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Micrographia by Robert Hooke

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From the consideration of the proprieties of which impressions, we may
collect these short definitions of Colours: That _Blue is an impression on
the Retina of an oblique and confus'd pulse of light, whose weakest part
precedes, and whose strongest follows._ And, that _Red is an impression on
the Retina of an oblique and confus'd pulse of light, whose strongest part
precedes, and whose weakest follows._

Which proprieties, as they have been already manifested, in the Prisme and
falling drops of Rain, to be the causes of the colours there generated, may
be easily found to be the efficients also of the colours appearing in thin
_laminated_ transparent bodies; for the explication of which, all this has
been premised.

And that this is so, a little closer examination of the _Phaenomena_ and
the _Figure_ of the body, by this _Hypothesis_ will make evident.

For first (as we have already observed) the _laminated_ body must be of a
determinate thickness, that is, it must not be thinner then such a
determinate quantity; for I have always observ'd, that neer the edges of
those which are exceeding thin, the colours disappear, and the part grows
white; nor must it be thicker then another determinate quantity; for I have
likewise observ'd, that beyond such a thickness, no colours appear'd, but
the Plate looked white, between which two determinate thicknesses were all
the colour'd Rings; of which in some substances I have found ten or twelve,
in others not half so many, which I suppose depends much upon the
transparency of the _laminated_ body. Thus though the consecutions are the
same in the scumm or the skin on the top of metals; yet in those
consecutions in the same colour is not so often repeated as in the
consecutions in thin Glass, or in Sope-water, or any other more transparent
and glutinous liquor; for in these I have observ'd, _Red, Yellow, Green,
Blue, Purple; Red, Yellow, Green, Blue, Purple; Red, Yellow, Green, Blue,
Purple; Red, Yellow, &c._ to succeed each other, ten or twelve times, but
in the other more _opacous_ bodies the consecutions will not be half so
many.

And therefore secondly, the _laminated_ body must be transparent, and this
I argue from this, that I have not been able to produce any colour at all
with an _opacous_ body, though never so thin. And this I have often try'd,
by pressing small _Globule_ of _Mercury_ between two smooth Plates of
Glass, whereby I have reduc'd that body to a much greater thinness then was
requisite to exhibit the colours with a transparent body.

Thirdly, there must be a considerable reflecting body adjacent to the under
or further side of the _lamina_ or _plate_: for this I always found, that
the greater that reflection was, the more vivid were the appearing colours.

From which Observations, is most evident, that the reflection from the
under or further side of the body is the principal cause of the production
of these colours; which, that it is so, and how it conduces to that effect,
I shall further explain in the following Figure, which is here described of
a very great thickness, as if it had been view'd through the _Microscope_;
and 'tis indeed much thicker than any _Microscope_ (I have yet us'd) has
been able to shew me those colour'd plates of Glass, or _Muscovie-glass_,
which I have not without much trouble view'd with it, for though I have
endeavoured to magnifie them as much as the Glasses were capable of, yet
are they so exceeding thin, that I have not hitherto been able positively
to determine their thickness. This Figure therefore I here represent, is
wholy _Hypothetical_.

Let ABCDHFE in the sixth Figure be a _frustum_ of _Muscovy-glass_, thinner
toward the end AE, and thicker towards DF. Let us first suppose the Ray
aghb coming from the Sun, of some remote luminous object to fall
_obliquely_ on the thinner plate BAE, part therefore is reflected back by
cghd, the first _Superficies_; whereby the perpendicular pulse ab is after
reflexion propagated by cd, cd, equally remote from each other with ab, ab,
so that ag + gc, or bh + hd are either of them equal to aa, as is also cc,
but the body BAE being transparent, a part of the light of this Ray is
refracted in the surface AB, and propagated by gikh to the surface EF,
whence it is reflected and refracted again by the surface AB. So that after
two refractions and one reflection, there is propagated a kind of fainter
Ray emnf, whose pulse is not only weaker by reason of the two refractions
in the surface AB, but by reason of the time spent in passing and repassing
between the two surfaces AB and EF, ef which is this fainter or weaker
pulse comes behind the pulse cd; so that hereby (the surfaces AB, and EF
being so neer together, that the eye cannot _discriminate_ them from one)
this confus'd or _duplicated_ pulse, whose strongest part precedes, and
whose weakest follows, does produce on the _Retina_, (or the _optick nerve_
that covers the bottom of the eye) the sensation of a _Yellow_.

