_He delivers a Course of Optical Lectures at Cambridge—Is elected
Fellow of the Royal Society—He communicates to them his Discoveries
on the different Refrangibility and Nature of Light—Popular Account
of them—They involve him in various Controversies—His Dispute with
Pardies—Linus—Lucas—Dr. Hooke and Mr. Huygens—The Influence of
these Disputes on the Mind of Newton._
Although Newton delivered a course of lectures on optics in the
University of Cambridge in the years 1669, 1670, and 1671, containing
his principal discoveries relative to the different refrangibility of
light, yet it is a singular circumstance, that these discoveries should
not have become public through the conversation or correspondence of
his pupils. The Royal Society had acquired no knowledge of them till
the beginning of 1672, and his reputation in that body was founded
chiefly on his reflecting telescope. On the 23d December, 1671, the
celebrated Dr. Seth Ward, Lord Bishop of Sarum, who was the author of
several able works on astronomy, and had filled the astronomical chair
at Oxford, proposed Mr. Newton as a Fellow of the Royal Society.
The satisfaction which he derived from this circumstance appears to
have been considerable; and in a letter to Mr. Oldenburg, of the 6th
January, he says, “I am very sensible of the honour done me by the
Bishop of Sarum in proposing me a candidate; and which, I hope, will
be further conferred upon me by my election into the Society; and if
so, I shall endeavour to testify my gratitude, by communicating what
my poor and solitary endeavours can effect towards the promoting your
philosophical designs.” His election accordingly took place on the
11th January, the same day on which the Society agreed to transmit a
description of his telescope to Mr. Huygens at Paris. The notice of his
election, and the thanks of the Society for the communication of his
telescope, were conveyed in the same letter, with an assurance that
the Society “would take care that all right should be done him in the
matter of this invention.” In his next letter to Oldenburg, written
on the 18th January, 1671–2, he announces his optical discoveries in
the following remarkable manner: “I desire that in your next letter
you would inform me for what time the Society continue their weekly
meetings; because if they continue them for any time, I am purposing
them, to be considered of and examined, an account of a philosophical
discovery which induced me to the making of the said telescope; and
I doubt not but will prove much more grateful than the communication
of that instrument; being in my judgment the oddest, if not the most
considerable detection which hath hitherto been made in the operations
of nature.”
This “considerable detection” was the discovery of the different
refrangibility of the rays of light which we have already explained,
and which led to the construction of his reflecting telescope. It
was communicated to the Royal Society in a letter to Mr. Oldenburg,
dated February 6th, and excited great interest among its members. The
“solemn thanks” of the meeting were ordered to be transmitted to its
author for his “very ingenious discourse.” A desire was expressed to
have it immediately printed, both for the purpose of having it well
considered by philosophers, and for “securing the considerable notices
thereof to the author against the arrogations of others;” and Dr. Seth
Ward, Bishop of Salisbury, Mr. Boyle, and Dr. Hooke were desired to
peruse and consider it, and to bring in a report upon it to the Society.
The kindness of this distinguished body, and the anxiety which they
had already evinced for his reputation, excited on the part of Newton
a corresponding feeling, and he gladly accepted of their proposal to
publish his discourse in the monthly numbers in which the Transactions
were then given to the world. “It was an esteem,” says he,[12] “of the
Royal Society for most candid and able judges in philosophical matters,
encouraged me to present them with that discourse of light and colours,
which since they have so favourably accepted of, I do earnestly desire
you to return them my cordial thanks. I before thought it a great
favour to be made a member of that honourable body; but I am now more
sensible of the advantages; for believe me, sir, I do not only esteem
it a duty to concur with you in the promotion of real knowledge; but a
great privilege, that, instead of exposing discourses to a prejudiced
and common multitude, (by which means many truths have been baffled and
lost), I may with freedom apply myself to so judicious and impartial
an assembly. As to the printing of that letter, I am satisfied in
their judgment, or else I should have thought it too straight and
narrow for public view. I designed it only to those that know how to
improve upon hints of things; and, therefore, to spare tediousness,
omitted many such remarks and experiments as might be collected by
considering the assigned laws of refractions; some of which I believe,
with the generality of men, would yet be almost as taking as any I
described. But yet, since the Royal Society have thought it fit to
appear publicly, I leave it to their pleasure: and perhaps to supply
the aforesaid defects, I may send you some more of the experiments to
second it (if it be so thought fit), in the ensuing Transactions.”
