The minor Discoveries and Inventions of Newton—His Researches on Heat—On Fire and Flame—On Elective Attraction—On the Structure of Bodies—His supposed Attachment to Alchymy—His Hypothesis respecting Ether as the Cause of Light and Gravity—On the Excitation of Electricity in Glass—His Reflecting Sextant invented before 1700—His Reflecting Microscope—His Prismatic Reflector as a Substitute for the small Speculum of Reflecting Telescopes—His Method of varying the Magnifying Power of Newtonian Telescopes—His Experiments on Impressions on the Retina.

In the preceding chapters we have given an account of the principal labours of Sir Isaac Newton; but there still remain to be noticed several of his minor discoveries and inventions, which could not properly be introduced under any general head.

The most important of these, perhaps, are his chymical researches, which he seems to have pursued with more or less diligence from the time when he first witnessed the practical operations of chymistry during his residence at the apothecary’s at Grantham. His first chymical experiments were probably made on the alloys of metals, for the purpose of obtaining a good metallic composition for the specula of reflecting telescopes. In his paper on thin plates he treats of the combinations of solids and fluids; but he enters more largely on these and other subjects in the queries published at the end of his Optics.

One of his most important chymical papers is his Tabula quantitatum et graduum caloris, which was published in the Philosophical Transactions. This short paper contains a comparative scale of temperature from that of melting ice to that of a small kitchen coal-fire. The following are the principal points of the scale, the intermediate degrees of heat having been determined with great care.

The first column of this table contains the degrees of heat in arithmetical progression, and the second in geometrical progression,—the second degree being twice as great as the first, and so on. It is obvious from this table, that the heat at which equal parts of tin and bismuth melt is four times greater than that of blood-heat, the heat of melting lead eight times greater, and the heat of a small coal-fire sixteen times greater.

This table was constructed by the help of a thermometer, and of red-hot iron. By the former he measured all heats as far as that of melting tin; and by the latter he measured all the higher heats. For the heat which heated iron loses in a given time is as the total heat of the iron; and therefore, if the times of cooling are taken equal, the heats will be in a geometrical progression, and may therefore be easily found by a table of logarithms.

He found by a thermometer constructed with linseed oil, that if the oil, when the thermometer was placed in melting snow, occupied a space of 1000 parts, the same oil, rarefied with one degree of heat, or that of the human body, occupied a space of 10256; in the heat of water beginning to boil, a space of 10705; in the heat of water boiling violently, 10725; in the heat of melted tin beginning to cool, and putting on the consistency of an amalgam, 11516, and when the tin had become solid, 11496. Hence the oil was rarefied in the ratio of 40 to 39 by the heat of the human body; of 15 to 14 by the heat of boiling water; of 15 to 13 in the heat of melting tin beginning to solidify; and of 23 to 20 in the same tin when solid. The rarefaction of air was, with the same heat, ten times greater than that of oil, and the rarefaction of oil fifteen times greater than that of spirit of wine. By making the heats of oil proportional to its rarefaction, and by calling the heat of the human body 12 parts, we obtain the heat of water beginning to boil, 33; of water boiling violently, 34; of melted tin beginning to solidify, 72; and of the same become solid, 70.

Sir Isaac then heated a sufficiently thick piece of iron till it was red-hot; and having fixed it in a cold place, where the wind blew uniformly, he put upon it small pieces of different metals and other fusible bodies, and noted the times of cooling, till all the particles, having lost their fluidity, grew cold, and the heat of the iron was equal to that of the human body. Then, by assuming that the excesses of the heats of the iron and of the solidified particles of metal above the heat of the atmosphere, were in geometrical progression when the times were in arithmetical progression, all the heats were obtained. The iron was placed in a current of air, in order that the air heated by the iron might always be carried away by the wind, and that cold air might replace it with a uniform motion; for thus equal parts of the air were heated in equal times, and received a heat proportional to that of the iron. But the heats thus found had the same ratio to one another with the heats found by the thermometer; and hence he was right in assuming that the rarefactions of the oil were proportional to its heats.

