Experiments and Considerations Touching Colours (1664) by Robert Boyle

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CHAP. II.

1 To come then in the first place to our more general Considerations, I
shall begin with saying something as to the Importance of examining the
Colours of Bodies. For there are some, especially _Chymists_, who think,
that a considerable diversity of Colours does constantly argue an equal
diversity of Nature, in the Bodies wherein it is conspicuous; but I confess
I am not altogether of their mind; for not to mention changeable Taffaties,
the blew and golden necks of Pidgeons, and divers Water-fowl, Rainbows
Natural and Artificial, and other Bodies, whose Colours the Philosophers
have been pleased to call not Real, but Apparent and Phantastical; not to
insist on these, I say, (for fear of needlesly engaging in a Controversie)
we see in Parrots, Goldfinches, and divers other Birds, not only that the
contiguous feathers which are probably as near in properties as place, are
some of them Red, and others White, some of them Blew, & others Yellow,
_&c._ but that in the several parts of the self-same feather there may
often be seen the greatest disparity of Colours; and so in the leaves of
Tulips, July-flowers, and some other Vegetables the several leaves, and
even the several parts of the same leaf, although no difference have been
observed in their other properties, are frequently found painted with very
different Colours. And such a variety we have much more admired in that
lovely plant which is commonly, and not unjustly call'd the _Marvayl of
Peru_; for of divers scores of fine Flowers, which in its season that gaudy
Plant does almost daily produce, I have scarce taken notice of any two that
were dyed perfectly alike. But though _Pyro_: such things as these, among
others, keep mee from daring to affirm, that the Diversity and change of
Colours does _alwaies_ argue any great difference or alteration, betwixt,
or in, the Bodies, wherein it is to be discerned, yet that _oftentimes_ the
Alteration of Colours does signifie considerable Alterations in the
disposition of parts of Bodies, may appear in the Extraction of Tinctures,
and divers other Chymical Operations, wherein the change of Colours is the
chief, and sometimes the only thing, by which the Artist regulates his
proceeding, and is taught to know when 'tis seasonable for him to leave
off. Instances of this sort are more obvious in divers sorts of fruits, as
Cherries, Plums, &c. wherein, according as the Vegetable sap is sweetned,
or otherwise ripened, by passing from one degree to another of Maturation,
the external part of the fruit passes likewise from one to another Colour.
But one of the noblest Instances I have met with of this kind, is not so
obvious; and that is the way of tempering Steel to make Gravers, Drills,
Springs, and other Mechanical Instruments, which we have divers times both
made Artificers practise in our presence, and tryed our selves, after the
following manner, First, the slender Steel to be tempered is to be hardened
by heating as much of it as is requisite among glowing Coals, till it be
glowing hot, but it must not be quenched assoon as it is taken from the
fire (for that would make it too brittle, and spoil it) but must be held
over a bason of water, till it descend from a White heat to a Red one,
which assoon as ever you perceive, you must immediately quench as much as
you desire to harden in the cold water. The Steel thus hardened, will, if
it be good, look somewhat White and must be made bright at the end, that
its change of Colours may be there conspicuous; and then holding it so in
the flame of a Candle, that the bright end may be, for about half an inch,
or more, out of the flame, that the smoak do not stain or sully the
brightness of it, you shall after a while see that clean end, which is
almost contiguous to the flame, pass very nimbly from one Colour to
another, as from a brighter Yellow, to a deeper and reddish Yellow, which
Artificers call a _sanguine_, and from that to a fainter first, and then a
a deeper Blew. And to bring home this Experiment to our present purpose, it
is found by daily Experience, that each of these succeeding Colours argue
such a change made in the texture of the Steel, that if it be taken from
the flame, and immediately quenched in the tallow (whereby it is setled in
whatever temper it had before) when it is Yellow, it is of such a hardness
as makes it fit for Gravers Drills, and such like tools; but if it be kept
a few minutes longer in the flame till it grow Blew, it becomes much
softer, and unfit to make Gravers for Metalls, but fit to make Springs for
Watches, and such like Instruments, which are therefore commonly of that
Colour; and if the Steel be kept in the flame, after that this deep Blew
hath disclosed it self, it will grow so soft, as to need to be new hardened
again, before it can be brought to a temper, fit for Drills or Penknives.
And I confess _Pyro._ I have taken much pleasure to see the Colours run
along from the parts of the Steel contiguous to the flame, to the end of
the Instrument, and succeed one another so fast, that if a man be not
vigilant, to thrust the Steel into the tallow at the very nick of time, at
which it has attain'd its due Colour, he shall miss of giving his tool the
right temper. But because the flame of a Candle is offensive to my weak
eyes, and because it is apt to either black or sully the contiguous part of
the Steel which is held in it, and thereby hinder the change of Colours
from being so long and clearly discern'd, I have sometimes made this
Experiment by laying the Steel to be tempered upon a heated bar of Iron,
which we finde also to be employ'd by some Artificers in the tempering of
such great Instruments, as are too big to be soon heated sufficiently by
the flame of a Candle. And you may easily satisfie your self _Pyro_: of the
differing hardness and toughness, which is ascribed to Steel temper'd at
different Colours, if you break but some slender wires of Steel so
temper'd, and observe how they differ in brittleness, and if with a file
you also make tryal of their various degrees of hardness.