Following the order which Newton himself adopted, we have, in the
preceding chapter, given an account of the leading doctrine of the
different refrangibility of light, and of the attempts to improve the
reflecting telescope which that discovery suggested. We shall now,
therefore, endeavour to make the reader acquainted with the other
discoveries respecting colours which he at this time communicated to
the Royal Society.
[Illustration: _Fig. 4._]
Having determined, by experiments already described, that a beam of
white light, as emitted from the sun, consisted of seven different
colours, which possess different degrees of refrangibility, he measured
the relative extent of the coloured spaces, and found them to have
the proportions shown in _fig. 4_, which represents the _prismatic_
spectrum, and which is nothing more than an elongated image of the sun
produced by the rays being separated in different degrees from their
original direction, the _red_ being refracted _least_, and the _violet
most_ powerfully.
If we consider light as consisting of minute particles of matter,
we may form some notion of its decomposition by the prism from the
following popular illustration. If we take steel filings of seven
different degrees of fineness and mix them together, there are two ways
in which we may conceive the mass to be decomposed, or, what is the
same thing, all the seven different kinds of filings separated from
each other. By means of seven sieves of different degrees of fineness,
and so made that the finest will just transmit the finest powder and
detain all the rest, while the next in fineness transmits the two
finest powders and detains all the rest, and so on, it is obvious that
all the powders may be completely separated from each other. If we
again mix all the steel filings, and laying them upon a table, hold
high above them a flat bar magnet, so that none of the filings are
attracted, then if we bring the magnet nearer and nearer, we shall come
to a point where the finest filings are drawn up to it. These being
removed, and the magnet brought nearer still, the next finest powders
will be attracted, and so on till we have thus drawn out of the mass
all the powders in a separate state. We may conceive the bar magnet to
be inclined to the surface of the steel filings, and so moved over the
mass, that at the end nearest to them the heaviest or coarsest will
be attracted, and all the remotest and the finest or lighter filings,
while the rest are attracted to intermediate points, so that the seven
different filings are not only separated, but are found adhering in
separate patches to the surface of the flat magnet. The first of these
methods, with the sieves, may represent the process of decomposing
light, by which certain rays of white light are absorbed, or stifled,
or stopped in passing through bodies, while certain other rays are
transmitted. The second method may represent the process of decomposing
light by refraction, or by the attraction of certain rays farther from
their original direction than other rays, and the different patches of
filings upon the flat magnet may represent the spaces on the spectrum.
When a beam of white light is decomposed into the seven different
colours of the spectrum, any particular colour, when once separated
from the rest, is not susceptible of any change, or farther
decomposition, whether it is refracted through prisms or reflected from
mirrors. It may become fainter or brighter, but Newton never could, by
any process, alter its colour or its refrangibility.
Among the various bodies which act upon light, it is conceivable
that there might have been some which acted least upon the violet
rays and most upon the red rays. Newton, however, found that this
never took place; but that the same degree of refrangibility always
belonged to the same colour, and the same colour to the same degree of
refrangibility.
Having thus determined that the seven different colours of the spectrum
were original or simple, he was led to the conclusion that _whiteness_
or white light is a compound of all the seven colours of the spectrum,
in the proportions in which they are represented in _fig. 4_. In order
to prove this, or what is called the recomposition of white light out
of the seven colours, he employed three different methods.
