I trust I shall not be accused of affectation when I declare that I find myself unable to express all that I felt upon the loss of such a "Guide, Philosopher, and Friend." I shall, therefore, not say one word of my own, but adopt those of an eminent friend, which he uttered with an abrupt felicity: "He has made a chasm, which not only nothing can fill up, but which nothing has a tendency to fill up. Johnson is dead. Let us go to the next best: there is n.o.body; no man can be said to put you in mind of Johnson."
SIR DAVID BREWSTER
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 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 Princ.i.p.al 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 pa.s.sion 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-gla.s.s, and in the way he obtained a plate of air, or of s.p.a.ce, varying from the thinnest possible edge at the centre of the object-gla.s.s where it touched the plane surface to a considerable thickness at the circ.u.mference of the lens. When the light was allowed to fall upon the object-gla.s.s, every different thickness of the plate of air between the object-gla.s.ses gave different colours, so that the point where the two object-gla.s.ses touched one another was the centre of a number of concentric coloured rings. Now, as the curvature of the object-gla.s.s 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 gla.s.s 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 gla.s.s _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 quant.i.ty 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 s.p.a.ces, 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 pa.s.ses 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 mult.i.tude 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...o...b..t. 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 a.s.sumption, 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...o...b..t. For this purpose it was necessary to compare the s.p.a.ce through which heavy bodies fall in a second at the surface of the earth with the s.p.a.ce 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 a.s.sumption would act upon the moon would be one-sixth more than required to retain it in its...o...b..t.
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 s.p.a.ce. 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 ent.i.tled "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...o...b..t 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.