Experiments and Considerations Touching Colours (1664) by Robert Boyle

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28. By what has been hitherto discours'd, _Pyrophilus_, we may be assisted
to judge of that famous Controversie which was of Old disputed betwixt the
_Epicureans_ and other _Atomists_ on the one side, and most other
_Philosophers_ on the other side. The former Denying Bodies to be Colour'd
in the Dark, and the Latter making Colour to be an Inherent quality, as
well as Figure, Hardness; Weight, or the like. For though this Controversie
be Reviv'd, and hotly Agitated among the _Moderns_, yet I doubt whether it
be not in great part a Nominal dispute, and therefore let us, according to
the Doctrine formerly deliver'd, Distinguish the Acceptions of the word
Colour, and say, that if it be taken in the Stricter Sense, the
_Epicureans_ seem to be in the Right, for if Colour be indeed, though not
according to them, but Light Modify'd, how can we conceive that it can
Subsist in the Dark, that is, where it must be suppos'd there is no Light;
but on the other side, if Colour be consider'd as a certain Constant
Disposition of the Superficial parts of the Object to Trouble the Light
they Reflect after such and such a Determinate manner, this Constant, and,
if I may so speak, Modifying disposition persevering in the Object, whether
it be Shin'd upon or no, there seems no just reason to deny, but that in
this Sense, Bodies retain their Colour as well in the Night as Day; or, to
Speak a little otherwise, it may be said, that Bodies are Potentially
Colour'd in the Dark, and Actually in the Light. But of this Matter
discoursing more fully elsewhere, as 'tis a difficulty that concerns
Qualities in general, I shall forbear to insist on it here.

* * * * *

CHAP. IV

1. Of greater Moment in the Investigation 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, Whether 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 Colours, 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 Importance, 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 Refractions chiefly, with a
concurrence sometimes 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 commonly 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
reputed 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
Diversifications 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 Genuine, as to the
Duration, which has induc'd some Learned Men to call the former rather
Evanid than Fantastical. But as the Ingenious _Gassendus_ does somewhere
Judiciously 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 comparison 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 accounted
Genuine Colours, and yet suddenly Degenerate, and lose their Nature.

4. A greater Disparity betwixt Emphatical 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 Diaphanous Bodies, and
principally by Refraction, I say Principally rather than Solely, because 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 somewhat 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 Agitated 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 Colour 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 Reflection, for that Whiteness is so produc'd,
we shall anon make it probable. But if on the other side it be said, that
the Whiteness of Froth is an Emphatical Colour, then it must no longer be
said, that Fantastical Colours require a certain Position of the Luminary
and the Eye, and must be Vary'd or Destroy'd by the Change thereof, since
Froth appears White, whether 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 Tenacious enough, yet without Destroying its Transparency,
or Staining it with any Colour, you may give the Little Films, whereof 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 outlast 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
usually 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 Emphatical, because it appears not that the
Nature 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 Emphatical too, which I shall shew anon to be Producible, barely
by such another change in Black Horn? and yet this so easily acquir'd
Whiteness seems to be as truly its Colour as the Blackness was before, and
at least is more Permanent than the Greenness 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 according 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 variously 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 allow that in all Cases
the Apparition of Emphatical Colours requires a Determinate 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 trajected
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 Before or Behind it; and though there may appear some Little
Variation in the Colours of the Rain-bow, beheld from Differing parts of
the Room, yet such a Diversity may be also observ'd by an Attentive Eye in
Real Colours, look'd upon under the like Circumstances, Nor will it follow,
that because there remains no Footsteps 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 Reflection 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
Coiour'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
Colours 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, together with which the Colours of the Images 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
Judging or Estimative faculty, which Mistakingly 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
Reason 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 declare.

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

* * * * *

CHAP. V.