[Illustration: _Fig. 5._]
When the beam RR was separated into its elementary colours by the prism
ABC, he received the colours on another prism BCB′, held either close
to the first or a little behind it, and by the opposite refraction of
this prism they were all refracted back into a beam of white light BW,
which formed a white circular image on the wall at W, similar to what
took place before any of the prisms were placed in its way.
The other method of recomposing white light consisted in making the
spectrum fall upon a lens at some distance from it. When a sheet of
white paper was held behind the lens, and removed to a proper distance,
the colours were all refracted into a circular spot, and so blended as
to reproduce light so perfectly white as not to differ sensibly from
the direct light of the sun.
The last method of recomposing white light was one more suited to
vulgar apprehension. It consisted in attempting to compound a white by
mixing the coloured powders used by painters. He was aware that such
colours, from their very nature, could not compose a pure white; but
even this imperfection in the experiment he removed by an ingenious
device. He accordingly mixed one part of _red lead_, four parts of
_blue bice_, and a proper proportion of _orpiment_ and _verdigris_.
This mixture was _dun_, like wood newly cut, or like the human skin. He
now took one-third of the mixture and rubbed it thickly on the floor
of his room, where the sun shone upon it through the opened casement,
and beside it, in the shadow, he laid a piece of white paper of the
same size. “Then going from them to the distance of twelve or eighteen
feet, so that he could not discern the unevenness of the surface of
the powder nor the little shadows let fall from the gritty particles
thereof; the powder appeared intensely white, so as to transcend even
the paper itself in whiteness.” By adjusting the relative illumination
of the powders and the paper, he was able to make them both appear of
the very same degree of whiteness. “For,” says he, “when I was trying
this, a friend coming to visit me, I stopped him at the door, and
before I told him what the colours were, or what I was doing, I asked
him which of the two whites were the best, and wherein they differed!
And after he had at that distance viewed them well, he answered, that
they were both good whites, and that he could not say which was best,
nor wherein their colours differed.” Hence Newton inferred that perfect
whiteness may be compounded of different colours.
As all the various shades of colour which appear in the material world
can be imitated by intercepting certain rays in the spectrum, and
uniting all the rest, and as bodies always appear of the same colour
as the light in which they are placed, he concluded, that the colours
of natural bodies are not qualities inherent in the bodies themselves,
but arise from the disposition of the particles of each body to stop or
absorb certain rays, and thus to reflect more copiously the rays which
are not thus absorbed.
No sooner were these discoveries given to the world than they were
opposed with a degree of virulence and ignorance which have seldom
been combined in scientific controversy. Unfortunately for Newton, the
Royal Society contained few individuals of pre-eminent talent capable
of appreciating the truth of his discoveries, and of protecting him
against the shafts of his envious and ignorant assailants. This eminent
body, while they held his labours in the highest esteem, were still
of opinion that his discoveries were fair subjects of discussion,
and their secretary accordingly communicated to him all the papers
which were written in opposition to his views. The first of these
was by a Jesuit named Ignatius Pardies, Professor of Mathematics at
Clermont, who pretended that the elongation of the sun’s image arose
from the inequal incidence of the different rays on the first face
of the prism, although Newton had demonstrated in his own discourse
that this was not the case. In April, 1672, Newton transmitted to
Oldenburg a decisive reply to the animadversions of Pardies; but,
unwilling to be vanquished, this disciple of Descartes took up a fresh
position, and maintained that the elongation of the spectrum might be
explained by the diffusion of light on the hypothesis of Grimaldi,
or by the diffusion of undulations on the hypothesis of Hook. Newton
again replied to these feeble reasonings; but he contented himself
with reiterating his original experiments, and confirming them by more
popular arguments, and the vanquished Jesuit wisely quitted the field.
