anticipated) Newton was induced subsequently to occupy himself with these new phenomena; but, in the mean time, he was exposed to several absurd attacks upon his experimental analysis of light. Such, for instance, was that of a Jesuit named Pardies, who pretended that the elongation of the refracted image, whence Newton inferred the unequal refrangibility of the rays, was produced entirely by a difference in their original incidences on the first face of the prism: a supposition, the inaccuracy of which the most simple calculation would have been sufficient to show; and which Newton had previously refuted in his own Memoir. But still more foolish was the assertion of one Linus, a physician of Liege, who pretended never to have been able to produce by refraction through a prism an elongated image, but only a round and colourless one; whence he concluded that Newton had been led into error by the accidental passage of some bright cloud, which had elongated and coloured the image; adding also that he himself should not have been astonished had the image been elongated in the longitudinal direction of the prism; but that, without violating the rules of optics, it was impossible to imagine its elongation in the transverse direction. This was accompanied by several authoritative remarks on the improbability of what he called the new hypothesis, which Newton had imagined simply to be a statement of facts. These absurdities, as soon as presented, were printed in the Philosophical Transactions; and Newton was obliged to take the trouble to answer them methodically, to prevent their being accredited by that envy which showed itself so eager to receive them. He was compelled to reply to Huygens, who, though really a man of talent, made objections as unphilosophical nearly as the others, since he compared the properties discovered experimentally by Newton with an hypothesis of his own on the nature of light, in the same manner as Hooke had compared them with his hypothesis, and Pardies and Linus with the ancient ones. In vain did Newton reply that he neither advanced nor admitted any hypothesis whatever, but that his sole object was to establish and connect facts by means of the laws of nature. This severe and abstract method of reasoning was then too little understood. It is scarcely conceivable into what de

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tails he was obliged to enter in the discussion; and such was the disgust with which this inspired him, that he gave up his previous intention of printing his lectures on Optics with his treatise on Series, and determined to commit himself no more with the public. "I was," he afterwards wrote to Leibnitz, so persecuted with discussions arising from the publication of my theory of light, that I blamed my own imprudence for parting with so substantial a blessing as my quiet, to run after a shadow.” It was, perhaps, the remembrance of these inconsiderate objections of Huygens, that afterwards inclined Newton to regard less favourably than he ought to have done, the law of double refraction in Iceland spar, discovered by this eminent mathematician, probably by experiment after Newton's own manner, though he presented it as a deduction from his own favourite system, and as a confirmation of it. It is easy to understand how much Newton must have been grieved by the opposition of so illustrious an adversary as Huygens, since he might at least have hoped to have been understood and appreciated by minds accustomed to the severity of mathematical investigations. Nevertheless, before quitting the lists, Newton wished finally to complete the account of the results which he had obtained, and of the views which he had formed on the nature of light. This was the object of a later paper addressed to the Royal Society.+

We there find an experimental analysis of the colours observed in thin plates-phenomena, which, as we have said, had been previously pointed out and described by Hooke, but without his having either measured the spaces occupied by the colours, or determined the law which they followed. Newton first measured the spaces with admirable precision and nicety, and thence derived the physical laws by which all these results are connected with, and may be deduced from each other.

This treatise, united with his first paper on the analysis of Light, afterwards served as a base for the grand work published in 1704, under the name of Newton's Treatise on Optics; with this difference, however, that in the latter work the experimental investigation of

the phenomena is more extensive and more strictly separated from all hypothesis. The new experiments with which Newton enriched it, relate principally to the colours observed in the thick plates of all bodies, when they are presented in a proper manner to the incident ray. Newton reduces them to the same laws as those of the phenomena in thin plates; and then considering these laws as established facts equally certain with the particular experiments from which they are deduced, yet far more universal, he unites them all in one general property of light, each peculiarity of which is characterized with such exactness, as to make the general property a pure expression for all the observed laws. The essence of this property is, that each particle of light, from the instant when quits the radiating body whence it emanates, is subject periodically and at equidistant intervals, to a continual alternation of dispositions to be reflected from or to be transmitted through the surfaces of the diaphanous bodies it meets with; so that, for instance, if such a surface presents itself to the luminous particle during one of the alternations when the tendency to reflection is in force, which Newton has appropriately termed the fit of easy reflection, this tendency makes it yield more easily to the reflecting power of the surface; while, on the other hand, it yields with more difficulty when it is in the contrary phase, which Newton has termed the fit of easy transmission. We have here an admirable example of the universal application of scientific definitions when framed in strict accordance with experiment. For, though the term fits, inasmuch as it seems to imply a physical property, is applicable in its first intention to material particles only, and thus involves the assumption of the materiality of light, (a fact of which we may reasonably doubt, though Newton has never treated it as doubtful,) yet the characteristics of these fits are described in such exact conformity with experiment, that they would exist without any change, even were it discovered that light is constituted in any other manner that it consists, for instance, in the propagation of undulations: such is the point of view in which Newton regards these fits in his Optics, 1704, limiting himself to deduce from them his profound inductions, on the intimate constitution of bodies, and on the cause which renders them apt to reflect