Another short chymical paper by Sir Isaac Newton has been published by Dr. Horsley. It is entitled De Natura Acidorum, but is principally occupied with a number of brief opinions on chymical subjects. This paper was written later than 1687, as it bears a reference to the Principia; and the most important facts which it contains seem to have been more distinctly reproduced in the queries at the end of the Optics.

The most important of these queries relate to fire, flame, and electric attractions, and as they were revised in the year 1716 and 1717, they may be regarded as containing the most matured opinions of their author. Fire he regards 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 his long query on elective attractions, he considers the small particles of bodies as acting upon one another at distances so minute as to escape observation. When salt of tartar deliquesces, he supposes that this arises from an attraction between the saline particles and the aqueous particles held in solution in the atmosphere, and to the same attraction he ascribes it that the water will not distil from the salt of tartar without great heat. For the same reason sulphuric acid attracts water powerfully, and parts with it with great difficulty. When this attractive force becomes very powerful, as in the union between sulphuric acid and water, so as to make the particles “coalesce with violence,” and rush towards one another with an accelerated motion, heat is produced by the mixture of the two fluids. In like manner, he explains the production of flame from the mixture of cold fluids,—the action of fulminating powders,—the combination of iron filings with sulphur,—and all the other chymical phenomena of precipitation, combination, solution, and crystallization, and the mechanical phenomena of cohesion and capillary attraction. He ascribes hot springs, volcanoes, fire-damps, mineral coruscations, earthquakes, hot suffocating exhalations, hurricanes, lightning, thunder, fiery meteors, subterraneous explosions, land-slips, ebullitions of the sea, and waterspouts, to sulphureous steams abounding in the bowels of the earth, and fermenting with minerals, or escaping into the atmosphere, where they ferment with acid vapours fitted to promote fermentation.

In explaining the structure of solid bodies, he is of 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 divers successions, until the progression end in the biggest particles, on which the operations in chymistry 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 pression, without any sliding of its parts, it is hard and elastic, returning to its figure with a force rising from the mutual attraction of its parts. If the parts slide upon one another, the body is malleable or 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.”

Sir Isaac then supposes, that, as the attractive force of bodies can reach but to a small distance from them, “a repulsive virtue ought to succeed;” and he considers such a virtue as following from the reflection of the rays of light, the rays being repelled without the immediate contact of the reflecting body, and also from the emission of light, the ray, as soon as it is shaken off from a shining body by the vibrating motion of the parts of the body, getting beyond the reach of attraction, and being driven away with exceeding great velocity by the force of reflection.113

Many of the chymical views which Sir Isaac thus published in the form of queries were in his own lifetime illustrated and confirmed by Dr. Stephen Hales, in his book on Vegetable Statics,—a work of great originality, which contains the germ of some of the finest discoveries in modern chymistry.

Although there is no reason to suppose that Sir Isaac Newton was a believer in the doctrines of alchymy, yet we are informed by the Reverend Mr. Law that he had been a diligent student of Jacob Behmen’s writings, and that there were found among his papers copious abstracts from them in his own handwriting.114 He states also that Sir Isaac, together with one Dr. Newton, his relation, had, in the earlier part of his life, set up furnaces, and were for several months at work in quest of the philosopher’s tincture. These statements may receive some confirmation from the fact, that there exist among the Portsmouth papers many sheets, in Sir Isaac’s own writing, of Flammel’s Explication of Hieroglyphic Figures, and in another hand, many sheets of William Yworth’s Processus Mysterii Magni Philosophicus, and also from the manner in which Sir Isaac requests Mr. Aston to inquire after one Borry in Holland, who always went clothed in green, and who was said to possess valuable secrets; but Mr. Law has weakened the force of his own testimony, when he asserts that Newton borrowed the doctrine of attraction from Behmen’s first three propositions of eternal nature.