And secondly, this _Yellow_ will appear so much the deeper, by how much the
further back towards the middle between cd and cd the spurious pulse ef is
remov'd, as in 2 where the surface BC being further remov'd from EF, the
weaker pulse ef will be nearer to the middle, and will make an impression
on the eye of a _Red_.

But thirdly, if the two reflecting surfaces be yet further remov'd asunder
(as in 3 CD and EF are) then will the weaker pulse be so farr behind, that
it will be more then half the distance between cd and cd. And in this case
it will rather seem to precede the following stronger pulse, then to follow
the preceding one, and consequently a _Blue_ will be generated. And when
the weaker pulse is just in the middle between two strong ones, then is a
deep and lovely _Purple_ generated; but when the weaker pulse ef is very
neer to cd, then is there generated a _Green_, which will be _bluer_, or
_yellower_, according as the _approximate_ weak pulse does precede or
follow the stronger.

Now fourthly, if the thicker Plate chance to be cleft into two thinner
Plates, as CDFE is divided into two Plates by the surface GH then from the
composition arising from the three reflections in the surfaces CD, GH, and
EF, there will be generated several compounded or mixt colours, which will
be very differing, according as the proportion between the thicknesses of
those two divided Plates CDHG, and GHFE are varied.

And _fifthly_, if these surfaces CD and FE are further remov'd asunder, the
weaker pulse will yet lagg behind much further, and not onely be
_coincident_ with the second, cd, but lagg behind that also, and that so
much the more, by how much the thicker the Plate be; so that by degrees it
will be _coincident_ with the third cd backward also, and by degrees, as
the Plate grows thicker with a fourth, and so onward to a fifth, sixth,
seventh, or eighth; so that if there be a thin transparent body, that from
the greatest thinness requisite to produce colours, does, in the manner of
a Wedge, by degrees grow to the greatest thickness that a Plate can be of,
to exhibit a colour by the reflection of Light from such a body, there
shall be generated several consecutions of colours, whose order from the
thin end towards the thick, shall be _Yellow, Red, Purple, Blue, Green;
Yellow, Red, Purple, Blue, Green; Yellow, Red, Purple, Blue, Green;
Yellow_, &c. and these so often repeated, as the weaker pulse does lose
paces with its _Primary_, or first pulse, and is _coincident_ with a
second, third, fourth, fifth, sixth, &c. pulse behind the first. And this,
as it is _coincident_, or follows from the first _Hypothesis_ I took of
colours, so upon experiment have I found it in multitudes of instances that
seem to prove it. One thing which seems of the greatest concern in this
_Hypothesis_, is to determine the greatest or least thickness requisite for
these effects, which, though I have not been wanting in attempting, yet so
exceeding thin are these coloured Plates, and so imperfect our
_Microscope_, that I have not been hitherto successfull, though if my
endeavours shall answer my expectations, I shall hope to gratifie the
curious Reader with some things more remov'd beyond our reach hitherto.

Thus have I, with as much brevity as I was able, endeavoured to explicate
(_Hypothetically_ at least) the causes of the _Phaenomena_ I formerly
recited, on the consideration of which I have been the more particular.

First, because I think these I have newly given are capable of explicating
all the _Phaenomena_ of colours, not onely of those appearing in the
_Prisme_, Water-drop, or Rainbow, and in _laminated_ or plated bodies, but
of all that are in the world, whether they be fluid or solid bodies,
whether in thick or thin, whether transparent, or seemingly opacous, as I
shall in the next Observation further endeavour to shew. And secondly,
because this being one of the two ornaments of all bodies discoverable by
the sight, whether looked on with, or without a _Microscope_, it seem'd to
deserve (somewhere in this Tract, which contains a description of the
Figure and Colour of some minute bodies) to be somewhat the more intimately
enquir'd into.