or transmit a particular colour. But in his paper of 1675, he connected these properties with a very bold physical hypothesis, so general, that, from it, he deduced the nature of light and of heat, and the explanation of all the phenomena of combination or motion which appear to result from certain intangible and imponderable principles. As this hypothesis (mentioned only in the History of the Royal Society) is little known, and as it appears to have been constantly connected with Newton's thoughts on the constitution of the universe, we may here give a summary of it. We do this without the intention either of defending or combating it, but in order that the reader may see precisely in what the general views of Newton from this time forward consisted, and how, while they continued unchanged by lapse of time, he made a more or less explicit declaration of them according to circumstances. Newton, in the first place, excuses himself for proposing a conjecture as to the nature of light, declaring that he does not need one, and that the properties which he has discovered being physical facts, their being explicable or not by this or that hypothesis, could not in any degree add to or take away from their certainty;* "but," says he, "because I have observed the heads of some great virtuosos to run much upon hypotheses, I will give one which I should be inclined to consider as the most probable, if I were obliged to adopt one." He then admits, nearly as Descartes had previously done, the existence of a fluid imperceptible to our senses, which extends everywhere in space, and penetrates all bodies, with different degrees of density. He supposes this fluid to be more dense in bodies which contain in the same volume a less number of constituent material particles; he supposes also that the density of this fluid varies around each different body, and even around each constituent particle, increasing rapidly near their surface, and afterwards more slowly, though by insensible degrees, as the distance from the surface becomes greater. This fluid (which Newton calls aetherial medium or ather, in order to characterize by this denomination its extreme tenuity) he also considered as highly elastic; and consequently by the effort which it makes to spread, that it presses against itself, and against the material parts of

Birch, Hist. R:S, vol. iii, p. 249.

other bodies, with an energy more or less powerful according to its actual density, and thus that all these bodies continually tend towards one another; the inequality of the pressure urging them always to pass from the denser into the rarer parts of the æther. Conformably to his opinion respecting the disposition of the æther around each body, and around each of its material constituent particles, he considered that the variations of its density between a body and a vacuum, or between one body and another neighbouring body, were not sudden and discontinuous, but gradual and progressive; and from being very rapid near the surfaces, where the nature or density of the matter instantaneously changes, they a little farther become so slow as soon to cease to be perceptible beyond certain limits of thickness inappreciable to our senses. If, then, this æther be disturbed or agitated, in any one point, by any cause whatever, producing a vibratory move ment, this motion must transmit itself by undulations through all the rest of the medium, in the same way that sound is transmitted through air, but much more rapidly, by reason of the æther's greater elasticity; and, if those undulations, successively reiterated, happen to encounter in their passage the material particles forming the substance of any body, they will agitate them with considerable force, by the quick and periodical repetition of their successive impressions, in precisely the same way that we see solid bodies, and sometimes even the whole mass of a large building, tremble under reiterated impulses of the weak undulations in the air, excited by the sounds of an organ, or by the rolling of a drum.

Now Newton does not suppose that light immediately results from the impression produced by these undulations on the nervous membrane of the retina, as Descartes and Hooke had previously done, and as, in general, has been done by all those who have followed the same system. The principal reason which Newton gives for rejecting this supposition is, that a motion excited in, and transmitted through, an elastic fluid which reposes on another fluid of a different density, does not seem capable of being reflected in the first fluid at their surface of common separation, without being in part transmitted into the second; whereas, in many cases, light, propagated into the interior of bodies, is totally reflected at their second

