image. Having taken a board with a small hole in it, he placed
it behind the face BC of the prism, and close to it, so that he could
transmit through the hole any one of the colours in MN, and keep back
all the rest. When the hole, for example, was near C, no other light
but the red fell upon the wall at N. He then placed behind N another
board with a hole in it, and behind this board he placed another prism,
so as to receive the red light at N, which passed through this hole in
the second board. He then turned round the first prism ABC so as to
make all the colours pass in succession through these two holes, and he
marked their places on the wall. From the variation of these places, he
saw that the _red_ rays at N were less refracted by the second prism
than the _orange_ rays, the _orange_ less than the _yellow_, and so on,
the _violet_ being more refracted than all the rest.
Hence he drew the grand conclusion, _that light was not homogeneous,
but consisted of rays, some of which were more refrangible than others_.
As soon as this important truth was established, Sir Isaac saw that a
lens which refracts light exactly like a prism must also refract the
differently coloured rays with different degrees of force, bringing
the violet rays to a focus nearer the glass than the red rays. This is
shown in _fig. 2_, where LL is a convex lens, and S, L, SL rays of the
sun falling upon it in parallel directions. The violet rays existing in
the white light SL being more refrangible than the rest, will be more
refracted or bent, and will meet at V, forming there a violet image of
the sun. In like manner the yellow rays will form an image of the sun
at Y, and so on, the red rays, which are the least refrangible, being
brought to a focus at R, and there forming a red image of the sun.
[Illustration: _Fig. 2._]
Hence, if we suppose LL to be the object-glass of a telescope directed
to the sun, and MM an eye-glass through which the eye at E sees
magnified the image or picture of the sun formed by LL, it cannot see
distinctly all the different images between R and V. If it is adjusted
so as to see distinctly the _yellow_ image at Y, as it is in the
figure, it will not see distinctly either the _red_ or _violet_ images,
nor indeed any of them but the yellow one. There will consequently be a
distinct yellow image, with indistinct images of all the other colours,
producing great confusion and indistinctness of vision. As soon as Sir
Isaac perceived this result of his discovery, he abandoned his attempts
to improve the refracting telescope, and took into consideration the
principle of reflection; and as he found that rays of all colours were
reflected regularly, so that the angle of reflection was equal to the
angle of incidence, he concluded that, upon this principle, _optical
instruments might be brought to any degree of perfection imaginable_,
provided a reflecting substance could be found which could polish as
finely as glass, and reflect as much light as glass transmits, and
provided a method of communicating to it a parabolic figure could be
obtained. These difficulties, however, appeared to him very great,
and he even thought them insuperable when he considered that, as any
irregularity in a reflecting surface makes the rays deviate five or
six times more from their true path than similar irregularities in a
refracting surface, a much greater degree of nicety would be required
in figuring reflecting specula than refracting lenses.
Such was the progress of Newton’s optical discoveries, when he was
forced to quit Cambridge in 1666 by the plague which then desolated
England, and more than two years elapsed before he proceeded any
farther. In 1668 he resumed the inquiry, and having thought of a
delicate method of polishing, proper for metals, by which, as he
conceived, “the figure would be corrected to the last,” he began to put
this method to the test of experiment. At this time he was acquainted
with the proposal of Mr. James Gregory, contained in his _Optica
Promota_, to construct a reflecting telescope with two concave specula,
the largest of which had a hole in the middle of the larger speculum,
to transmit the light to an eye-glass;[11] but he conceived that it
would be an improvement on this instrument to place the eye-glass at
the side of the tube, and to reflect the rays to it by an oval plane
speculum. One of these instruments he actually executed with his own
hands; and he gave an account of it in a letter to a friend, dated
February 23d, 1668–9, a letter which is also remarkable for containing
the first allusion to his discoveries respecting colours. Previous to
this he was in correspondence on the subject with Mr. Ent, afterward
Sir George Ent, one of the original council of the Royal Society, an
eminent medical writer of his day, and President of the College of
Physicians. In a letter to Mr. Ent he had promised an account of his
telescope to their mutual friend, and the letter to which we now allude
contained the fulfilment of that promise. The telescope was six inches
long. It bore an aperture in the large speculum something more than
an inch, and as the eye-glass was a plano-convex lens, whose focal
length was one-sixth or one-seventh of an inch, it magnified about
forty times, which, as Newton remarks, was more than any six-foot
tube (meaning refracting telescopes) could do with distinctness. On
account of the badness of the materials, however, and the want of a
good polish, it represented objects less distinct than a six-feet tube,
though he still thought it would be equal to a three or four feet tube
directed to common objects. He had seen through it Jupiter distinctly
with his four satellites, and also the horns or moon-like phases of
Venus, though this last phenomenon required some niceness in adjusting
the instrument.