Another combatant soon sprung up in the person of one Francis Linus,
a physician in Liege,[13] who, on the 6th October, 1674, addressed a
letter to a friend in London, containing animadversions on Newton’s
doctrine of colours. He boldly affirms, that in a perfectly clear sky
the image of the sun made by a prism is never elongated, and that the
spectrum observed by Newton was not formed by the true sunbeams, but by
rays proceeding from some bright cloud. In support of these assertions,
he appeals to frequently repeated experiments on the refractions and
reflections of light which he had exhibited thirty years before to
Sir Kenelm Digby, “who took notes upon them;” and he unblushingly
states, that, if Newton had used the same industry as he did, he
would never have “taken so impossible a task in hand, as to explain
the difference between the length and breadth of the spectrum by the
received laws of refraction.” When this letter was shown to Newton, he
refused to answer it; but a letter was sent to Linus referring him
to the answer to Pardies, and assuring him that the experiments on
the spectrum were made when there was no bright cloud in the heavens.
This reply, however, did not satisfy the Dutch experimentalist. On the
25th February, 1675, he addressed another letter to his friend, in
which he gravely attempts to prove that the experiment of Newton was
not made in a clear day;—that the prism was not close to the hole,—and
that the length of the spectrum was not perpendicular, or parallel to
the length of the prism. Such assertions could not but irritate even
the patient mind of Newton. He more than once declined the earnest
request of Oldenburg to answer these observations; he stated, that,
as the dispute referred to matters of fact, it could only be decided
before competent witnesses, and he referred to the testimony of those
who had seen his experiments. The entreaties of Oldenburg, however,
prevailed over his own better judgment, and, “lest Mr. Linus should
make the more stir,” this great man was compelled to draw up a long
and explanatory reply to reasonings utterly contemptible, and to
assertions altogether unfounded. This answer, dated November 13th,
1675, could scarcely have been perused by Linus, who was dead on the
15th December, when his pupil Mr. Gascoigne, took up the gauntlet,
and declared that Linus had shown to various persons in Liege the
experiment which proved the spectrum to be circular, and that Sir
Isaac could not be more confident on his side than they were on the
other. He admitted, however, that the different results might arise
from different ways of placing the prism. Pleased with the “handsome
genius of Mr. Gascoigne’s letter,” Newton replied even to it, and
suggested that the spectrum seen by Linus may have been the circular
one, formed by one reflexion, or, what he thought more probable, the
circular one formed by two refractions, and one intervening reflection
from the base of the prism, which would be coloured if the prism was
not an isosceles one. This suggestion seems to have enlightened the
Dutch philosophers. Mr. Gascoigne, having no conveniences for making
the experiments pointed out by Newton, requested Mr. Lucas of Liege to
perform them in his own house. This ingenious individual, whose paper
gave great satisfaction to Newton, and deserves the highest praise,
confirmed the leading results of the English philosopher; but though
the refracting angle of his prism was 60° and the refractions equal, he
never could obtain a spectrum whose length was more than from _three_
to _three and a half_ times its breadth, while Newton found the length
to be _five_ times its breadth. In our author’s reply, he directs his
attention principally to this point of difference. He repeated his
measures with each of the three angles of _three_ different prisms, and
he affirmed that Mr. Lucas might _make sure to find the image as long
or longer than he had yet done_, by taking a prism with plain surfaces,
and with an angle of 66° or 67°. He admitted that the smallness of
the angle in Mr. Lucas’s prism, viz. 60°, did not account for the
shortness of the spectrum which he obtained with it; and he observed
in one of his own prisms that the length of the image was greater in
proportion to the refracting angle than it should have been; an effect
which he ascribes to its having a greater refractive power. There is
every reason to believe that the prism of Lucas had actually a less
dispersive power than that of Newton; and had the Dutch philosopher
measured its refractive power instead of guessing it, or had Newton
been less confident than he was[14] that all other prisms must give
a spectrum of the same length as his in relation to its refracting
angle and its index of refraction, the invention of the achromatic
telescope would have been the necessary result. The objections of Lucas
drove our author to experiments which he had never before made,—to
measure accurately the lengths of the spectra with different prisms of
different angles and different refractive powers; and had the Dutch
philosopher maintained his position with more obstinacy, he would have
conferred a distinguished favour upon science, and would have rewarded
Newton for all the vexation which had sprung from the minute discussion
of his optical experiments.