1. There are you know, _Pyrophilus_, besides those Obsolete Opinions about
Colours which have been long since Rejected, very Various Theories that
have each of them, even at this day, Eminent Men for its Abetters; for the
Peripatetick Schools, though they dispute amongst themselves divers
particulars concerning Colours, yet in this they seem Unanimously enough to
Agree, that Colours are Inherent and Real Qualities, which the Light doth
but Disclose, and not concurr to Produce. Besides there are _Moderns_, who
with a slight Variation adopt the Opinion of _Plato_, and as he would have
Colour to be nothing but a Kind of Flame consisting of Minute Corpuscles as
it were Darted by the Object against the Eye, to whose Pores their
Littleness and Figure made them congruous, so these would have Colour to be
an Internal Light of the more Lucid parts of the Object, Darkned and
consequently Alter'd by the Various Mixtures of the less Luminous parts.
There are also others, who in imitation of some of the Ancient _Atomists_,
make Colour not to be Lucid steam, but yet a Corporeal _Effluvium_ issuing
out of the Colour'd Body, but the Knowingst of these have of late Reform'd
their Hypothesis, by acknowledging and adding that some External Light is
necessary to Excite, and as _they_ speak, Sollicit these Corpuscles of
Colour as _they_ call them, and Bring them to the Eye. Another and more
principal Opinion of the _Modern_ Philosophers, to which this last nam'd
may by a Favourable explication be reconcil'd, is that which derives
Colours from the Mixture of Light and Darkness, or rather Light and
Shadows. And as for the _Chymists_ 'tis known, that the generality of them
ascribes the Origine of Colours to the Sulphureous Principle in Bodies,
though I find, as I elsewhere largely shew, that some of the Chiefest of
them derive Colours rather from Salt than Sulphur, and others, from the
third Hypostatical Principle, _Mercury_. And as for the _Cartesians_ I need
not tell you, that they, supposing the Sensation of Light to bee produc'd
by the Impulse made upon the Organs of Sight, by certain extremely Minute
and Solid Globules, to which the Pores of the Air and other Diaphanous
bodies are pervious, endeavour to derive the Varieties of Colours from the
Various Proportion of the Direct Progress or Motion of these Globules to
their Circumvolution or Motion about their own Centre, by which Varying
Proportion they are by this Hypothesis suppos'd qualify'd to strike the
Optick Nerve after several Distinct manners, so to produce the perception
of Differing Colours.

2. Besides these six principal Hypotheses, _Pyrophilus_, there may be some
others, which though Less known, may perhaps as well as thesc deserve to be
taken into consideration by you; but that I should copiously debate any of
them at present, I presume you will not expect, if you consider the Scope
of these Papers, and the Brevity I have design'd in them, and therefore I
shall at this time only take notice to you in the general of two or three
things that do more peculiarly concern the Treatise you have now in your
hands.

3. And first, though the Embracers of the Several Hypotheses I have been
naming to you, by undertaking each Sect of them to explicate Colours
indefinitely, by the particular Hypotheses they maintain, seem to hold it
forth as the only Needful Theory about that Subject, yet for my part I
doubt whether any one of all these Hypotheses have a right to be admitted
Exclusively to all others, for I think it Probable, that Whiteness and
Blackness may be explicated by Reflection alone without Refraction, as
you'l find endeavour'd in the Discourse you'l meet with e're long Of the
Origine of Whiteness and Blackness, and on the other side, since I have not
found that by any Mixture of White and True Black, (for there is a Blewish
Black which many mistake for a Genuine) there can be a Blew, a Yellow, or a
Red, to name no other Colours, produced, and since we do find that these
Colours may be produc'd in the Glass-prism and other Transparent bodies, by
the help of Refractions, it seems that Refraction is to be taken in into
the Explication of some Colours, to whose Generation they seem to concurr,
either by making a further or other Commixture of Shades with the Refracted
Light, or by some other way not now to be discours'd. And as it seems not
improbable, that in case the Pores of the Air, and other Diaphanous bodies
be every where almost fill'd with such _Globuli_ as the _Cartesians_
suppose, the Various kind of Motion of these _Globuli_, may in many cases
have no small stroak in Varying our Perception of Colour, so without the
Supposition of these _Globuli_, which 'tis not so easie to evince, I think
we may probably enough conceive in general, that the Eye may be Variously
affected, not only by the Entire Beams of Light that fall upon it as they
are such, but by the Order, and by the Degree of Swiftness, and in a word
by the Manner according to which the Particles that compose each Particular
Beam arrive at the Sensory, so that whatever be the Figure of the Little
Corpuscles, of which the Beams of Light consist, not only the Celerity or
Slowness of their Revolution or Rotation in reference to their Progressive
Motion, but their more Absolute Celerity, their Direct or Undulating
Motion, and other Accidents, which may attend their Appulse to the Eye, may
fit them to make Differing Impressions on it.