On the 7th December, 1675, Sir Isaac Newton communicated to the Royal Society a paper entitled An hypothesis explaining properties of light, in which he, for the first time, introduces his opinions respecting ether, and employs them to explain the nature of light, and the cause of gravity. “He was induced,” he says, “to do this, because he had observed the heads of some great virtuosos to run much upon hypotheses, and he therefore gave one which he was inclined to consider as the most probable, if he were obliged to adopt one.”115

This hypothesis seems to have been afterward a subject of discussion between him and Mr. Boyle, to whom he promised to communicate his opinion more fully in writing. He accordingly addressed to him a long letter, dated February 28th, 1678–9, in which he explains his views respecting ether, and employs them to account for the refraction of light,—the cohesion of two polished pieces of metal in an exhausted receiver,—the adhesion of quicksilver to glass tubes,—the cohesion of the parts of all bodies,—the cause of filtration,—the phenomena of capillary attraction,—the action of menstrua on bodies,—the transmutation of gross compact substances into aerial ones,—and the cause of gravity. From the language used in this paper, we should be led to suppose that Sir Isaac had entirely forgotten that he had formerly treated the general subject of ether, and applied it to the explanation of gravity. “I shall set down,” says he, “one conjecture more which came into my mind now as I was writing this letter; it is about the cause of gravity,” which he goes on to explain;116 and he concludes by saying, that “he has so little fancy to things of this nature, that, had not your encouragement moved me to it, I should never, I think, thus far have set pen to paper about them.”

These opinions, however, about the existence of ether, Newton seems to have subsequently renounced; for in the manuscript in the possession of Dr. J.C. Gregory, which we have already mentioned, and which was written previous to 1702, he states, that ether is neither obvious to our senses, nor supported by any arguments, but is a gratuitous assumption, which, if we are to trust to reason and to our senses, must be banished from the nature of things; and he goes on to establish, by various arguments, the validity of this opinion. This renunciation of his former hypothesis probably arose from his having examined more carefully some of the phenomena which he endeavoured to explain by it. Those of capillary attraction, for example, he had ascribed to the ether “standing rarer in the very sensible cavities of the capillary tubes than without them,” whereas he afterward discovered their true cause, and ascribed them to the reciprocal attraction of the tube and the fluid. But, however this may be, there can be no doubt that he resumed his early opinions before the publication of his Optics, which may be considered as containing his views upon this subject.

The queries which contain these opinions are the 18th–24th, all of which appeared for the first time in the second English edition of the Optics. 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 subtiler medium than air;” and he considers this medium as the same with that 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. This ethereal medium, according to our author, is exceedingly more rare and more elastic than air. It pervades all bodies, and is expanded through all the heavens. It is much rarer within the dense bodies of the sun, stars, planets, and comets, than in the celestial spaces between them, and also more rare within glass, water, &c. than in the free and open spaces void of air and other grosser bodies. In passing out of glass, water, &c. and other dense bodies into empty space, it grows denser and denser by degrees, and this gradual condensation extends to some distance from the bodies. Owing to its great elasticity, and, consequently, its efforts to spread in all directions, it presses against itself, and, consequently, against the solid particles of bodies, so as to make them continually approach to one another, the body being impelled from the denser parts of the medium towards the rarer with all that power which we call gravity.

In employing this medium to explain the nature of light, Newton does not suppose, with Descartes, Hooke, Huygens, and others, that light is nothing more than the impression of those undulations on the retina. He regards light as a peculiar substance, composed of heterogeneous particles thrown off with great velocity, and 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 their alternate fits of easy reflection and transmission.

Hence, if a ray of light falls upon a transparent body, in which the ether consists of strata of variable density, the particles of light acted upon by the vibrations which they create will be urged with an accelerated velocity in entering the body, while their velocity will be retarded in quitting it. In this manner he conceives the phenomena of refraction to be produced, and he shows how in such a case the refraction would be regulated by the law of the sines.