Although Newton considered this little instrument as in itself
contemptible, yet he regarded it as an “epitome of what might be done;”
and he expressed his conviction that a six-feet telescope might be made
after this method, which would perform as well as a sixty or a hundred
feet telescope made in the common way; and that if a common refracting
telescope could be made of the “purest glass exquisitely polished,
with the best figure that any geometrician (Descartes, &c.) hath or
can design,” it would scarcely perform better than a common telescope.
This, he adds, may seem a paradoxical assertion, yet he continues, “it
is the necessary consequence of some experiments which I have made
concerning the nature of light.”
The telescope now described possesses a very peculiar interest, as
being the first reflecting one which was ever executed and directed to
the heavens. James Gregory, indeed, had attempted, in 1664 or 1665, to
construct his instrument. He employed Messrs. Rives and Cox, who were
celebrated glass-grinders of that time, to execute a concave speculum
of six feet radius, and likewise a small one; but as they had failed
in polishing the large one, and as Mr. Gregory was on the eve of going
abroad, he troubled himself no farther about the experiment, and the
tube of the telescope was never made. Some time afterward, indeed, he
“made some trials both with a little concave and convex speculum,” but,
“possessed with the fancy of the defective figure, he would not be at
the pains to fix every thing in its due distance.”
Such were the earliest attempts to construct the reflecting telescope,
that noble instrument which has since effected such splendid
discoveries in astronomy. Looking back from the present advanced state
of practical science, how great is the contrast between the loose
specula of Gregory and the fine Gregorian telescopes of Hadley, Short,
and Veitch,—between the humble six-inch tube of Newton and the gigantic
instruments of Herschel and Ramage.
The success of this first experiment inspired Newton with fresh zeal,
and though his mind was now occupied with his optical discoveries,
with the elements of his method of fluxions, and with the expanding
germ of his theory of universal gravitation, yet with all the ardour
of youth he applied himself to the laborious operation of executing
another reflecting telescope with his own hands. This instrument,
which was better than the first, though it lay by him several years,
excited some interest at Cambridge; and Sir Isaac himself informs us,
that one of the fellows of Trinity College had completed a telescope
of the same kind, which he considered as somewhat superior to his own.
The existence of these telescopes having become known to the Royal
Society, Newton was requested to send his instrument for examination
to that learned body. He accordingly transmitted it to Mr. Oldenburg
in December, 1671, and from this epoch his name began to acquire that
celebrity by which it has been so peculiarly distinguished.
On the 11th of January, 1672, it was announced to the Royal Society
that his reflecting telescope had been shown to the king, and had
been examined by the president, Sir Robert Moray, Sir Paul Neale, Sir
Christopher Wren, and Mr. Hook. These gentlemen entertained so high an
opinion of it, that, in order to secure the honour of the contrivance
to its author, they advised the inventor to send a drawing and
description of it to Mr. Huygens at Paris. Mr. Oldenburg accordingly
drew up a description of it in Latin, which, after being corrected
by Mr. Newton, was transmitted to that eminent philosopher. This
telescope, of which the annexed is an accurate drawing, is carefully
preserved in the library of the Royal Society of London, with the
following inscription:—
“_Invented by Sir Isaac Newton and made with his own hands, 1671._”
[Illustration: _Fig. 3._
_Sir Isaac Newton’s Reflecting Telescope._]
It does not appear that Newton executed any other reflecting telescopes
than the two we have mentioned. He informs us that he repolished and
greatly improved a fourteen-feet object-glass, executed by a London
artist, and having proposed in 1678 to substitute glass reflectors in
place of metallic specula, he tried to make a reflecting telescope
on this principle four feet long, and with a magnifying power of
150. The glass was wrought by a London artist, and though it seemed
well finished, yet, when it was quicksilvered on its convex side, it
exhibited all over the glass innumerable inequalities, which gave an
indistinctness to every object. He expresses, however, his conviction
that nothing but good workmanship is wanting to perfect these
telescopes, and he recommends their consideration “to the curious in
figuring glasses.”