4. Secondly, For these and the like Considerations, _Pyrophilus_, I must
desire that you would look upon this little Treatise, not as a Discourse
written Principally to maintain any of the fore-mention'd Theories,
Exclusively to all others, or substitute a New one of my Own, but as the
beginning of a History of Colours, upon which, when you and your Ingenious
friends shall have Enrich'd it, a Solid Theory may be safely built. But yet
because this History is not meant barely for a Register of the things
recorded in it, but for an _Apparatus_ to a sound and comprehensitive
Hypothesis, I thought fit, so to temper the whole Discourse, as to make it
as conducible, as conveniently I can to that End, and therefore I have not
scrupled to let you see that I was willing, as to save you the labour of
Cultivating some Theories that I thought would never enable you to reach
the Ends you aim at, so to contract your Enquiries into a Narrow compass,
for both which purposes I thought it requisite to do these two things, the
_One_, to set down some Experiments which by the help of the Reflections
and Insinuations that attend them, may assist you to discover the
Infirmness and Insufficiency both of the common Peripatetick Doctrine, and
of the now more applauded Theory of the _Chymists_ about Colour, because
those two Doctrines having Possess'd themselves, the one of the most part
of the Schools, and the other of the Esteem of the Generality ef Physicians
and other Learned Men, whose Professions and Ways of Study do not exact
that they should Scrupulously examine the very First and Simplest
Principles of Nature, I fear'd it would be to little purpose, without doing
something to discover the Insufficiency of these Hypotheses, that I should,
(which was the _Other_ thing I thought requisite for me to do) set down
among my other Experiments those in the greatest Number, that may let you
see, that, till I shall be Better Inform'd, I encline to take Colour to be
a Modification of Light, and would invite you chiefly to Cultivate that
Hypothesis, and Improve it to the making out of the Generation of
Particular Colours, as I have Endeavour'd to apply it to the Explication of
Whiteness and Blackness.

5. Thirdly. But, _Pyrophilus_, though this be at present the Hypothesis I
preferr, yet I propose it but in a General Sense, teaching only that the
Beams of Light, Modify'd by the Bodies whence they are sent (Reflected or
Refracted) to the Eye, produce there that Kind of Sensation, Men commonly
call Colour; But whether I think this Modification of the Light to be
perform'd by Mixing it with Shades, or by Varying the Proportion of the
Progress and Rotation of the _Cartesian Globuli Caelestes_, or by some other
way which I am not now to mention, I pretend not here to Declare. Much less
do I pretend to Determine, or scarce so much as to Hope to know all that
were requisite to be Known, to give You, or even my Self, a perfect account
of the Theory of Vision and Colours, for in Order to such an undertaking I
would first Know what Light is, and if it be a Body (as a Body or the
Motion of a Body it seems to be) what Kind of Corpuscles for Size and Shape
it consists of, with what Swiftness they move Forwards, and Whirl about
their own Centres. Then I would Know the Nature of Refraction, which I take
to be one of the Abstrusest things (not to explicate Plausibly, but to
explicate Satisfactorily) that I have met with in Physicks; I would further
Know what Kind and what Degree of Commixture of Darkness or Shades is made
by Refractions or Reflections, or both, in the Superficial particles of
those Bodies, that being Shin'd upon, constantly exhibit the one, for
Instance, a Blew, the other a Yellow, the third a Red Colour; I would
further Know why this Contemperation of Light and Shade, that is made, for
Example, by the Skin of a Ripe Cherry, should exhibit a Red, and not a
Green, and the Leaf of the same Tree should exhibit a Green rather than a
Red; and indeed, Lastly, why since the Light that is Modify'd into these
Colours consists but of Corpuscles moved against the _Retina_ or Pith of
the Optick Nerve, it should there not barely give a Stroak, but produce a
Colour, whereas a Needle wounding likewise the Eye, would not produce
Colour but Pain. These, and perhaps other things I should think requisite
to be Known, before I should judge my Self to have fully Comprehended the
True and Whole Nature of Colours; and therefore, though by making the
Experiments and Reflections deliver'd in this Paper, I have endeavour'd
somewhat to Lessen my Ignorance in this Matter, and think it far more
Desireable to discover a Little, than to discover Nothing, yet I pretend
but to make it Probable by the Experiments I mention, that some Colours may
be Plausibly enough Explicated in the General by the Doctrine here
propos'd; For whensoever I would Descend to the Minute and Accurate
Explication of Particulars, I find my Self very Sensible of the great
Obscurity of things, without excepting those which we never see but when
they are Enlightned, and confess with _Scaliger_[5], _Latet natura haec_,
(says he, Speaking of that of Colour) _& sicut aliarum rerum species in
profundissima caligine inscitiae humanae._

