Letter from Newton to Henry Oldenburg, dated 15 February 1675/6
Feb 15. 167. Cambridge
Sr
I thank you for giving me notice of ye objection wch some have made. If I understand it right they meane that colour may proceed from the different velocities wch æthereall pulses or rays of light \may/ have as they come immediately from ye Sun. |But| I|i|f this be their meaning, they propound not an objection but an Hypothesis to explain my Theory. For ye better understanding of this I shall desire you to consider that I put not ye different refrangibility of rays to be ye primary{illeg} \internal or for{illeg} {formal} \or essential// cause of colours, but only the means whereby rays of different colours are separated. Neither do I say what is that cause, either of colour \or/ of different refrangibility, but leave these to be explained by Hypotheses, & only say that rays wch differ in colour, differ also in refrangibility, & yt different refrangibility conduces to ye production of colour no other way then by causing a different refraction & thereby \a/ separating|on| of those rays wch had different colours {illeg}|b|efore, but could not a{illeg}|p|pear in their own colours till they were separated. Suppose red & blew pouders (as Minium & Bise) were equally mixed, ye compound would be neither a good red nor a good blew but a midli{illeg}|n|g dirty colour. Suppose further this mixture was put into water, & after ye water had been well stirred, the pouders left to subside; if ye red was much more ponderous then ye blew, it would subside fastest & leave most of ye blew to subside after it; & by consequence ye heape would appear red at bottom & blew at top, & of intermediate colours between. Here then are various col{illeg}|o|urs produced out of a dirty colour by means of different gravity, & yet that different gravity was not ye {formal} \internal/ cause of those colours, but only ye cause of ye separation of ye particles of several colours. And so it is in ye production of colours by ye Prism: the different refrangibility \of rays/ is no otherwise ye cause of colour in this case then ye different gravity of ye pouders was in ye other; it only causes a divers refraction of ye rays originally qualified to exhibit divers colours, & by that divers refraction they are separated {illeg}|&| when separated they must needs exhibit each their own colol|u|rs wch they could not do in ye mixture while mixed. Had I supposed \different/ refrangibility ye internal or {formal} cause of colours, it would have been strangely precarious, & scarcely intelligible; but to make it only ye cause of separation of rays indowed wth diverent {sic} colours is nothing but experiment, & all yt I have asserted in my writings. In like manner where I make different reflexibility ye cause of different colours (as in ye case of thin transparent plates) I say not that it is their internal cause but only the means of their separation. For I apprehend that all ye Phænomena of colours in ye world result from nothing but separations or mixtures of difform rays & that different refrangibility {illeg}|&| reflexibility are only ye means by wch those separations or mixtures {illeg}|a|re made.
Th{illeg}|i|s being apprehended, I presume you will easily see t{illeg}|h|at you have not sent me an objection but only an Hypothesis to explain my Theory by. For to this \suppose different velocities of ye rays ye principle of colour/ is only to assign a cause of ye the different colours wch rays \are originally disposed to exhibit & do/ {illeg}|ex|hibit when separated by different refractions. And though this should be ye true es{illeg}|s|ential cause of those different colours yet it hinders not but that the different refrangibility ray of ye rays may be ye their accidental cause by causing \making/ a separation of pulses of different swiftness. Yea so far is this Hypothesis from contradicting me that if it be supposed it infers all my Theory. For if it {illeg}|b|e true, then is ye sun's light an aggregate of heterogeneal rays such are originally disposed to exhibit various colours: then is ye whitenes of y{illeg}|t| Sun light a mixture of those colours, being ye result of \the mixture of/ those unequal colorific motions: then is there nothing requisite for ye production of colours but a separation of these rays so that ye swiftest may go to one place by themselves & ye slowest to another by themselves, or one sort be stifled & another remain: then must all ye phænomena of colours proceed from ye separations of these rays of unequal swift{illeg}|n|ess because while they continue blended together, as in ye sun's original light, they can exhibit no other colour but white: & lastly; then must various refrangibility & reflexibility be ye instrumental causes of ye Phænomena of colour, those two being ye proper means whereby difform rays are separated.
Were I to apply this Hypothesis to my notions I would say therefore yt ye slowest pulses being weakest are more easily turned out of ye way by any refracting superficies then ye swiftest, & so cæteris paribus are more refracted: and that ye Prism by refracting them more separates them from ye swiftest, & the{illeg}n they being freed from ye alloy of o{n}e another strike ye sence distinctly each wth their own motions apart & so beget different sensations of colour both f different both from one another & from that wch they begat while mixed together; suppose ye swiftest ye strongest colour, red; and ye slowest ye weakest, blew.
To all this I might add concerning ye different swiftness of rays that I my self have formerly applyed it to my notions in mentioning other Hypotheses, as you may see in my answer to Mr Hook sect. 4, & I think also in ye Hypothesis I lately sent you. I say I applyed it in other Hypotheses; for in this of Mr Hook I think it is much more natural to suppose ye pulses equally swift & to differ only in bigness, because it is so in ye air, & ye laws of undulation are wthout do{illeg}|u|bt ye same in æther that they are in air.
Having thus answered, as I conceive, your objection in particu{illeg}|l|ar; I shall now for a conclusion remind you of what I have formerly said in general to ye sa{illeg}|me| purpose: so that I may at once cut off all objections that may be raised for ye future either f{illeg}|r|om this or any other Hypothesis whatever. If you consider what I said both in my second letter to P. Pardies & in my answer to Mr Hook sect 4. {illeg} concerning ye application of all Hypotheses to my Theory you may thence gather this general rule. That in any Hypothesis whence ye rays may be supposed to have any originall diversita|i|es, whether as to size or figure or motion or force or quality or any thing els imaginable wch may suffice to difference those rays in colour & refrangibility, there is no need to seek for other causes if these effects then {illeg}(colours & different refrangibility) then those original diversities. This rule being laid down, I argue thus. In any {illeg}i cal Hypothesis whatever that can be propounded, light as it comes from ye Sun must be supposed either homogeneal or heterogeneal. If ye last, then {illeg} is that Hypothesis comprehended in this general rule & so cannot be against me: if the first then must refractions have a power to modify light so as to change yt it's colorifick qualification & refrangibility; wch is against experience.
Since ye writing of this I receivd yor other letter. I thank you for yor account of Mr Berchhenshaw's scale of Musick though I have not so much skill in that science as to understand it well. If you should register ye papers in your hand before you return them, I would desire you to leave out ye last paragraph of ye Hypothesis where I mention Mr Hook & Grimaldo together: but since you are to receive those papers again (that of ye observations at least; for ye Hypot{illeg}|he|sis I am more inclind to suppress,) I suppose it will not be necessary that you should put your self to ye trouble of registring them.
I remain
Sr
Yor humble Servant
Is. Newton
