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Mr. HOOKE'S considerations upon Mr. NEWTON'S discourse on light and colours were read. Mr. Hooke was thanked for the pains taken in bringing in such ingenious reflections; and it was ordered, that this paper should be registred3[1] , and a copy of it immediately sent to Mr. NEWTON: and that in the mean time the printing of Mr. NEWTON'S discourse by itself might go on, and if he did not contradict it; and that Mr. HOOKE'S paper might be printed afterwards, it not being thought fit to print them together, lest Mr. NEWTON should look upon it as a disrespect, in printing so sudden a refutation of a discourse of his, which had met with so much applause at the Society but a few days before.

Mr. HOOKE'S paper was as follows:

"I have perused the discourse of Mr. NEWTON about colours and refractions, and I was not a little pleased with the niceness and curiosity of his observations. But, tho' I wholly agree with him as to the truth of those he hath alledged, <11> as having, by many hundreds of trials, found them so; yet as to his hypothesis of solving the phenomæna of colours thereby, I confess, I cannot see yet any undeniable argument to convince me of the certainty threof. For all the experiments and observations I have hitherto made, nay, and even those very experiments, which he alledgeth, do seem to me to prove, that white is nothing but a pulse or motion, propagated through an homogeneous, uniform and transparent medium: and that colour is nothing but the disturbance of that light, by the communication of that pulse to other transparent mediums, that is, by the refraction thereof: that whiteness and blackness are nothing but the plenty or scarcity of the undisturbed rays of light: and that the two colours (than the which there are not more uncompounded in nature) are nothing but the effects of a compounded pulse, or disturbed propagation of motion caused by refraction.

But, how certain soever I think myself of my hypothesis (which I did not take up without first trying some hundreds of experiments) yet I should be very glad to meet with one experimentum crucis from Mr. NEWTON, that should divorce me from it. But it is not that, which he so calls, will do the turn; for the same phænomenom will be solved by my hypothesis, as well as by his, without any manner of difficulty or straining: nay, I will undertake to shew another hypothesis, differing from both his and mine, that shall do the same thing.

That the ray of light is as it were split or rarified by refraction, is most certain; and that thereby a differing pulse is propagated, both on those sides, and in all the middle parts of the ray, is easy to be conceived: and also, that differing pulses or compound motions should make differing impressions on the eye, brain, or sense, is also easy to be conceived: and that, whatever refracting medium does again reduce it to its primitive simple motion by destroying the adventitious, does likewise restore it to its primitive whiteness and simplicity.

But why there is a neccessity, that all those motions, or whatever else it be that makes colours, should be originally in the simple rays of light, I do not yet understand the necessity of, no more than that all those sounds must be in the air of the bellows, which are afterwards heard to issue from the organ-pipes; or in the string, which are afterwards, by different stoppings and strikings produced; which string (by the way) is a pretty representation of the shape of a refracted ray to the eye; and the manner of it may be somewhat imagined by the similitude thereof: for the ray is like the string, strained between the luminous object and the eye, and the stop or fingers is like the refracting surface, on the one side of which the string hath no motion, on the other a vibrating one. Now we may say indeed and imagine, that the rest or streightness of the string is caused by the cessation of motions, or coalition of all vibrations; and that all the vibrations are dormant in it: but yet it seems more natural to me to imagine it the other way.

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And I am a little troubled, that this supposition should make Mr. NEWTON wholly lay aside the thoughts of improving telescopes and microscopes by refractions; since it is not improbable, but that he, that hath made so very good an improvement of telescopes by his own trials upon reflection, would, if he had prosecuted it, have done more by refraction. And that reflection is not the only way of improving telescopes, I may possibly hereafter shew some proof of. The truth is, the difficulty of removing that inconvenience of the splitting of the ray, and consequently of the effect of colours, is very great; but yet not insuperable. I have made many trials, both for telescopes and microscopes by reflection, which I have mentioned in my Micrographia, but deserted it as to telescopes, when I considered, that the focus of the spherical concave is not a point but a line, and that the rays are less true reflected to a point by a concave, than refracted by a convex; which made me seek that by refraction, which I found could not rationally be expected by reflection: nor indeed could I find any effect of it by one of six foot radius, which, about seven or eight years since, Mr. REEVE made for Mr. GREGORY, with which I made several trials; but it now appears it was for want of a good encheiria (from which cause many good experiments have been lost) both which considerations discouraged me from attempting further that way; especially since I found the parabola much more difficult to describe, than the hyperbola or ellipsis. And I was wholly taken from the thoughts of it, by lighting on divers ways, which in theory answered all I could wish for; tho' having much more business, I could not attend to bring them into use for telescopes; tho' for microscopes I have a good while used it. Thus much as to the preamble; I shall now consider the propositions themselves.