* * * * *


Observ. X. _Of _Metalline_, and other real Colours._

Having in the former Discourse, from the Fundamental cause of Colour, made
it probable, that there are but two Colours, and shewn, that the _Phantasm_
of Colour is caus'd by the sensation of the _oblique_ or uneven pulse of
Light which is capable of no more varieties than two that arise from the
two sides of the _oblique_ pulse, though each of those be capable of
infinite gradations or degrees (each of them beginning from _White_, and
ending the one in the deepest _Scarlet_ or _Yellow_, the other in the
deepest _Blue_) I shall in this _Section_ set down some Observations which
I have made of other colours, such as _Metalline_ powders tinging or
colour'd bodies and several kinds of tinctures or ting'd liquors, all
which, together with those I treated of in the former Observation will, I
suppose, comprise the several subjects in which colour is observ'd to be
inherent, and the several manners by which it _inheres_, or is apparent in
them. And here I shall endeavour to shew by what composition all kind of
compound colours are made, and how there is no colour in the world but may
be made from the various degrees of these two colours, together with the
intermixtures of _Black_ and _White_.

And this being so, as I shall anon shew, it seems an evident argument to
me, that all colours whatsoever, whether in fluid or solid, whether in very
transparent or seemingly _opacous_, have the same efficient cause, to wit,
some kind of _refraction_ whereby the Rays that proceed from such bodies,
have their pulse _obliquated_ or confus'd in the manner I explicated in the
former _Section_; that is, a _Red_ is caus'd by a duplicated or confus'd
pulse, whose strongest pulse precedes, and a weaker follows: and a _Blue_
is caus'd by a confus'd pulse, where the weaker pulse precedes, and the
stronger follows. And according as these are, more or less, or variously
mixt and compounded, so are the _sensations_, and consequently the
_phantasms_ of colours _diversified_.

To proceed therefore; I suppose, that all transparent colour'd bodies,
whether fluid or solid, do consist at least of two parts, or two kinds of
substances, the one of a substance of a somewhat differing _refraction_
from the other. That one of these substances which may be call'd the
_tinging_ substance, does consist of distinct parts, or particles of a
determinate bigness which are _disseminated_, or dispers'd all over the
other: That these particles, if the body be equally and uniformly colour'd,
are evenly rang'd and dispers'd over the other contiguous body; That where
the body is deepest ting'd, there these particles are rang'd thickest, and
where 'tis but faintly ting'd, they are rang'd much thinner, but uniformly.
That by the mixture of another body that unites with either of these, which
has a differing refraction from either of the other, quite differing
effects will be produc'd, that is, the _consecutions_ of the confus'd
pulses will be much of another kind, and consequently produce other
_sensations_ and _phantasms_ of colours, and from a _Red_ may turn to a
_Blue_, or from a _Blue_ to a _Red_, &c.

Now, that this may be the better understood, I shall endeavour to explain
my meaning a little more sensible by a _Scheme_: Suppose we therefore in
the seventh _Figure_ of the sixth _Scheme_, that ABCD represents a Vessel
holding a ting'd liquor, let IIIII, &c. be the clear liquor, and let the
tinging body that is mixt with it be EE, &c. FF, &c. GG, &c. HH, &c. whose
particles (whether round, or some other determinate Figure is little to our
purpose) are first of a determinate and equal bulk. Next, they are rang'd
into the form of _Quincunx_, or _Equilaterotriangular_ order, which that
probably they are so, and why they are so, I shall elsewhere endeavour to
shew. Thirdly, they are of such a nature, as does either more easily or
more difficultly transmit the Rays of light then the liquor; if more
easily, a _Blue_ is generated, and if more difficultly, a _Red_ or
_Scarlet_.

And first, let us suppose the tinging particles to be of a substance that
does more _impede_ the Rays of light, we shall find that the pulse or wave
of light mov'd from AD to BC, will proceed on, through the containing
_medium_ by the pulses or waves KK, LL, MM, NN, OO; but because several of
these Rays that go to the constitution of these pulses will be slugged or
stopped by the tinging particles E, F, G, H; therefore there shall be
_secundary_ and weak pulse that shall follow the Ray, namely PP which will
be the weaker: first, because it has suffer'd many refractions in the
impeding body; next, for that the Rays will be a little dispers'd or
confus'd by reason of the refraction in each of the particles, whether
_round_ or _angular_; and this will be more evident, if we a little more
closely examine any one particular tinging _Globule_.

