Sir David Brewster, a distinguished physicist, was born at
Jedburgh, on December 11, 1781. He was educated at Edinburgh
University, and was licensed as a clergyman of the Church of
Scotland by the Presbytery of Edinburgh. Nervousness in the
pulpit compelled him to retire from clerical life and devote
himself to scientific work, and in 1808 he became editor of
the "Edinburgh Encyclopaedia." His chief scientific interest
was optics, and he invented the kaleidoscope, and improved
Wheatstone's stereoscope by introducing the divided lenses. In
1815 he was elected a member of the Royal Society, and, later,
was awarded the Rumford gold and silver medals for his
discoveries in the polarisation of light. In 1831 he was
knighted. From 1859 he held the office of Principal of
Edinburgh University until his death on February 10, 1868. The
"Life of Sir Isaac Newton" appeared in 1831, when it was first
published in Murray's "Family Library." Although popularly
written, not only does it embody the results of years of
investigation, but it throws a unique light on the life of the
celebrated scientist. Brewster supplemented it in 1855 with
the much fuller "Memoirs of the Life, Writings, and
Discoveries of Sir Isaac Newton."
_I.--The Young Scientist_
Sir Isaac Newton was born at the hamlet of Woolsthorpe on December 25,
1642. His father, a yeoman farmer, died a few months after his marriage,
and never saw his son.
When Isaac was three years old his mother married again, and he was
given over to the charge of his maternal grandmother. While still a boy
at school, his mechanical genius began to show itself, and he
constructed various mechanisms, including a windmill, a water-clock, and
a carriage put in motion by the person who sat in it. He was also fond
of drawing, and wrote verses. Even at this age he began to take an
interest in astronomy. In the yard of the house where he lived he traced
the varying movements of the sun upon the walls of the buildings, and by
means of fixed pins he marked out the hourly and half-hourly
subdivisions.
At the age of fifteen his mother took him from school, and sent him to
manage the farm and country business at Woolsthorpe, but farming and
marketing did not interest him, and he showed such a passion for study
that eventually he was sent back to school to prepare for Cambridge.
In the year 1660 Newton was admitted into Trinity College, Cambridge.
His attention was first turned to the study of mathematics by a desire
to inquire into the truth of judicial astrology, and he is said to have
discovered the folly of that study by erecting a figure with the aid of
one or two of the problems in Euclid. The propositions contained in
Euclid he regarded as self-evident; and, without any preliminary study,
he made himself master of Descartes' "Geometry" by his genius and
patient application. Dr. Wallis's "Arithmetic of Infinites," Sanderson's
"Logic," and the "Optics" of Kepler, were among the books which he
studied with care; and he is reported to have found himself more deeply
versed in some branches of knowledge than the tutor who directed his
studies.
In 1665 Newton took his Bachelor of Arts degree, and in 1666, in
consequence of the breaking out of the plague, he retired to
Woolsthorpe. In 1668 he took his Master of Arts degree, and was
appointed to a senior fellowship. And in 1669 he was made Lucasian
professor of mathematics.
During the years 1666-69, Newton was engaged in optical researches which
culminated in his invention of the first reflecting telescope. On
January 11, 1761, 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 Murray, Sir Paul Neale, and Sit Christopher
Wren.
In the course of his optical researches, Newton discovered the different
refrangibility of different rays of light, and in his professorial
lectures during the years 1669, 1670, and 1671 he announced his
discoveries; but not till 1672 did he communicate them to the Royal
Society. 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. The most distinguished of his
opponents were Robert Hooke and Huyghens. Both attacked his theory from
the standpoint of the undulatory theory of light which they upheld.
_II.--The Colours of Natural Bodies_
In examining the nature and origin of colours as the component parts of
white light, the attention of Newton was directed to the explanation of
the colours of natural bodies. His earliest researches on this subject
were communicated, in his "Discourse on Light and Colours," to the Royal
Society in 1675.
Dr. Hooke had succeeded in splitting a mineral substance called mica
into films of such extreme thinness as to give brilliant colours. One
plate, for example, gave a yellow colour, another a blue colour, and the
two together a deep purple, but as plates which produced this colour
were always less than the twelve-thousandth part of an inch thick it was
quite impracticable, by any contrivance yet discovered, to measure their
thickness, and determine the law according to which the colours varied
with the thickness of the film. Newton surmounted this difficulty by
laying a double convex lens, the radius of the curvature of each side of
which was fifty feet, upon the flat surface of a plano-convex
object-glass, and in the way he obtained a plate of air, or of space,
varying from the thinnest possible edge at the centre of the
object-glass where it touched the plane surface to a considerable
thickness at the circumference of the lens. When the light was allowed
to fall upon the object-glass, every different thickness of the plate of
air between the object-glasses gave different colours, so that the point
where the two object-glasses touched one another was the centre of a
number of concentric coloured rings. Now, as the curvature of the
object-glass was known, it was easy to calculate the thickness of the
plate of air at which any particular colour appeared, and thus to
determine the law of the phenomena.