In order that the ethereal medium may produce the fits of easy reflection and transmission, he conceives that its vibrations must be swifter than light. He computes its elasticity to be 490,000,000,000 times greater than that of air, in proportion to its density, and about 600,000,000 times more rare than water, from which he infers that the resistance which it would oppose to the motions of the planets would not be sensible in 10,000 years. He considers that the functions of vision and hearing may be performed chiefly by the vibrations of this medium, executed in the bottom of the eye, or in the auditory nerve by the rays of light, and propagated through the solid, pellucid, and uniform capillamenta of the optic or auditory nerves into the place of sensation; and he is of opinion that animal motion may be performed by the vibrations of the same medium, excited in the brain by the power of the will, and propagated from thence by the solid, pellucid, and uniform capillamenta of the nerves into the muscles for contracting and dilating them.

In the registers of the Royal Society there exist several letters117 on the excitation of electricity in glass, which were occasioned by an experiment of this kind having been mentioned in Sir Isaac’s hypothesis of light. The society had ordered the experiment to be tried at their meeting of the 16th December, 1675; but, in order to secure its success, Mr. Oldenburg wrote to Sir Isaac for a more particular account of it. Sir Isaac being thus “put upon recollecting himself a little farther about it,” remembers that he made the experiment with a glass fixed at the distance of the 1/3d of an inch from one end of a brass hoop, and only the 1/8th of an inch from the other. Small pieces of thin paper were then laid upon the table; when the glass was laid above them and rubbed, the pieces of paper leaped from the one part of the glass to the other, and twirled about in the air. Notwithstanding this explicit account of the experiment, it entirely failed at the Royal Society, and the secretary was desired to request the loan of Sir Isaac’s apparatus, and to inquire whether or not he had secured the papers from being moved by the air, which might have somewhere stole in. In a letter, dated 21st December, Sir Isaac recommended to the society to rub the glass “with stuff whose threads may rake its surface, and, if that will not do, to rub it with the fingers’ ends to and fro, and knock them as often upon the glass.” These directions enabled the society to succeed with the experiment on the 13th January, 1676, when they used a scrubbing brush of short hog’s bristles, and the heft of a knife made with whalebone.

Among the minor inventions of Sir Isaac Newton, we must enumerate his reflecting instrument for observing the moon’s distance from the fixed stars at sea. The description of this instrument was communicated to Dr. Halley in the year 1700; but, either from having mislaid the manuscript, or from attaching no value to the invention, he never communicated it to the Royal Society, and it remained among his papers till after his death in 1742, when it was published in the Philosophical Transactions. The following is Sir Isaac’s own description of it as communicated to Dr. Halley.

“In the annexed figure PQRS denotes a plate of brass, accurately divided in the limb DQ, into ½ degrees, ½ minutes, and 1/12 minutes, by a diagonal scale; and the ½ degrees, and ½ minutes, and 1/12 minutes, counted for degrees, minutes, and 1/6 minutes. AB is a telescope three or four feet long, fixed on the edge of that brass plate. G is a speculum fixed on the brass plate perpendicularly as near as may be to the object-glass of the telescope, so as to be inclined forty-five degrees to the axis of the telescope, and intercept half the light which would otherwise come through the telescope to the eye. CD is a moveable index turning about the centre C, and, with its fiducial edge, showing the degrees, minutes, and 1/6 minutes on the limb of the brass plate PQ; the centre C must be over against the middle of the speculum G. H is another speculum, parallel to the former, when the fiducial edge of index falls on 0° 0' 0; so that the same star may then appear through the telescope in one and the same place, both by the direct rays and by the reflexed ones; but if the index be turned, the star shall appear in two places, whose distance is showed on the brass limb by the index.