surface, and again returns into their interior without the smallest part of it going out. Newton, therefore, admits that light consists of a peculiar substance different from the æther, but composed of heterogeneous particles, which, springing in all directions from shining bodies, with an excessive though measurable velocity, agitate the æther in their passage, and excite in it undulations; by the meeting of which, they become liable to be in their turn accelerated or retarded. ..Newton does not attempt to characterize the essence of these particles, but merely the faculty that he attributes to them of agitating the æther, and of being agitated by it; and finally he adds,* “those that will, may suppose it, multitudes of unimaginable small and swift corpuscules of various sizes springing from shining bodies at great distances one after another; but yet without any sensible interval of time; and continually urged forward by a principle of motion, which, in the beginning, accelerates them till the resistance of the ætherial medium equal the force of that principle, much after the manner that bodies let fall in water are accelerated, till the resistance of the water equals the force of gravity." Be this as it may, the independence of the particles of light and of æther being admitted, as well as their mutual reaction, Newton takes the case of a ray of light moving through a space in which the ætherial medium is composed of strata of unequal density; and applying to the particles of this ray the general principle established above, he concludes that they ought to be pressed, urged, or generally acted upon, so as to go from the denser to the rarer strata of æther; whence they must receive an accelerated velocity, if this tendency conspire with the proper motion of the ray; and a retarded velocity, if it be contrary to it; and generally a curvilinear deviation when the proper motion of the ray and the impression produced by the elastic medium are oblique to one another.

This is precisely what must happen when rays of light pass from one transparent homogeneous body into another, since the æther is there supposed to be of different densities; and the deviation of the rays takes place only near the common surface of the two bodies, where the sensible variation of density begins, whence results the phenomenon

Birch, Hist. R. S. vol. iii. pp. 254, 5.

of refraction.* "Now," says Newton, "if the motion of the ray be supposed in this passage to be increased or diminished in a certain proportion, according to the difference of the densities of the ætherial mediums, and the addition or detraction of the motion be reckoned in the perpendicular from the refracting superficies, as it ought to be, the sines of incidence and refraction will be proportional, according to what Descartes has demonstrated." This explanation of refraction is exactly the same as Newton afterwards reproduced in the Principia, though without there pronouncing any opinion on the nature of the disturbing force. It is, however, probable, that in his Memoir he deduced it by simple induction, rather than by a mathematical investigation; for it does not appear that, at this epoch, he was acquainted with the calculation of curvilinear motions. It is, however, important to remark, that from this time he had formed a conception of the doctrine of universal gravitation; for he takes care to point out that the unequal density of the æther, at different distances from the surface of bodies, suffices to determine their mutual tendency towards one another; a consideration which he again brought forward in the Queries annexed to his Optics (in 1704), after he had discovered the laws of the system of the world. Nevertheless we may infer, that in 1675, he had not yet formed the idea of attractions at small distances, since, in his paper addressed to the Royal Society, he imagines that the ascent of liquids in capillary tubes is caused by the air being more rare in confined than in open spaces, and the more rare in proportion as the spaces are more confined. While in the Queries he attributes these phenomena to their true cause, viz. to the reciprocal attractions of the tubes and of the fluid; though, even at this later period, he did not know how to calculate their effect. It was reserved for LAPLACE to complete this investigation.

After having thus considered the simple transmission of rays in ætherial strata of unequal der.sities, Newton examines the modifications produced during this transmission, by their meeting with undulations originally excited in the æther itself, according as such undulations may favour or oppose the actual motion of the luminous particles;

Birch, Hist. R.S. vol. iii. p. 256,

and by this re-action he is enabled to explain the intermittances in reflection and refraction, which take place in thin plates. We may observe in his Optics, that he has never abandoned this idea; for though in that work he has maintained the most complete reserve with regard to the nature of light, yet, after characterizing the fits as a purely abstract physical property, he gives as a method of rendering it sensible, the same manner of conceiving it that he had given in his Memoir of 1675; the same idea is reproduced in several of the Queries, particularly in the 17th, and those following to the 24th, where Newton asks, as in the paper presented to the Royal Society, if this same æther be not also sufficient to produce universal gravitation, and even all the phenomena of animal motion? Finally, in his paper, he endeavours to apply the same principles to the inflections, undergone by rays of light on passing near the extremities of bodies; which he, in like manner, explains by variations in the density of the æther. It is always thus that he has represented these inflections, both in the Principia, printed in 1687, and in the Queries.

From these examples, taken together, we may see that Newton did not "several times change his ideas on light," as has been asserted by some writers, but that, always preserving the same opinion, he has explained it more or less fully, as different occasions demanded.