For a period of fifty years this recommendation excited no notice.
At last Mr. James Short of Edinburgh, an artist of consummate skill,
executed about the year 1730 no fewer than six reflecting telescopes
with glass specula, three of fifteen inches, and three of nine inches
in focal length. He found it extremely troublesome to give them a true
figure with parallel surfaces; and several of them when finished turned
out useless, in consequence of the veins which then appeared in the
glass. Although these instruments performed remarkably well, yet the
light was fainter than he expected, and from this cause, combined with
the difficulty of finishing them, he afterward devoted his labours
solely to those with metallic specula.
At a later period, in 1822, Mr. G. B. Airy of Trinity College, and
one of the distinguished successors of Newton in the Lucasian chair,
resumed the consideration of glass specula, and demonstrated that the
aberration both of figure and of colour might be corrected in these
instruments. Upon this ingenious principle Mr. Airy executed more
than one telescope, but though the result of the experiment was such
as to excite hopes of ultimate success, yet the construction of such
instruments is still a desideratum in practical science.
Such were the attempts which Sir Isaac Newton made to construct
reflecting telescopes; but notwithstanding the success of his labours,
neither the philosopher nor the practical optician seems to have had
courage to pursue them. A London artist, indeed, undertook to imitate
these instruments; but Sir Isaac informs us, that “he fell much short
of what he had attained, as he afterward understood by discoursing
with the under workmen he had employed.” After a long period of fifty
years, John Hadley, Esq. of Essex, a Fellow of the Royal Society, began
in 1719 or 1720 to execute a reflecting telescope. His scientific
knowledge and his manual dexterity fitted him admirably for such a
task, and, probably after many failures, he constructed two large
telescopes about five feet three inches long, one of which, with a
speculum six inches in diameter, was presented to the Royal Society in
1723. The celebrated Dr. Bradley and the Rev. Mr. Pound compared it
with the great Huygenian refractor 123 feet long. It bore as high a
magnifying power as the Huygenian telescope: it showed objects equally
distinct, though not altogether so clear and bright, and it exhibited
every celestial object that had been discovered by Huygens,—the five
satellites of Saturn, the shadow of Jupiter’s satellites on his disk,
the black list in Saturn’s ring, and the edge of his shadow cast on the
ring. Encouraged and instructed by Mr. Hadley, Dr. Bradley began the
construction of reflecting telescopes, and succeeded so well that he
would have completed one of them, had he not been obliged to change his
residence. Some time afterward he and the Honourable Samuel Molyneux
undertook the task together at Kew, and attempted to execute specula
about twenty-six inches in focal length; but notwithstanding Dr.
Bradley’s former experience, and Mr. Hadley’s frequent instructions,
it was a long time before they succeeded. The first good instrument
which they finished was in May, 1724. It was twenty-six inches in focal
length; but they afterward completed a very large one of eight feet,
the largest that had ever been made. The first of these instruments was
afterward elegantly fitted up by Mr. Molyneux, and presented to his
majesty John V. King of Portugal.
The great object of these two able astronomers was to reduce the
method of making specula to such a degree of certainty that they
could be manufactured for public sale. Mr. Hauksbee had indeed made a
good one about three and a half feet long, and had proceeded to the
execution of two others, one of six feet, and another of twelve feet
in focal length; but Mr. Scarlet and Mr. Hearne, having received all
the information which Mr. Molyneux had acquired, constructed them
for public sale; and the reflecting telescope has ever since been an
article of trade with every regular optician.
As Sir Isaac Newton was at this time President of the Royal Society,
he had the high satisfaction of seeing his own invention become an
instrument of public use, and of great advantage to science, and
he no doubt felt the full influence of this triumph of his skill.