“By this instrument the distance of the moon from any fixed star is thus observed: view the star through the perspicil by the direct light, and the moon by the reflexed (or on the contrary); and turn the index till the star touch the limb of the moon, and the index shall show on the brass limb of the instrument the distance of the star from the moon’s limb; and though the instrument shake by the motion of the ship at sea, yet the moon and star will move together as if they did really touch one another in the heavens; so that an observation may be made as exactly at sea as at land.

“And by the same instrument may be observed exactly the altitudes of the moon and stars, by bringing them to the horizon; and thereby the latitude and times of observation may be determined more exactly than by the ways now in use.

“In the time of the observation, if the instrument move angularly about the axis of the telescope, the star will move in a tangent of the moon’s limb, or of the horizon; but the observation may notwithstanding be made exactly, by noting when the line described by the star is a tangent to the moon’s limb, or to the horizon.

“To make the instrument useful, the telescope ought to take in a large angle; and to make the observation true, let the star touch the moon’s limb, not on the outside, but on the inside.”

This ingenious contrivance is obviously the very same invention as that which Mr. Hadley produced in 1731, and which, under the name of Hadley’s Quadrant, has been of so great service in navigation. The merit of its first invention must therefore be transferred to Sir Isaac Newton.

In the year 1672, Sir Isaac communicated to Mr. Oldenburg his design for a microscope, which he considered to be as capable of improvement as the telescope, and perhaps more so, because it requires only one speculum. This microscope is shown in the annexed diagram, where AB is the object-metal, CD the eye-glass, F their common focus, and O the other focus of the metal in which the object is placed. This ingenious idea has been greatly improved in modern times by Professor Amici, who makes AB a portion of an ellipsoid, whose foci are O and F, and who places a small plain speculum between O and AB, in order to reflect the object, which is placed on one side AP, for the purpose of being illuminated.

In another letter to Mr. Oldenburg, dated July 11th in the same year, he suggests another improvement in microscopes, which is to “illuminate the object in a darkened room with the light of any convenient colour not too much compounded: for by that means the microscope will, with distinctness, bear a deeper charge and larger aperture, especially if its construction be such as I may hereafter describe.”118 This happy idea I have some years ago succeeded in realizing, by illuminating microscopic objects with the light of a monochromatic lamp, which discharges a copious flame of pure yellow light of definite refrangibility.119

In order to remedy the evils arising from the weak reflecting power of speculum metal, and from its tarnishing by exposure to the air, Sir Isaac proposed to substitute for the small oval speculum a triangular prism of glass or crystal ABC. Its side AB ba he supposes to perform the office of that metal, by reflecting towards the eye-glass the light which comes from the concave speculum DF, fig.13, whose light he supposes to enter into this prism at its side CB bc, and lest any colours should be produced by the refraction of these planes, it is requisite that the angles of the prism at Aa and Bb be precisely equal. This may be done most conveniently, by making them half right angles, and consequently the third angle at Cc a right one. The plane AB ba will reflect all the light incident upon it; but in order to exclude unnecessary light, it will be proper to cover it all over with some black substance excepting two circular spaces of the planes Ac and Bc, through which the useful light may pass. The length of the prism should be such that its sides Ac and Bc may be square, and so much of the angles B and b as are superfluous ought to be ground off, to give passage for as much light as is possible from the object to the speculum.

One great advantage of this prism, which cannot be obtained from the oval metal, is, that without using two glasses the object may be erected, and the magnifying power of the telescope varied at pleasure, by merely varying the distances of the speculum, the prism, and the eye-glass. This will be understood from fig.16, where AI represents the great concave speculum, EF the eye-glass, and BCD the prism of glass, whose sides BC and CD are not flat, but spherically convex. The rays which come from G, the focus of the great speculum AI, will, by the refraction of the first side BC, be reduced to parallelism, and after reflection from the base CD, will be made by the refraction of the next side BD to converge to the focus H of the eye-glass EF. If we now bring the prism BCD nearer the image at G, the point H will recede from BD, and the image formed there will be greater than that at G, and if we remove the prism BCD from G, the point H will approach to BD, and the image at H will be less than that at G. The prism BCD performs the same part as a convex lens, G and H being its conjugate foci, and the relative size of the images formed at these points being proportional to their distance from the lens. This construction would be a good one for varying optically the angular distance of a pair of wires placed in the focus of the eye-glass EF; and by bisecting the lenticular prism BCD, and giving the halves a slight inclination, we should be able to separate and to close the two images or disks which would thus be produced, and thus form a double image micrometer.