Suppose we therefore AB in the eighth _Figure_ of the sixth _Scheme_, to
represent a tinging _Globule_ or particle which has a greater refraction
than the liquor in which it is contain'd: Let CD be a part of the pulse of
light which is _propagated_ through the containing _medium_; this pulse
will be a little stopt or impeded by the _Globule_, and so by that time the
pulse is past to EF that part of it which has been impeded by passing
through the _Globule_, will get but to LM, and so that pulse which has been
_propagated_ through the _Globule_, to wit, LM, NO, PQ, will always come
behind the pulses EF, GH, IK, &c.

Next, by reason of the greater impediment in AB, and its _Globular_ Figure,
the Rays that pass through it will be dispers'd, and very much scatter'd.
Whence CA and DB which before went _direct_ and _parallel_, will after the
refraction in AB, _diverge_ and spread by AP, and BQ; so that as the Rays
do meet with more and more of these tinging particles in their way, by so
much the more will the pulse of light further lagg behind the clearer
pulse, or that which has fewer refractions, and thence the deeper will the
colour be, and the fainter the light that is trajected through it; for not
onely many Rays are reflected from the surfaces of AB, but those Rays that
get through it are very much disordered.

By this _Hypothesis_ there is no one experiment of colour that I have yet
met with, but may be, I conceive, very rationably solv'd, and perhaps, had
I time to examine several particulars requisite to the demonstration of it,
I might prove it more than probable, for all the experiments about the
changes and mixings of colours related in the Treatise of Colours,
published by the _Incomparable_ Mr. _Boyle_, and multitudes of others which
I have observ'd, do so easily and naturally flow from those principles,
that I am very apt to think it probable, that they own their production to
no other _secundary_ cause: As to instance in two or three experiments. In
the twentieth Experiment, this _Noble Authour_ has shewn that the deep
_bluish purple-colour_ of _Violets_, may be turn'd into a _Green_, by
_Alcalizate Salts_, and to a _Red_ by acid; that is, a _Purple_ consists of
two colours, a deep _Red_, and a deep _Blue_; when the _Blue_ is diluted,
or altered, or destroy'd by _acid Salts_, the _Red_ becomes predominant,
but when the _Red_ is diluted by _Alcalizate_, and the _Blue_ heightned,
there is generated a _Green_; for of a _Red_ diluted, is made a _Yellow_,
and _Yellow_ and _Blue_ make a _Green_.

Now, because the _spurious_ pulses which cause a _Red_ and a _Blue_, do the
one follow the clear pulse, and the other precede it, it usually follows,
that those _Saline_ refracting bodies which do _dilute_ the colour of the
one, do deepen that of the other. And this will be made manifest by almost
all kinds of _Purples_, and many sorts of _Greens_, both these colours
consisting of mixt colours; for if we suppose A and A in the ninth Figure,
to represent two pulses of clear light, which follow each other at a
convenient distance, AA, each of which has a _spurious_ pulse preceding it,
as BB, which makes a _Blue_, and another following it, as CC, which makes a
_Red_, the one caus'd by tinging particles that have a greater refraction,
the other by others that have a less refracting quality then the liquor or
_Menstruum_ in which these are dissolv'd, whatsoever liquor does so alter
the refraction of the one, without altering that of the other part of the
ting'd liquor, must needs very much alter the colour of the liquor; for if
the refraction of the _dissolvent_ be increas'd, and the refraction of the
tinging particles not altered, then will the preceding _spurious_ pulse be
shortned or stopt, and not out-run the clear pulse so much; so that BB will
become EE, and the _Blue_ be _diluted_, whereas the other _spurious_ pulse
which follows will be made to lagg much more, and be further behind AA than
before, and CC will become _ff_, and so the _Yellow_ or _Red_ will be
heightned.

