Experiments and considerations touching colours first occasionally written, among some other essays to a friend, and now suffer'd to come abroad as the beginning of an experimental history of colours / by the Honourable Robert Boyle ...

CHAP. IV.

1. OF greater Moment in the Investi∣gation of the Nature of Colours is the Controversie, Whether those of the Rain-bow, and those that are often seen in Clouds, before the Rising, or after the Setting of the Sun; and in a word, Whe∣ther those other Colours, that are wont to be call'd Emphatical, ought or ought not to be accounted True Colours. I need not tell you that the Negative is the Common Opinion, especially in the Schools, as may appear by that Vulgar distinction of Co∣lours, whereby these under Consideration are term'd Apparent, by way of Opposition

to those that in the other Member of the Distinction are call'd True or Genuine. This question I say seems to me of Impor∣tance, upon this Account, that it being commonly Granted, (or however, easie enough to be Prov'd) that Emphatical Colours are Light it self Modify'd by Re∣fractions chiefly, with a concurrence some∣times of Reflections, and perhaps some other Accidents depending on these two; if these Emphatical Colours be resolv'd to be Genuine, it will seem consequent, that Colours, or at least divers of them, are but Diversify'd Light, and not such Real and Inherent qualities as they are com∣monly thought to be.

2. Now since we are wont to esteem the Echoes and other Sounds of Bodies, to be True Sounds, all their Odours to be True Odours, and (to be short) since we judge other Sensible Qualities to be True ones, because they are the proper Objects of some or other of our Senses, I see not why Emphatical Colours, being the proper and peculiar Objects of the Organ of Sight, and capable to Affect it as Truly and as Powerfully as other Colours, should be re∣puted but Imaginary ones.

And if we have (which perchance you'l allow) formerly evinc'd Colour, (when

the word is taken in its more Proper sense) to be but Modify'd Light, there will be small Reason to deny these to be true Colours, which more manifestly than others disclose themselves to be produc'd by Di∣versifications of the Light.

3. There is indeed taken notice of a Difference betwixt these Apparent colours, and those that are wont to be esteem'd Ge∣nuine, as to the Duration, which has in∣duc'd some Learned Men to call the former rather Evanid than Fantastical. But as the Ingenious Gassendus does somewhere Judi∣ciously observe, if this way of Arguing were Good, the Greeness of a Leaf ought to pass for Apparent, because, soon Fading into a Yellow, it scarce lasts at all, in com∣parison of the Greeness of an Emerauld. I shall add, that if the Sun-beams be in a convenient manner trajected through a Glass-prism, and thrown upon some well∣shaded Object within a Room, the Rain-bow thereby Painted on the Surface of the Body that Terminates the Beams, may oftentimes last longer than some Colours I have produc'd in certain Bodies, which would justly, and without scruple be ac∣counted Genuine Colours, and yet suddenly Degenerate, and lose their Nature.

4. A greater Disparity betwixt Em∣phatical

Colours, and others, may perhaps be taken from this, that Genuine Colours seem to be produc'd in Opacous Bodies by Reflection, but Apparent ones in Diapha∣nous Bodies, and principally by Refraction, I say Principally rather than Solely, be∣cause in some cases Reflection also may concurr, but still this seems not to conclude these Latter Colours not to be True ones. Nor must what has been newly said of the Differences of True and Apparent Colours, be interpreted in too Unlimited a Sense, and therefore it may perhaps some∣what Assist you, both to Reflect upon the two fore-going Objections, and to judge of some other Passages which you'l meet with in this Tract, if I take this Occasion to observe to you, that if Water be Agi∣tated into Froth, it exhibits you know a White colour, which soon after it Loses upon the Resolution of the Bubbles into Air and Water, now in this case either the Whiteness of the Froth is a True Co∣lour or not, if it be, then True Colours, supposing the Water pure and free from Mixtures of any thing Tenacious, may be as Short-liv'd as those of the Rain-bow; also the Matter, wherein the Whiteness did Reside, may in a few moments perfectly Lose all foot-steps or remains of it. And

besides, even Diaphanous Bodies may be capable of exhibiting True Colours by Re∣flection, for that Whiteness is so pro∣duc'd, we shall anon make it probable. But if on the other side it be said, that the Whiteness of Froth is an Emphatical Co∣lour, then it must no longer be said, that Fantastical Colours require a certain Po∣sition of the Luminary and the Eye, and must be Vary'd or Destroy'd by the Change thereof, since Froth appears White, whe∣ther the Sun be Rising or Setting, or in the Meridian, or any where between it and the Horizon, and from what (Neighbouring) place soever the Beholders Eye looks upon it. And since by making a Liquor Tena∣cious enough, yet without Destroying its Transparency, or Staining it with any Co∣lour, you may give the Little Films, where∣of the Bubbles consist, such a Texture, as may make the Froth last very many Hours, if not some Days, or even Weeks, it will render it somewhat Improper to assign Duration for the Distinguishing Character to Discriminate Genuine from Fantastical Colours. For such Froth may much out∣last the Undoubtedly true Colours of some of Nature's Productions, as in that Gaudy Plant not undeservedly call'd the Mervail of Peru, the Flowers do often Fade, the