Such was the termination of his disputes with the Dutch philosophers,
and it can scarcely be doubted that it cost him more trouble to detect
the origin of his adversaries’ blunders, than to establish the great
truths which they had attempted to overturn.
Harassing as such a controversy must have been to a philosopher
like Newton, yet it did not touch those deep-seated feelings which
characterize the noble and generous mind. No rival jealousy yet
pointed the arguments of his opponents;—no charges of plagiarism were
yet directed against his personal character. These aggravations of
scientific controversy, however, he was destined to endure; and in the
dispute which he was called to maintain both against Hooke and Huygens,
the agreeable consciousness of grappling with men of kindred powers was
painfully imbittered by the personality and jealousy with which it was
conducted.
Dr. Robert Hooke was about seven years older than Newton, and was
one of the ninety-eight original or unelected members of the Royal
Society. He possessed great versatility of talent, yet, though his
genius was of the most original cast, and his acquirements extensive,
he had not devoted himself with fixed purpose to any particular branch
of knowledge. His numerous and ingenious inventions, of which it
is impossible to speak too highly, gave to his studies a practical
turn which unfitted him for that continuous labour which physical
researches so imperiously demand. The subjects of light, however, and
of gravitation seem to have deeply occupied his thoughts before Newton
appeared in the same field, and there can be no doubt that he had made
considerable progress in both of these inquiries. With a mind less
divergent in its pursuits, and more endowed with patience of thought,
he might have unveiled the mysteries in which both these subjects were
enveloped, and preoccupied the intellectual throne which was destined
for his rival; but the infirm state of his health, the peevishness
of temper which this occasioned, the number of unfinished inventions
from which he looked both for fortune and fame, and, above all, his
inordinate love of reputation, distracted and broke down the energies
of his powerful intellect. In the more matured inquiries of his rivals
he recognised, and often truly, his own incompleted speculations; and
when he saw others reaping the harvest for which he had prepared the
ground, and of which he had sown the seeds, it was not easy to suppress
the mortification which their success inspired. In the history of
science, it has always been a difficult task to adjust the rival claims
of competitors, when the one was allowed to have completed what the
other was acknowledged to have begun. He who commences an inquiry, and
publishes his results, often goes much farther than he has announced
to the world, and, pushing his speculations into the very heart of the
subject, frequently submits them to the ear of friendship. From the
pedestal of his published labours his rival begins his researches,
and brings them to a successful issue; while he has in reality done
nothing more than complete and demonstrate the imperfect speculations
of his predecessor. To the world, and to himself, he is no doubt in the
position of the principal discoverer: but there is still some apology
for his rival when he brings forward his unpublished labours; and some
excuse for the exercise of personal feeling, when he measures the speed
of his rival by his own proximity to the goal.
The conduct of Dr. Hooke would have been viewed with some such feeling,
had not his arrogance on other occasions checked the natural current
of our sympathy. When Newton presented his reflecting telescope to
the Royal Society, Dr. Hooke not only criticised the instrument with
undue severity, but announced that he possessed an infallible method of
perfecting all kinds of optical instruments, so that “whatever almost
hath been in notion and imagination, or desired in optics, may be
performed with great facility and truth.”
Hooke had been strongly impressed with the belief, that light
consisted in the undulations of a highly elastic medium pervading
all bodies; and, guided by his experimental investigation of the
phenomena of diffraction, he had even announced the great _principle
of interference_, which has performed such an important part in
modern science. Regarding himself, therefore, as in possession of
the true theory of light, he examined the discoveries of Newton in
their relation to his own speculative views, and, finding that their
author was disposed to consider that element as consisting of material
particles, he did not scruple to reject doctrines which he believed to
be incompatible with truth. Dr. Hooke was too accurate an observer not
to admit the general correctness of Newton’s observations. He allowed
the existence of different refractions, the unchangeableness of the
simple colours, and the production of white light by the union of all
the colours of the spectrum; but he maintained that the different
refractions arose from the splitting and rarefying of ethereal pulses,
and that there are only two colours in nature, viz. _red_ and _violet_,
which produce by their mixture all the rest, and which are themselves
formed by the two sides of a split pulse or undulation.