2 But _Pyrophilus_, I must not at present any further prosecute the
Consideration of the importance of Experiments about Colours, not only
because you will in the following papers finde some instances, that would
here be presented you out of their due place, of the use that may be made
of such Experiments, in discovering in divers bodies, what kind the salt
is, that is predominant in them; but also because a speculative Naturalist
might justly enough allege, that as Light is so pleasing an object, as to
be well worth our looking on, though it discover'd to us nothing but its
self; so modifi'd Light called Colour, were worth our contemplation, though
by understanding its Nature we should be taught nothing else. And however,
I need not make either you or my self excuses for entertaining you on the
subject I am now about to treat of, since the pleasure _Pyro_: takes in
mixing and laying on of Colours, will I presume keep him, and will (I am
sure) keep mee from thinking it troublesome to set down, especially after
the tedious processes (about other matters) wherewith I fear I may have
tyr'd him, some easie, and not unpleasant Experiments relating to that
subject.

3 But, before we descend to the more particular considerations, we are to
present you concerning Colours, I presume it will be seasonable to propose
at the very entrance a Distinction; the ignorance or neglect of which,
seems to mee to have frequently enough occasioned either mistakes or
confusion in the Writings of divers Modern Philosophers; for Colour may be
considered, either as it is a quality residing in the body that is said to
be coloured, or to modifie the light after such or such a manner; or else
as the Light it self, which so modifi'd, strikes upon the organ of sight,
and so causes that Sensation which we call Colour; and that this latter may
be look'd upon as the more proper, though not the usual acception of the
word Colour, will be made probable by divers passages in the insuing part
of our discourse; and indeed it is the Light it self, which after a certain
manner, either mingled with shades, or some other waies troubled, strikes
our eyes, that does more immediately produce that motion in the organ, upon
whose account men say they see such or such a Colour in the object; yet,
because there is in the body that is said to be coloured, a certain
disposition of the superficial particles, whereby it sends the Light
reflected, or refracted, to our eyes thus and thus alter'd, and not
otherwise, it may also in some sense be said, that Colour depends upon the
visible body; and therefore we shall not be against that way of speaking of
Colours that is most used among the Modern Naturalists, provided we be
allowed to have recourse when occasion shall require to the premis'd
distinction, and to take the more immediate cause of Colour to be the
modifi'd Light it self, as it affects the Sensory; though the disposition
also of the colour'd body, as that modifies the Light, may be call'd by
that name Metonimically (to borrow a School term) or Efficiently, that is
in regard of its turning the Light, that rebounds from it, or passes thorow
it, into this or that particular Colour.