By accurate measurements Newton found that the thickness of air at which
the most luminous parts of the first rings were produced were, in parts
of an inch, as 1, 3, 5, 7, 9, and 11 to 178,000.
If the medium or the substance of the thin plate is water, as in the
case of the soap-bubble, which produces beautiful colours according to
its different degrees of thinness, the thicknesses at which the most
luminous parts of the ring appear are produced at 1/1.336 the thickness
at which they are produced in air, and, in the case of glass or mica, at
1/1.525 at thickness, the numbers 1.336, 1.525 expressing the ratio of
the sines of the angles of incidence and refraction which produce the
colours.
From the phenomena thus briefly described, Newton deduced that
ingenious, though hypothetical, property of light called its "fits of
easy reflection and transmission." This property consists in supposing
that every particle of light from its first discharge from a luminous
body possesses, at equally distant intervals, dispositions to be
reflected from, and transmitted through, the surfaces of the bodies upon
which it is incident. Hence, if a particle of light reaches a reflecting
surface of glass _when in its fit of easy reflection_, or in its
disposition to be reflected, it will yield more readily to the
reflecting force of the surface; and, on the contrary, if it reaches the
same surface _while in a fit of easy transmission_, or in a disposition
to be transmitted, it will yield with more difficulty to the reflecting
force.
The application of the theory of alternate fits of transmission and
reflection to explain the colours of thin plates is very simple.
Transparency, opacity and colour were explained by Newton on the
following principles.
Bodies that have the greatest refractive powers reflect the greatest
quantity of light from their surfaces, and at the confines of equally
refracting media there is no reflection.
The least parts of almost all natural bodies are in some measure
transparent.
Between the parts of opaque and coloured bodies are many spaces, or
pores, either empty or filled with media of other densities.
The parts of bodies and their interstices or pores must not be less than
of some definite bigness to render them coloured.
The transparent parts of bodies, according to their several sizes,
reflect rays of one colour, and transmit those of another on the same
ground that thin plates do reflect or transmit these rays.
The parts of bodies on which their colour depend are denser than the
medium which pervades their interstices.
The bigness of the component parts of natural bodies may be conjectured
by their colours.
_Transparency_ he considers as arising from the particles and their
intervals, or pores, being too small to cause reflection at their common
surfaces; so that all light which enters transparent bodies passes
through them without any portion of it being turned from its path by
reflexion.
_Opacity_, he thinks, arises from an opposite cause, _viz._, when the
parts of bodies are of such a size to be capable of reflecting the light
which falls upon them, in which case the light is "stopped or stifled"
by the multitude of reflections.
The _colours_ of natural bodies have, in the Newtonian hypothesis, the
same origin as the colours of thin plates, their transparent particles,
according to their several sizes, reflecting rays of one colour and
transmitting those of another.
Among the optical discoveries of Newton those which he made on the
inflection of light hold a high place. They were first published in his
"Treatise on Optics," in 1707.
_III--The Discovery of the Law of Gravitation_
From the optical labours of Newton we now proceed to the history of his
astronomical discoveries, those transcendent deductions of human reason
by which he has secured to himself an immortal name, and vindicated the
intellectual dignity of his species.
In the year 1666, Newton was sitting in his garden at Woolsthorpe,
reflecting on the nature of gravity, that remarkable power which causes
all bodies to descend towards the centre of the earth. As this power
does not sensibly diminish at the greatest height we can reach he
conceived it possible that it might reach to the moon and affect its
motion, and even hold it in its orbit. At such a distance, however, he
considered some diminution of the force probable, and in order to
estimate the diminution, he supposed that the primary planets were
carried round the sun by the same force. On this assumption, by
comparing the periods of the different planets with their distances from
the sun, he found that the force must decrease as the squares of the
distances from the sun. In drawing this conclusion he supposed the
planets to move in circular orbits round the sun.
Having thus obtained a law, he next tried to ascertain if it applied to
the moon and the earth, to determine if the force emanating from the
earth was sufficient, if diminished in the duplicate ratio of the moon's
distance, to retain the moon in its orbit. For this purpose it was
necessary to compare the space through which heavy bodies fall in a
second at the surface of the earth with the space through which the
moon, as it were, falls to the earth in a second of time, while
revolving in a circular orbit. Owing to an erroneous estimate of the
earth's diameter, he found the facts not quite in accordance with the
supposed law; he found that the force which on this assumption would act
upon the moon would be one-sixth more than required to retain it in its
orbit.