First then, Mr. NEWTON alledgeth, that as rays of light differ in refrangibility, so they differ in their disposition to exibit this or that colour: with which I do in the main agree; that is, that the ray by refraction is, as it were, split or rarified, and that the one side, namely that which is most refracted, gives a blue, and that which is least a red: the intermediate are the dilutings and intermixtures of those two, which I thus explain. The motion of light in an uniform medium, in which it is generated, is propagated by simple and uniform pulses or waves, which are at right angles with the line of direction; but falling obliquely on the refracting meduim, it receives another impression or motion, which disturbs the former motion, somewhat like the vibration of a string: and that, which was before a line, now becomes a triangular superficies, in which the pulse is not propagated at right angles with its line of direction, but ascew, as I have more at large explained in my Micrographia; and that, which makes excursions on the one side, impresses a compound motion on the bottom of the eye, of which we have the imagination of red; and that, which makes excursions on the other, causes a sensation, which we imagine a blue; and so of all the intermediate dilutings of those colours. Now, that the intermediate are nothing but the dilutings of those two primary, I hope I have sufficiently proved by the experiment of the two wedge-like boxes, described in my Micrographia. Upon this account I cannot assent to the latter part of <13> the proposition, that colours are not qualifications of light, derived from refractions, or reflections of natural bodies, but original and connate properties, &c.

The second proposition I wholly allow, not exactly in the sense there meant, but with my manner of expressing it; that is, that part of the split ray, which is most bent, exhibits a blue, that which is least, a red, and the middle parts midling colours; and that those parts will always exhibit those colours till the compound motions are destroyed, and reduced by other motions to one simple and uniform pulse as it was at first.

And this will easily explain and give a reason of the phænomena of the third proposition, to which I do readily assent in all cases, except where the split ray is made by another refraction, to become intire and uniform, again to diverge and separate, which explains his fourth proposition.

But as to the fifth, that there are an indefinite variety of primary or original colours, amongst which are yellow, green, violet, purple, orange, &c. and an infinite number of intermediate gradations, I cannot assent thereunto, as suppposing it wholly useless to multiply entities without necessity, since I have elsewhere shewn, that all the varieties of colours in the world may be made of two. I agree in the sixth, but cannot approve of his way of explicating the seventh. How the split ray being made doth produce a clear and uniform light, I have before shewed; that is, by being united thereby from a superficial motion, which is susceptible of two, to a lineary, which is susceptible of one only motion; and it is as easy to conceive how all those motions again appear after the rays are again split or rarified. He, that shall but a little consider the undulations on the surface of a small river of water, in a gutter, or the like, will easily see the whole manner curiously exemplified.

The eighth proposition I cannot at all assent to, for the reasons above; and the reasons of the blue flame of brimstone, of the yellow of a candle, the green of copper, and the various colours of the stars, and other luminous bodies, I take to proceed from quite another cause, easily explained by my former hypothesis.

I agree with the observations of the ninth, tenth, and eleventh, though not with his theory, as finding it not absolutely necessary, being as easily and naturally explained and solved by my hypothesis.

The reason of the phænomena of my experiment, which he alledgeth, is as easily solvable by my hypothesis as by his; as are also those, which are mentioned in the thirteenth. I do not therefore see any absolute necessity to believe his theory demonstrated, since I can assure Mr. NEWTON, I cannot only solve all the phænomena of light and colours by the hypoythesis I have formerly printed, and now explicate them by, but by two or three other very dif <14> fering from it, and from this, which he hath described in his ingenious discourse.

Nor would I be understood to have said all this against his theory, as it is an hypothesis; for I do most readily agree with them in every part thereof, and esteem it very subtil and ingenious, and capable of solving all the phænomena of colours: but I cannot think it to be the only hypothesis, nor so certain as mathematical demonstrations.

But grant his first proposition, that light is a body, and that as many colours as degrees thereof as there may be, so many sorts of bodies there may be, all which compounded together would make white; and grant further, that all luminous bodies are compounded of such substances condensed, and that whilst they shine, they do continually send out an indefinite quantity thereof, every way in orbem, which in a moment of time doth disperse itself to the utmost and most indefinite bounds of the universe; granting these, I say, I do suppose there will be no great difficulty to demonstrate all the rest of his curious theory: though yet, methinks, all the coloured bodies in the world compounded together should not make a white body, and I should be glad to see an experiment of that kind done on the other side. If my supposition be granted, that light is nothing but a simple and uniform motion, or pulse of a homogeneous and adopted (that is a transparent) medium, propagated from the luminous body in orbem, to all imaginable distances in a moment of time, and that that motion is first begun by some other kind of motion in the luminous body; such as by the dissolution of sulphureous bodies by the air, or by the working of the air, or the several component parts one upon another, in rotten wood, or putrifying fish, or by an external stroke, as in diamond, sugar, the sea-water, or two flints or crystal rubbed together; and that this motion is propagated through all bodies susceptible thereof, but is blended or mixt with other adventitious motions, generated by the obliquity of the stroke upon a refracting body; and that, so long as those motions remain distinct in the same part of the medium or propagated ray, so long they produce the same effect, but when blended by other motions, they produce other effects: and supposing, that by a direct contrary motion to the newly impressed, that adventitious one be destroyed and reduced to the first simple motion; I believe Mr. NEWTON will think it no difficult matter, by my hypothesis, to solve all the phænomena, not only of the prism, tinged liquors, and solid bodies, but of the colours of plated bodies, which seem to have the greatest difficulty. It is true, I can, in my supposition, conceive the white or unifrom motion of light to be compounded of the compound motions of all the other colours, as in any one strait and uniform motion may be compounded of thousands of compound motions, in the same manner as DESCARTES explicates the reason of the refraction; but I see no necessity of it. If Mr. NEWTON hath any argument, that he supposes as absolute demonstration of his theory, I should be <15> very glad to be convinced by it, the phænomena of light and colours being, in my opinion, as well worthy of contemplation, as any thing else in the world."

[1] 3 Register, vol. iv. p. 148.

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