Life of Sir Isaac Newton

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 Encyclopædia.” 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 Infinités,” 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.