Because of this incongruity he let the matter drop for a time. But, in
1679, his mind again reverted to the subject; and in 1682, having
obtained a correct measurement of the diameter of the earth, he repeated
his calculations of 1666. In the progress of his calculations he saw
that the result which he had formerly expected was likely to be
produced, and he was thrown into such a state of nervous irritability
that he was unable to carry on the calculation. In this state of mind he
entrusted it to one of his friends, and he had the high satisfaction of
finding his former views amply realised. The force of gravity which
regulated the fall of bodies at the earth's surface, when diminished as
the square of the moon's distance from the earth, was found to be
exactly equal to the centrifugal force of the moon as deduced from her
observed distance and velocity.
The influence of such a result upon such a mind may be more easily
conceived than described. The whole material universe was opened out
before him; the sun with all his attending planets; the planets with all
their satellites; the comets wheeling in every direction in their
eccentric orbits; and the system of the fixed stars stretching to the
remotest limits of space. All the varied and complicated movements of
the heavens, in short, must have been at once presented to his mind as
the necessary result of that law which he had established in reference
to the earth and the moon.
After extending this law to the other bodies of the system, he composed
a series of propositions on the motion of the primary planets about the
sun, which was sent to London about the end of 1683, and was soon
afterwards communicated to the Royal Society.
Newton's discovery was claimed by Hooke, who certainly aided Newton to
reach the truth, and was certainly also on the track of the same law.
Between 1686 and 1687 appeared the three books of Newton's immortal
work, known as the "Principia." The first and second book are entitled
"On the Motion of Bodies," and the third "On the System of the World."
In this great work Newton propounds the principle that "every particle
of matter in the universe is attracted by, or gravitates to, every other
particle of matter with a force inversely proportional to the squares of
their distances." From the second law of Kepler, namely, the
proportionality of the areas to the times of their description, Newton
inferred that the force which keeps a planet in its orbit is always
directed to the sun. From the first law of Kepler, that every planet
moves in an ellipse with the sun in one of its foci, he drew the still
more general inference that the force by which the planet moves round
that focus varies inversely as the square of its distance from the
focus. From the third law of Kepler, which connects the distances and
periods of the planets by a general rule, Newton deduced the equality of
gravity in them all towards the sun, modified only by their different
distances from its centre; and in the case of terrestrial bodies, he
succeeded in verifying the equality of action by numerous and accurate
experiments.
By taking a more general view of the subject, Newton showed that a conic
section was the only curve in which a body could move when acted upon by
a force varying inversely as the square of the distance; and he
established the conditions depending on the velocity and the primitive
position of the body which were requisite to make it describe a
circular, an elliptical, a parabolic, or a hyperbolic orbit.
It still remained to show whether the force resided in the centre of
planets or in their individual particles; and Newton demonstrated that
if a spherical body acts upon a distant body with a force varying as the
distance of this body from the centre of the sphere, the same effect
will be produced as if each of its particles acted upon the distant body
according to the same law.
Hence it follows that the spheres, whether they are of uniform density,
or consist of concentric layers of varying densities, will act upon each
other in the same manner as if their force resided in their centres
alone. But as the bodies of the solar system are nearly spherical, they
will all act upon one another and upon bodies placed on their surface,
as if they were so many centres of attraction; and therefore we obtain
the law of gravity, that one sphere will act upon another sphere with a
force directly proportional to the quantity of matter, and inversely as
the square of the distance between the centres of the spheres. From the
equality of action and reaction, to which no exception can be found,
Newton concluded that the sun gravitates to the planets and the planets
to their satellites, and the earth itself to the stone which falls upon
its surface, and consequently that the two mutually gravitating bodies
approach one another with velocities inversely proportional to their
quantities of matter.
Having established this universal law, Newton was able not only to
determine the weight which the same body would have at the surface of
the sun and the planets, but even to calculate the quantity of matter in
the sun and in all the planets that had satellites, and also to
determine their density or specific gravity.
With wonderful sagacity Newton traced the consequences of the law of
gravitation. He showed that the earth must be an oblate spheroid, formed
by the revolution of an ellipse round its lesser axis. He showed how the
tides were caused by the moon, and how the effect of the moon's action
upon the earth is to draw its fluid parts into the form of an oblate
spheroid, the axis of which passes through the moon. He also applied the
law of gravitation to explain irregularities in the lunar motions, the
precession of the equinoctial points, and the orbits of comets.
In the "Principia" Newton published for the first time the fundamental
principle of the fluxionary calculus which he had discovered about
twenty years before; but not till 1693 was his whole work communicated
to the mathematical world. This delay in publication led to the
historical controversy between him and Leibnitz as to priority of
discovery.