The phenomena of diffraction, how ever, were still too imperfectly known, and observed with too little detail for enabling Newton to see precisely whe ther they agreed or not with his hypothesis. We have reason to believe that, in order to study these properties, he then made a number of experiments, to be afterwards inserted at the end of the Optics; for he there introduces them as part of an investigation which he had formerly undertaken, but from which his thoughts were now so far estranged, that he had lost the taste for resuming it. These observations, like all his others, are presented as matters of fact, without relation to any system. When the hypothesis of Newton on the nature of light was presented, in 1675, to the Royal Society, Hooke, as usual, put in his claims to it. Newton, however, did not again waste his time and repose in a controversy on the subject, but contented himself with writing to Olden burg (21st December), in order to make

him see the injustice of that jealous individual. He first clearly shows that his fundamental idea has nothing in common with that of Hooke, inasmuch as the latter supposes light to consist in the undulations themselves of the æther, transmitted to the organ of vision; while the light of Newton is a substance entirely distinct, which, thrown into the æther, impresses upon, or receives from it, peculiar motions, by means of which it acts upon us. "As to the observations of Hooke on the colours in thin plates, I avow," says Newton, "that I have made use of them, and thank him for the same; but he left me to find out and make such experiments about it, as might inform me of the manner of the production of those colours, to ground an hypothesis on; he having given no further insight to it than this, that the colour depended on some certain thickness of the plate; though what that thickness was at every colour, he confesses, in his Micrography, he had attempted in vain to learn; and, therefore, seeing I was left to measure it myself, I suppose he will allow me to make use of what I took the pains to find out; and this I hope may vindicate me from what Mr. Hooke has been pleased to charge me with."* Happily this time the discussion proceeded no further; and Oldenburg had sufficient influence, as well as sufficient sense, to prevent its obtaining notoriety. From this time till the year 1679, four years afterwards, Newton communicated nothing to the Royal Society. Oldenburg, whose kindness had ever encouraged him, unfortunately died in this interval, and was succeeded in the secretaryship by Hooke, an appointment little likely to remove an apprehension of new disputes. We may imagine, however, that Newton did not remain idle; and, in fact, in this interval, it appears, he was principally occupied with astronomical observations. At last, 28th November, 1679, he had occasion to write to Hooke about a System of Physical Astronomy, on which the Royal Society had asked his opinion. In his letter he proposed, as a matter deserving attention, to verify the motion of the earth by direct experiment, viz. by letting bodies fall from a considerable height, and then observing if they follow exactly a vertical direction; for if the earth

Birch, Hist. R. S. vol. iii. p. 279. Ibid, vol, iii. p. 512.

turns, since the rotatory velocity at the point of departure must be greater than that at the foot of the vertical, they will be found to deviate from this line towards the east, instead of following it exactly as they would do if the earth did not revolve. This ingenious idea being very favourably received, Hooke was charged to put it into effect. On reflection, Hooke immediately added the remark, that wherever the direction of gravity is oblique to the axis of the earth's rotation, i. e. in all parts of the earth, except at the equator, bodies, in falling, change parallels, and approach the equator: so that in Europe, for instance, the deviation does not take place, rigorously speaking, to the east, but to the south-east of the point of departure. Hooke communicated this remark to Newton, who immediately recognized its correctness in theory; but, in addition to this, Hooke assured the Royal Society that, on repeating the experiment several times, he had actually found that the deviation took place constantly towards the south-east; an accordance which would appear very simple, if Hooke's remarks were merely theoretical; but which must appear very extraordinary if he intended to speak of an actual observed deviation reckoned from the foot of the vertical; for in this case, according to the formulæ of LAPLACE, the tendency to the south is of the second order, relative to the absolute deviation; and in Hooke's observations this very slight deviation must have been excessively difficult to ascertain, since his experiments were made in the open air. It was this, however, which led Newton to consider whether the elliptical motion of the planets could result from a force varying inversely as the square of the distance, and if so, under what circumstances such a result would ensue. In fact, in proposing to the Royal Society his curious experiment, he had considered the motion of the heavy body as determined by a force of constant intensity, and had concluded the trajectory to be a spiral,* doubtless, because he imagined the body to fall in a resisting medium, such as the air. Hooke, who for a long time had adopted the hypothesis of a force decreasing as the squares of the distance from the centre, replied that the trajectory ought

Vide Newton's original Letters in the Biographia Britannica, article Hooke, p. 2659.