Among the minor and detached labours of Sir Isaac, we must not omit his curious experiments on the action of light upon the retina. Locke seems to have wished his opinion respecting a fact stated in Boyle’s Book on Colours, and in a letter from Cambridge, dated June 30th, 1691, he communicated to his friend the following very remarkable observations made by himself.

“The observation you mention in Mr. Boyle’s book of colours I once made upon myself with the hazard of my eyes. The manner was this; I looked a very little while upon the sun in the looking-glass with my right eye, and then turned my eyes into a dark corner of my chamber, and winked, to observe the impression made, and the circles of colours which encompassed it, and how they decayed by degrees, and at last vanished. This I repeated a second and a third time. At the third time, when the phantasm of light and colours about it were almost vanished, intending my fancy upon them to see their last appearance, I found, to my amazement, that they began to return, and by little and little to become as lively and vivid as when I had newly looked upon the sun. But when I ceased to intend my fancy upon them, they vanished again. After this, I found, that as often as I went into the dark, and intended my mind upon them, as when a man looks earnestly to see any thing which is difficult to be seen, I could make the phantasm return without looking any more upon the sun; and the oftener I made it return, the more easily I could make it return again. And at length, by repeating this without looking any more upon the sun, I made such an impression on my eye, that, if I looked upon the clouds, or a book, or any bright object, I saw upon it a round bright spot of light like the sun, and, which is still stranger, though I looked upon the sun with my right eye only, and not with my left, yet my fancy began to make an impression upon my left eye, as well as upon my right. For if I shut my right eye, or looked upon a book or the clouds with my left eye, I could see the spectrum of the sun almost as plain as with my right eye, if I did but intend my fancy a little while upon it; for at first, if I shut my right eye, and looked with my left, the spectrum of the sun did not appear till I intended my fancy upon it; but by repeating, this appeared every time more easily. And now, in a few hours’ time, I had brought my eyes to such a pass, that I could look upon no bright object with either eye but I saw the sun before me, so that I durst neither write nor read; but to recover the use of my eyes, shut myself up in my chamber made dark, for three days together, and used all means to divert my imagination from the sun. For if I thought upon him, I presently saw his picture, though I was in the dark. But by keeping in the dark, and employing my mind about other things, I began in three or four days to have some use of my eyes again; and, by forbearing to look upon bright objects, recovered them pretty well, though not so well but that, for some months after, the spectrum of the sun began to return as often as I began to meditate upon the phenomena, even though I lay in bed at midnight with my curtains drawn. But now I have been very well for many years, though I am apt to think, if I durst venture my eyes, I could still make the phantasm return by the power of my fancy. This story I tell you, to let you understand, that in the observation related by Mr. Boyle, the man’s fancy probably concurred with the impression made by the sun’s light to produce that phantasm of the sun which he constantly saw in bright objects. And so your question about the cause of this phantasm involves another about the power of fancy, which I must confess is too hard a knot for me to untie. To place this effect in a constant motion is hard, because the sun ought then to appear perpetually. It seems rather to consist in a disposition of the sensorium to move the imagination strongly, and to be easily moved, both by the imagination and by the light, as often as bright objects are looked upon.”

These observations possess in many respects a high degree of interest. The fact of the transmission of the impression from the retina of the one eye to that of the other is particularly important; and it deserves to be remarked, as a singular coincidence, that I had occasion to observe and to describe the same phenomena above twenty years ago,120 and long before the observations of Sir Isaac were communicated to the scientific world.