In 1676 Newton had communicated to Leibnitz the fact that he had
discovered a general method of drawing tangents, concealing the method
in two sentences of transposed characters. In the following year
Leibnitz mentioned in a letter to Oldenburg (to be communicated to
Newton) that he had been for some time in possession of a method for
drawing tangents, and explains the method, which was no other than the
differential calculus. Before Newton had published a single word upon
fluxions the differential calculus had made rapid advances on the
Continent.
In 1704 a reviewer of Newton's "Optics" insinuated that Newton had
merely improved the method of Leibnitz, and had indeed stolen Leibnitz's
discovery; and this started a controversy which raged for years.
Finally, in 1713, a committee of the Royal Society investigated the
matter, and decided that Newton was the first inventor.
_IV.--Later Years of Newton's Life_
In 1692, when Newton was attending divine service, his dog Diamond upset
a lighted taper on his desk and destroyed some papers representing the
work of years. Newton is reported merely to have exclaimed: "O Diamond,
Diamond, little do you know the mischief you have done me!" But,
nevertheless, his excessive grief is said for a time to have affected
his mind.
In 1695 Newton was appointed Warden of the Mint, and his mathematical
and chemical knowledge were of eminent use in carrying on the recoinage
of the mint. Four years later he was made Master of the Mint, and held
this office during the remainder of his life. In 1701 he was elected one
of the members of parliament for Oxford University, and in 1705 he was
knighted.
Towards the end of his life Newton began to devote special attention to
the theological questions, and in 1733 he published a work entitled
"Observations upon the Prophecies of Daniel and the Apocalypse of St.
John," which is characterised by great learning and marked with the
sagacity of its distinguished author. Besides this religious work, he
also published his "Historical Account of Two Notable Corruptions of
Scripture," and his "Lexicon Propheticum."
In addition to theology, Newton also studied chemistry; and in 1701 a
paper by him, entitled "Scala graduum caloris," was read at the Royal
Society; while the queries at the end of his "Optics" are largely
chemical, dealing with such subjects as fire, flame, vapour, heat, and
elective attractions.
He regards fire as a body heated so hot as to emit light copiously; and
flame as a vapour, fume, or exhalation, heated so hot as to shine.
In explaining the structure of solid bodies, he is of the opinion "that
the smallest particles of matter may cohere by the strongest
attractions, and compose bigger particles of weaker virtue; and many of
these may cohere and compose bigger particles whose virtue is still
weaker; and so on for diverse successions, until the progression end in
the biggest particles on which the operations in chemistry and the
colours of natural bodies depend, and which, by adhering, compose bodies
of a sensible magnitude. If the body is compact, and bends or yields
inward to pressure without any sliding of its parts, it is hard and
elastic, returning to its figure with a force arising from the mutual
attraction of its parts.
"If the parts slide upon one another the body is malleable and soft. If
they slip easily, and are of a fit size to be agitated by heat, and the
heat is big enough to keep them in agitation, the body is fluid; and if
it be apt to stick to things it is humid; and the drops of every fluid
affect a round figure by the mutual attraction of their parts, as the
globe of the earth and sea affects a round figure by the mutual
attraction of its parts by gravity."
In a letter to Mr. Boyle (1678-79) Newton explains his views respecting
the ether. He considers that the ether accounts for the refraction of
light, the cohesion of two polished pieces of metal in an exhausted
receiver, the adhesion of quick-silver to glass tubes, the cohesion of
the parts of all bodies, the phenomena of filtration and of capillary
attraction, the action of menstrua on bodies, the transmutation of gross
compact substances into aerial ones, and gravity. If a body is either
heated or loses its heat when placed in vacuo, he ascribes the
conveyance of the heat in both cases "to the vibration of a much subtler
medium than air"; and he considers this medium also the medium by which
light is refracted and reflected, and by whose vibrations light
communicates heat to bodies and is put into fits of easy reflection and
transmission. Light, Newton regards as a peculiar substance composed of
heterogeneous particles thrown off with great velocity in all directions
from luminous bodies, and he supposes that these particles while passing
through the ether excite in it vibrations, or pulses, which accelerate
or retard the particles of light, and thus throw them into alternate
"fits of easy reflection and transmission." He computes the elasticity
of the ether to be 490,000,000,000 times greater than air in proportion
to its density.
In 1722, in his eightieth year, Newton began to suffer from stone; but
by means of a strict regimen and other precautions he was enabled to
alleviate the complaint, and to procure long intervals of ease. But a
journey to London on February 28, 1727, to preside at a meeting of the
Royal Society greatly aggravated the complaint. On Wednesday, March 15,
he appeared to be somewhat better. On Saturday morning he carried on a
pretty long conversation with Dr. Mead; but at six o'clock the same
evening he became insensible, and continued in that state until Monday,
the 20th, when he expired, without pain, between one and two o'clock in
the morning, in the eighty-fifth year of his age.
* * * * *