4 I know not whether I may not on this occasion add, that Colour is so far
from being an Inherent quality of the object in the sense that is wont to
be declar'd by the Schools, or even in the sense of some Modern Atomists,
that, if we consider the matter more attentively, we shall see cause to
suspect, if not to conclude, that though Light do more immediately affect
the organ of sight, than do the bodies that send it thither, yet Light it
self produces the sensation of a Colour, but as it produces such a
determinate kind of local motion in some part of the brain; which, though
it happen most commonly from the motion whereinto the slender strings of
the _Retina_ are put, by the appulse of Light, yet if the like motion
happen to be produc'd by any other cause, wherein the Light concurrs not at
all, a man shall think he sees the same Colour. For proof of this, I might
put you in mind, that 'tis usual for dreaming men to think they see the
Images that appear to them in their sleep, adorn'd some with this, and some
with that lively Colour, whilst yet, both the curtains of their bed, and
those of their eyes are close drawn. And I might add the confidence with
which distracted persons do oftentimes, when they are awake, think, they
see black fiends in places, where there is no black object in sight without
them. But I will rather observe, that not only when a man receives a great
stroak upon his eye, or a very great one upon some other part of his head,
he is wont to see, as it were, flashes of lightning, and little vivid, but
vanishing flames, though perhaps his eyes be shut: But the like apparitions
may happen, when the motion proceeds not from something without, but from
something within the body, provided the unwonted fumes that wander up and
down in the head, or the propagated concussion of any internal part in the
body, do cause about the inward extremities of the Optick Nerve, such a
motion as is wont to be there produc'd, when the stroak of the Light upon
the _Retina_ makes us conclude, that we see either Light, or such and such
a Colour: This the most ingenious _Des Cartes_ hath very well observ'd, but
because he seems not to have exemplifi'd it by any unobvious or peculiar
observation, I shall indeavour to illustrate this doctrine by a few
Instances.

5 And first, I remember, that having, through Gods goodness, been free for
several years, from troublesome Coughs, being afterwards, by an accident,
suddenly cast into a violent one, I did often, when I was awaked in the
night by my distempers, observe, that upon coughing strongly, it would seem
to mee, that I saw very vivid, but immediately disappearing flames, which I
took particular notice of, because of the conjecture I am now mentioning.

6 An excellent and very discreet person, very near ally'd both to you and
mee, was relating to mee, that some time since, whilst she was talking with
some other Ladies, upon a sudden, all the objects, she looked upon,
appeared to her dyed with unusual Colours, some of one kind, and some of
another, but all so bright and vivid, that she should have been as much
delighted, as surpriz'd with them, but that finding the apparition to
continue, she fear'd it portended some very great alteration as to her
health: As indeed the day after she was assaulted with such violence by
Hysterical and Hypocondrical Distempers, as both made her rave for some
daies, and gave her, during that time, a Bastard Palsey.

7 Being a while since in a Town, where the Plague had made great havock,
and inquiring of an ingenious man, that was so bold, as without much
scruple to visit those that were sick of it, about the odd symptomes of a
Disease that had swept away so many there; he told mee, among other things,
that he was able to tell divers Patients, to whom he was called, before
they took their beds, or had any evident symptomes of the Plague, that they
were indeed infected upon peculiar observations, that being asked, they
would tell him that the neighbouring objects, and particularly his cloths,
appear'd to them beautifi'd with most glorious Colours, like those of the
Rainbow, oftentimes succeeding one another; and this he affirm'd to be one
of the most usual, as well as the most early symptomes, by which this odd
Pestilence disclos'd it self: And when I asked how long the Patients were
wont to be thus affected, he answered, that it was most commonly for about
a day; and when I further inquired whether or no Vomits, which in that
Pestilence were usually given, did not remove this symptome (For some used
the taking of a Vomit, when they came ashore, to cure themselves of the
obstinate and troublesome giddiness caus'd by the motion of the ship)
reply'd, that generally, upon the evacuation made by the Vomit, that
strange apparition of Colours ceased, though the other symptomes were not
so soon abated, yet he added (to take notice of that upon the by, because
the observation may perchance do good) that an excellent Physician, in
whose company he was wont to visit the sick, did give to almost all those
to whom he was called, in the beginning before Nature was much weakened, a
pretty odd Vomit consisting of eight or ten dramms of Infusion of _Crocus
Metallorum_, and about half a dramm, or much more, of White Vitriol, with
such success, that scarce one of ten to whom it was seasonably administred,
miscarried.