Still, however, the reflecting telescope had not achieved any new
discovery in the heavens. The latest accession to astronomy had been
made by the ordinary refractors of Huygens, labouring under all the
imperfections of coloured light; and this long pause in astronomical
discovery seemed to indicate that man had carried to its farthest
limits his power of penetrating into the depths of the universe.
This, however, was only one of those stationary positions from which
human genius takes a new and a loftier elevation. While the English
opticians were thus practising the recent art of grinding specula, Mr.
James Short of Edinburgh was devoting to the subject all the energies
of his youthful mind. In 1732, and in the 22d year of his age, he
began his labours, and he carried to such high perfection the art of
grinding and polishing specula, and of giving them the true parabolic
figure, that, with a telescope fifteen inches in focal length, he
read in the Philosophical Transactions at the distance of 500 feet,
and frequently saw the five satellites of Saturn together,—a power
which was beyond the reach even of Hadley’s six-feet instrument. The
celebrated Maclaurin compared the telescopes of Short with those made
by the best London artists, and so great was their superiority, that
his small telescopes were invariably superior to larger ones from
London. In 1742, after he had settled as an optician in the metropolis,
he executed for Lord Thomas Spencer a reflecting telescope, twelve
feet in focal length, for 630_l._; in 1752 he completed one for the
King of Spain, at the expense of 1200_l._; and a short time before
his death, which took place in 1768, he finished the specula of the
large telescope which was mounted equatorially for the observatory of
Edinburgh by his brother Thomas Short, who was offered twelve hundred
guineas for it by the King of Denmark.
Although the superiority of these instruments, which were all of the
Gregorian form, demonstrated the value of the reflecting telescope, yet
no skilful hand had yet directed it to the heavens; and it was reserved
for Dr. Herschel to employ it as an instrument of discovery, to exhibit
to the eye of man new worlds and new systems, and to bring within
the grasp of his reason those remote regions of space to which his
imagination even had scarcely ventured to extend its power. So early as
1774 he completed a _five_-feet Newtonian reflector, and he afterward
executed no fewer than _two hundred_ 7 feet, _one hundred and fifty_
10 feet, and _eighty_ 20 feet specula. In 1781 he began a reflector
thirty feet long, and having a speculum thirty-six inches in diameter;
and under the munificent patronage of George III. he completed,
in 1789, his gigantic instrument forty feet long, with a speculum
_forty-nine and a half_ inches in diameter. The genius and perseverance
which created instruments of such transcendent magnitude were not
likely to terminate with their construction. In the examination of
the starry heavens, the ultimate object of his labours, Dr. Herschel
exhibited the same exalted qualifications, and in a few years he rose
from the level of humble life to the enjoyment of a name more glorious
than that of the sages and warriors of ancient times, and as immortal
as the objects with which it will be for ever associated. Nor was it
in the ardour of the spring of life that these triumphs of reason were
achieved. Dr. Herschel had reached the middle of his course before his
career of discovery began, and it was in the autumn and winter of his
days that he reaped the full harvest of his glory. The discovery of a
new planet at the verge of the solar system was the first trophy of his
skill, and new double and multiple stars, and new nebulæ, and groups of
celestial bodies were added in thousands to the system of the universe.
The spring-tide of knowledge which was thus let in upon the human mind
continued for a while to spread its waves over Europe; but when it sank
to its ebb in England, there was no other bark left upon the strand but
that of the Deucalion of Science, whose home had been so long upon its
waters.
During the life of Dr. Herschel, and during the reign, and within the
dominions of his royal patron, four new planets were added to the solar
system, but they were detected by telescopes of ordinary power; and we
venture to state, that since the reign of George III. no attempt has
been made to keep up the continuity of Dr. Herschel’s discoveries.
Mr. Herschel, his distinguished son, has indeed completed more
than one telescope of considerable size; Mr. Ramage, of Aberdeen,
has executed reflectors rivalling almost those of Slough;—and Lord
Oxmantown, an Irish nobleman of high promise, is now engaged on an
instrument of great size. But what avail the enthusiasm and the efforts
of individual minds in the intellectual rivalry of nations? When the
proud science of England pines in obscurity, blighted by the absence
of the royal favour, and of the nation’s sympathy;—when its chivalry
fall unwept and unhonoured;—how can it sustain the conflict against the
honoured and marshalled genius of foreign lands?