In reply to these observations, Newton wrote an able letter to
Oldenburg, dated June 11, 1672, in which he examined with great
boldness and force of argument the various objections of his opponent,
and maintained the truth of his doctrine of colours, as independent of
the two hypotheses respecting the origin and production of light. He
acknowledged his own partiality to the doctrine of the materiality of
light; he pointed out the defects of the undulatory theory; he brought
forward new experiments in confirmation of his former results; and he
refuted the opinions of Hooke respecting the existence of only two
simple colours. No reply was made to the powerful arguments of Newton;
and Hooke contented himself with laying before the Society his curious
observations on the colours of soap-bubbles, and of plates of air, and
in pursuing his experiments on the diffraction of light, which, after
an interval of two years, he laid before the same body.
After he had thus silenced the most powerful of his adversaries, Newton
was again called upon to defend himself against a new enemy. Christian
Huygens, an eminent mathematician and natural philosopher, who, like
Hooke, had maintained the undulatory theory of light, transmitted
to Oldenburg various animadversions on the Newtonian doctrine; but
though his knowledge of optics was of the most extensive kind, yet his
objections were nearly as groundless as those of his less enlightened
countryman. Attached to his own hypothesis respecting the nature
of light, namely, to the system of undulation, he seems, like Dr.
Hooke, to have regarded the discoveries of Newton as calculated to
overturn it; but his principal objections related to the composition
of colours, and particularly of white light, which he alleged could be
obtained from the union of two colours, _yellow_ and _blue_. To and
similar objections, Newton replied that the colours in question were
not simple yellows and blues, but were compound colours, in which,
together, all the colours of the spectrum were themselves blended;
and though he evinced some strong traces of feeling at being again
put upon his defence, yet his high respect for Huygens induced him to
enter with patience on a fresh development of his doctrine. Huygens
felt the reproof which the tone of this answer so gently conveyed,
and in writing to Oldenburg, he used the expression, that Mr. Newton
“maintained his doctrine with some concern.” To this our author
replied, “As for Mr. Huygens’s expression, I confess it was a little
ungrateful to me, to meet with objections which had been answered
before, without having the least reason given me why those answers
were insufficient.” But though Huygens appears in this controversy as
a rash objector to the Newtonian doctrine, it was afterward the fate
of Newton to play a similar part against the Dutch philosopher. When
Huygens published his beautiful law of double refraction in Iceland
spar, founded on the finest experimental analysis of the phenomena,
though presented as a result of the undulatory system, Newton not only
rejected it, but substituted for it another law entirely inconsistent
with the experiments of Huygens, which Newton himself had praised, and
with those of all succeeding philosophers.
The influence of these controversies on the mind of Newton seems to
have been highly exciting. Even the satisfaction of humbling all
his antagonists he did not feel as a sufficient compensation for the
disturbance of his tranquillity. “I intend,” says he,[15] “to be no
farther solicitous about matters of philosophy. And therefore I hope
you will not take it ill if you find me never doing any thing more in
that kind; or rather that you will favour me in my determination, by
preventing, so far as you can conveniently, any objections or other
philosophical letters that may concern me.” In a subsequent letter in
1675, he says, “I had some thoughts of writing a further discourse
about colours, to be read at one of your assemblies; but find it yet
against the grain to put pen to paper any more on that subject;” and
in a letter to Leibnitz, dated December the 9th, 1675, he observes, “I
was so persecuted with discussions arising from the publication of my
theory of light, that I blamed my own imprudence for parting with so
substantial a blessing as my quiet to run after a shadow.”