8 But to return to the consideration of Colours: As an apparition of them
may be produced by motions from within, without the assistance of an
outward object, so I have observed, that 'tis sometimes possible that the
Colour that would otherwise be produced by an outward object, may be
chang'd by some motion, or new texture already produced in the Sensory, as
long as that unusual motion, or new disposition lasts; for I have divers
times try'd, that after I have through a Telescope look'd upon the Sun,
though thorow a thick, red, or blew glass, to make its splendor supportable
to the eye, the impression upon the _Retina_, would be not only so vivid,
but so permanent, that if afterwards I turned my eye towards a flame, it
would appear to mee of a Colour very differing from its usual one. And if I
did divers times successively shut and open the same eye, I should see the
adventitious Colour, (if I may so call it) changed or impair'd by degrees,
till at length (for this unusual motion of the eye would not presently
cease) the flame would appear to mee, of the same hew that it did to other
beholders; a not unlike effect I found by looking upon the Moon, when she
was near full, thorow an excellent Telescope, without colour'd Glass to
screen my eye with; But that which I desire may be taken notice of, because
we may elsewhere have occasion to reflect upon it, and because it seems not
agreeable to what Anatomists and Optical Writers deliver, touching the
relation of the two eyes to each other, is this circumstance, that though
my Right eye, with which I looked thorow the Telescope, were thus affected
by the over-strong impression of the light, yet when the flame of a Candle,
or some other bright object appear'd to me of a very unusual Colour, whilst
look'd upon with the Discompos'd Eye, or (though not so notably) with both
eyes at once; yet if I shut that Eye, and looked upon the same object with
the other, it would appear with no other than its usual Colour, though if I
again opened, and made use of the Dazled eye, the vivid adventitious Colour
would again appear. And on this occasion I must not pretermit an
Observation which may perswade us, that an over-vehement stroak upon the
Sensory, especially if it be naturally of a weak constitution, may make a
more lasting impression than one would imagine, which impression may in
some cases, as it were, mingle with, and vitiate the action of vivid
objects for a long time after.

For I know a Lady of unquestionable Veracity, who having lately, by a
desperate fall, receiv'd several hurts, and particularly a considerable one
upon a part of her face near her Eye, had her sight so troubl'd and
disorder'd, that, as she hath more than once related to me, not only when
the next morning one of her servants came to her bed side, to ask how she
did, his cloaths appear'd adorn'd with such variety of dazling Colours,
that she was fain presently to command him to withdraw, but the Images in
her Hangings, did, for many daies after, appear to her, if the Room were
not extraordinarily darken'd, embellish'd with several offensively vivid
Colours, which no body else could see in them; And when I enquir'd whether
or no White Objects did not appear to her adorn'd with more luminous
Colours than others, and whether she saw not some which she could not now
well describe to any, whose eyes had never been distemper'd, she answer'd
mee, that sometimes she thought she saw Colours so new and glorious, that
they were of a peculiar kind, and such as she could not describe by their
likeness to any she had beheld either before or since, and that White
Objects did so much disorder her sight, that if several daies after her
fall, she look'd upon the inside of a Book, she fanci'd she saw there
Colours like those of the Rain-bow, and even when she thought her self
pretty well recover'd, and made bold to leave her Chamber, the coming into
a place where the Walls and Ceeling were whited over, made those Objects
appear to her cloath'd with such glorious and dazling Colours, as much
offended her sight, and made her repent her venturousness, and she added,
that this Distemper of her Eyes lasted no less than five or six weeks,
though, since that, she hath been able to read and write much without
finding the least Inconvenience in doing so. I would gladly have known,
whether if she had shut the Injur'd Eye, the _Phaenomena_ would have been
the same, when she employ'd only the other, but I heard not of this
accident early enough to satisfie that Enquiry.