same Day they are Blown; And I have often seen a Virginian Flower, which usu∣ally Withers within the compass of a Day; and I am credibly Inform'd; that not far from hence a curious Herborist has a Plant, whose Flowers perish in about an Hour. But if the Whiteness of Water turn'd into Froth must therefore be reputed Empha∣tical, because it appears not that the Na∣ture of the Body is Alter'd, but only that the Disposition of its Parts in reference to the Incident Light is Chang'd, why may not the Whiteness be accounted Empha∣tical too, which I shall shew anon to be Producible, barely by such another change in Black Horn? and yet this so easily ac∣quir'd Whiteness seems to be as truly its Colour as the Blackness was before, and at least is more Permanent than the Green∣ness of Leaves, the Redness of Roses, and; in short, than the Genuine Colours of the most part of Nature's Productions. It may indeed be further Objected, that accor∣ding as the Sun or other Luminous Body changes place, these Emphatical Colours alter or vanish. But not to repeat what I have just now said, I shall add, that if a piece of Cloath in a Drapers Shop (in such the Light being seldome Primary) be va∣riously Folded, it will appear of differing

Colours, as the Parts happen to be more Illuminated or more Shaded, and if you stretch it Flat, it will commonly exhibit some one Uniform Colour, and yet these are not wont to be reputed Emphatical, so that the Difference seems to be chiefly this, that in the Case of the Rain-bow, and the like, the Position of the Luminary Varies the Colour, and in the Cloath I have been mentioning, the Position of the Object does it. Nor am I forward to al∣low that in all Cases the Apparition of Emphatical Colours requires a Determi∣nate position of the Eye, for if Men will have the Whiteness of Froth Emphatical, you know what we have already Inferr'd from thence. Besides, the Sun-beams tra∣jected through a Triangular Glass, after the manner lately mention'd, will, upon the Body that Terminates them, Paint a Rain-bow, that may be seen whether the Eye be plac'd on the Right Hand of it or the Left, or Above or Beneath it, or Be∣fore or Behind it; and though there may appear some Little Variation in the Co∣lours of the Rain-bow, beheld from Diffe∣ring parts of the Room, yet such a Diver∣sity may be also observ'd by an Attentive Eye in Real Colours, look'd upon under the like Circumstances. Nor will it fol∣low,

that because there remains no Foot-steps of the Colour upon the Object, when the Prism is Remov'd, that therefore the Colour was not Real, since the Light was truly Modify'd by the Refraction and Re∣flection it Suffer'd in its Trajection through the Prism; and the Object in our case serv'd for a Specular Body, to Reflect that Colour to the Eye. And that you may not be Startled, Pyrophilus, that I should Venture to say, that a Rough and Colour'd Object may serve for a Speculum to Reflect the Artificial Rain-bow I have been mentioning, consider what usually happens in Darkned Rooms, where a Wall, or other Body conveniently Situated within, may so Reflect the Colours of Bodies, without the Room, that they may very clearly be Discern'd and Distinguish'd, and yet 'tis taken for granted, that the Co∣lours seen in a Darkned Room, though they leave no Traces of themselves upon the Wall or Body that Receives them, are the True Colours of the External Objects, to∣gether with which the Colours of the I∣mages are Mov'd or do Rest. And the Errour is not in the Eye, whose Office is only to perceive the Appearances of things, and which does Truly so, but in the Judg∣ing or Estimative faculty, which Mistaking∣ly

concludes that Colour to belong to the Wall, which does indeed belong to the Object, because the Wall is that from whence the Beams of Light that carry the Visible Species, do come in Straight Lines directly to the Eye, as for the same Rea∣son we are wont at a certain Distance from Concave Sphaerical Glasses, to perswade our selves that we see the Image come forth to Meet us, and Hang in the Air betwixt the Glass and Us, because the Reflected Beams that Compose the Image cross in that place, where the Image seems to be, and thence, and not from the Glass, do in Direct Lines take their Course to the Eye, and upon the like Cause it is, that divers Deceptions in Sounds and other Sensible Objects do depend, as we elsewhere de∣clare.

5. I know not, whether I need add, that I have purposely Try'd, (as you'l find some Pages hence, and will perhaps think somewhat strange) that Colours that are call'd Emphatical, because not Inherent in the Bodies in which they Appear, may be Compounded with one another, as those that are confessedly Genuine may. But when all this is said, Pyrophilus, I must Advertise you, that it is but Problematically Spoken, and that though I think the Opi∣nion

I have endeavour'd to fortifie Pro∣bable, yet a great part of our Discourse concerning Colours may be True, whether that Opinion be so or not.