[5] Exercitat. 325 Parag. 4

* * * * *

_THE_
_EXPERIMENTAL HISTORY_
_OF COLOURS._

* * * * *

PART. II.

_Of the Nature of Whiteness and_
_Blackness._

CHAP. I.

1. Though after what I have acknowledged, _Pyrophilus_, of the Abstruse
Nature of Colours in _particular_, you will easily believe, that I pretend
not to give you a Satisfactory account of Whiteness and Blackness; Yet not
wholly to frustrate your Expectation of my offering something by way of
Specimen towards the Explication of some Colours in particular, I shall
make choice of These as the most Simple Ones, (and by reason of their
mutual Opposition the Least hardly explicable) about which to present you
my Thoughts, upon condition you will take them at most to be my
Conjectures, not my Opinions.

2. When I apply'd my Self to consider, how the cause of Whiteness might be
explan'd by Intelligible and Mechanical Principles, I remembred not to have
met with any thing among the Antient _Corpuscularian_ Philosophers,
touching the Quality we call Whiteness, save that _Democritus_ is by
_Aristotle_ said to have ascrib'd the Whiteness of Bodies to their
Smoothness, and on the contrary their Blackness to their Asperity.[6] But
though about the Latter of those Qualities his Opinion be allowable, as we
shall see anon, yet that he heeds a Favourable Interpretation in what is
Deliver'd concerning the First, (at least if his Doctrine be not
Mis-represented in this point, as it has been in many others) we shall
quickly have Occasion to manifest. But amongst the _Moderns_, the most
Learned _Gassendus_ in his Ingenious Epistle publish'd in the Year 1642.
_De apparente Magnitudine solis humilis & sublimis_, reviving the
_Atomical_ Philosophy, has, though but Incidentally, deliver'd something
towards the Explication of Whiteness upon Mechanical Principles: And
because no Man that I know of, has done so before him, I shall, to be sure
to do him Right, give you his Sense in his own Words:[7] _Cogites velim_
(says he) _lucem quidem in Diaphano nullius coloris videri, sed in Opaco
tamen terminante Candicare, ac tanto magis, quanto densior seu collectior
fuerit. Deinde aquam non esse quidem coloris ex se candidi & radium tamen
ex ea reflexum versus oculum candicare. Rursus cum plana aquae Superficies
non nisi ex una parte eam reflexionem faciat: si contigerit tamen illam in
aliquot bullas intumescere, bullam unamquamque reflectionem facere, &
candoris speciem creare certa Superficiei parte. Ad haec Spumam ex aqua pura
non alia ratione videri candescere & albescerere quam quod sit congeries
confertissima minutissimarum bullarum, quarum unaquaeque suum radium
reflectit, unde continens candor alborve apparet. Denique Nivem nihil aliud
videri quam speciem purissimae spumae ex bullulis quam minutissimis &
confertissimis cohaerentis. Sed ridiculam me exhibeam, si tales meas nugas
uberius proponem._

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