9 Wherefore, I shall now add, that some years before, a person exceedingly
eminent for his profound Skil in almost all kinds of Philological Learning,
coming to advise with mee about a Distemper in his Eyes, told me, among
other Circumstances of it, that, having upon a time looked too fixedly upon
the Sun, thorow a Telescope, without any coloured Glass, to take off from
the dazling splendour of the Object, the excess of Light did so strongly
affect his Eye, that ever since, when he turns it towards a Window, or any
White Object, he fancies, he seeth a Globe of Light, of about the bigness
the Sun then appeared of to him, to pass before his Eyes: And having
Inquir'd of him, how long he had been troubled with this Indisposition, he
reply'd, that it was already nine or ten years, since the Accident, that
occasioned it, first befel him.

I could here subjoyn, _Pyrophilus_, some memorable Relations that I have
met with in the Account given us by the experienc'd _Epiphanius
Ferdinandus_, of the Symptomes he observ'd to be incident to those that are
bitten with the Tarantula, by which (Relations) I could probably shew, that
without any change in the Object, a change in the Instruments of Vision may
for a great while make some Colours appear Charming, and make others
Provoking, and both to a high degree, though neither of them produc'd any
such Effects before. These things, I say, I could here subjoyn in
confirmation of what I have been saying, to shew, that the Disposition of
the Organ is of great Importance in the Dijudications we make of Colours,
were it not that these strange Stories belonging more properly to another
Discourse, I had rather, (contenting my self to have given you an
Intimation of them here) that you should meet with them fully deliver'd
there.

* * * * *

CHAP. III.

But, _Pyrophilus_, I would not by all that I have hitherto discours'd, be
thought to have forgotten the Distinction (of Colour) that I mentioned to
you about the beginning of the third Section of the former Chapter; and
therefore, after all I have said of Colour, as it is modifi'd Light, and
immediately affects the Sensory, I shall now re-mind you, that I did not
deny, but that Colour might in some sense be consider'd as a Quality
residing in the body that is said to be Colour'd, and indeed the greatest
part of the following Experiments referr to Colour principally under that
Notion, for there is in the bodyes we call Colour'd, and chiefly in their
Superficial parts, a certain disposition, whereby they do so trouble the
Light that comes from them to our Eye, as that it there makes that distinct
Impression, upon whose Account we say, that the Seen body is either White
or Black, or Red or Yellow, or of any one determinate Colour. But because
we shall (God permiting) by the Experiments that are to follow some Pages
hence, more fully and particularly shew, that the Changes, and consequently
in divers places the Production and the appearance of Colours depends upon
the continuing or alter'd Texture of the Object, we shall in this place
intimate (and that too but as by the way) two or three things about this
Matter.

2. And first it is not without some Reason, that I ascribe Colour (in the
sense formerly explan'd) _chiefly_ to the Superficial parts of Bodies, for
not to question how much Opacous Corpuscles may abound even in those Bodies
we call Diaphanous, it seems plain that of Opacous bodies we do indeed see
little else than the Superficies, for if we found the beams of Light that
rebound from the Object to the Eye, to peirce deep into the Colour'd body,
we should not judge it Opacous, but either Translucid, or at least
Semi-diaphanous, and though the Schools seem to teach us that Colour is a
Penetrative Quality, that reaches to the Innermost parts of the Object, as
if a piece of Sealing-wax be broken into never so many pieces, the Internal
fragments will be as Red as the External surface did appear, yet that is
but a Particular Example that will not overthrow the Reason lately offer'd,
especially since I can alleage other Examples of a contrary Import, and two
or three Negative Instances are sufficient to overthrow the Generality of a
Positive Rule, especially if that be built but upon One or a Few Examples.
Not (then) to mention Cherries, Plums, and I know not how many other
Bodies, wherein the skin is of one Colour, and what it hides of another, I
shall name a couple of Instances drawn from the Colours of Durable bodies
that are thought far more Homogeneous, and have not parts that are either
Organical, or of a Nature approaching thereunto.

3 To give you the first Instance, I shall need but to remind you of what I
told you a little after the beginning of this Essay, touching the Blew and
Red and Yellow, that may be produc'd upon a piece of temper'd Steel, for
these Colours though they be very Vivid, yet if you break the Steel they
adorn, they will appear to be but Superficial; not only the innermost parts
of the Metall, but those that are within a hairs breadth of the
Superficies, having not any of these Colours, but retaining that of the
Steel it self. Besides that, we may as well confirm this Observation, as
some other particulars we elsewhere deliver concerning Colours, by the
following Experiment which we purposely made.

4 We took a good quantity of clean Lead, and melted it with a strong Fire,
and then immediately pouring it out into a clean Vessel of a convenient
shape and matter, (we us'd one of Iron, that the great and sudden Heat
might not injure it) and then carefully and nimbly taking off the Scum that
floated on the top, we perceiv'd, as we expected, the smooth and glossie
Surface of the melted matter, to be adorn'd with a very glorious Colour,
which being as Transitory as Delightfull, did almost immediately give place
to another vivid Colour, and that was as quickly succeeded by a third, and
this as it were chas'd away by a fourth, and so these wonderfully vivid
Colours successively appear'd and vanish'd, (yet the same now and then
appearing the second time) till the Metall ceasing to be hot enough to
afford any longer this pleasing Spectacle, the Colours that chanc'd to
adorn the Surface, when the Lead thus began to cool, remain'd upon it; but
were so Superficial, that how little soever we scrap'd off the Surface of
the Lead, we did in such places scrape off all the Colour, and discover
only that which is natural to the Metall it self, which receiving its
adventitious Colours, only when the heat was very Intense, and in that part
which was expos'd to the comparatively very cold Air, (which by other
Experiments seems to abound with subtil Saline parts, perhaps not uncapable
of working upon Lead so dispos'd:) These things I say, together with my
observing that whatever parts of the so strongly melted Lead were expos'd a
while to the Air, turn'd into a kind of Scum or Litharge, how bright and
clean soever they appear'd before, suggested to me some Thoughts or
Ravings, which I have not now time to acquaint You with. One that did not
know me, _Pyrophilus_, would perchance think I endeavour'd to impose upon
You by relating this Experiment, which I have several times try'd, but the
Reason why the _Phaenomena_ mention'd have not been taken notice of, may be,
that unless Lead be brought to a much higher degree of Fusion or Fluidity
than is usual, or than is indeed requisite to make it melt, the _Phaenomena_
I mention'd will scarce at all disclose themselves; And we have also
observ'd that this successive appearing and vanishing of vivid Colours, was
wont to be impair'd or determin'd whilst the Metal expos'd to the Air
remain'd yet hotter than one would readily suspect. And one thing I must
further Note, of which I leave You to search after the Reason, namely, that
the same Colours did not always and regularly succeed one another, as is
usually in Steel, but in the diversify'd Order mention'd in this following
Note, which I was scarce able to write down, the succession of the Colours
was so very quick, whether that proceeded from the differing degrees of
Heat in the Lead expos'd to the cool Air, or from some other Reason, I
leave you to examine.

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