A _Saline_ liquor therefore, mixt with another ting'd liquor, may alter the
colour of it several ways, either by altering the refraction of the liquor
in which the colour swims: or secondly by varying the refraction of the
coloured particles, by uniting more intimately either with some particular
_corpuscles_ of the tinging body, or with all of them, according as it has
a _congruity_ to some more especially, or to all alike: or thirdly, by
uniting and interweaving it self with some other body that is already
joyn'd with the tinging particles, with which substance it may have a
_congruity_, though it have very little with the particles themselves: or
fourthly, it may alter the colour of a ting'd liquor by dis-joyning certain
particles which were before united with the tinging particles, which though
they were somewhat _congruous_ to these particles, have yet a greater
_congruity_ with the newly _infus'd Saline menstruum_. It may likewise
alter the colour by further dissolving the tinging substance into smaller
and smaller _particles_, and so _diluting_ the colour; or by uniting
several _particles_ together as in precipitations, and so deepning it, and
some such other ways, which many experiments and comparisons of differing
trials together, might easily inform one of.

From these Principles applied, may be made out all the varieties of colours
observable, either in liquors, or any other ting'd bodies, with great ease,
and I hope intelligible enough, there being nothing in the _notion_ of
colour, or in the suppos'd production, but is very conceivable, and may be
possible.

The greatest difficulty that I find against this _Hypothesis_, is, that
there seem to be more distinct colours then two, that is, then Yellow and
Blue. This Objection is grounded on this reason, that there are several
Reds, which _diluted_, make not a Saffron or pale Yellow, and therefore
Red, or Scarlet seems to be a third colour distinct from a deep degree of
Yellow.

To which I answer, that Saffron affords us a deep Scarlet tincture, which
may be _diluted_ into as pale a Yellow as any, either by making a weak
solution of the Saffron, by infusing a small parcel of it into a great
quantity of liquor, as in spirit of Wine, or else by looking through a very
thin quantity of the tincture, and which may be heightn'd into the
loveliest Scarlet, by looking through a very thick body of this tincture,
or through a thinner parcel of it, which is highly _impregnated_ with the
tinging body, by having had a greater quantity of the Saffron dissolv'd in
a smaller parcel of the liquor.

Now, though there may be some particles of other tinging bodies that give a
lovely Scarlet also, which though _diluted_ never so much with liquor, or
looked on through never so thin a parcel of ting'd liquor, will not yet
afford a pale Yellow, but onely a kind of faint Red; yet this is no
argument but that those ting'd particles may have in them the faintest
degree of Yellow, though we may be unable to make them exhibit it; For that
power of being _diluted_ depending upon the divisibility of the ting'd
body, if I am unable to make the tinging particles so thin as to exhibit
that colour, it does not therefore follow, that the thing is impossible to
be done; now, the tinging particles of some bodies are of such a nature,
that unless there be found some way of comminuting them into less bulks
then the liquor does dissolve them into, all the Rays that pass through
them must necessarily receive a tincture so deep, as their appropriate
refractions and bulks compar'd with the proprieties of the dissolving
liquor must necessarily dispose them to empress, which may perhaps be a
pretty deep Yellow, or pale Red.

And that this is not _gratis dictum_, I shall add one instance of this
kind, wherein the thing is most manifest.

If you take Blue _Smalt_, you shall find, that to afford the deepest Blue,
which _caeteris paribus_ has the greatest particles or sands; and if you
further divide, or grind those particles on a Grindstone, or _porphyry_
stone, you may by _comminuting_ the sands of it, _dilute_ the Blue into as
pale a one as you please, which you cannot do by laying the colour thin;
for wheresoever any single particle is, it exhibits as deep a Blue as the
whole mass. Now, there are other Blues, which though never so much ground,
will not be _diluted_ by grinding, because consisting of very small
particles, very deeply ting'd, they cannot by grinding be actually
separated into smaller particles then the operation of the fire, or some
other dissolving _menstruum_, reduc'd them to already.

Thus all kind of _Metalline_ colours, whether _precipitated_, _sublim'd_,
_calcin'd_, or otherwise prepar'd, are hardly chang'd by grinding, as
_ultra marine_ is not more _diluted_; nor is _Vermilion_ or _Red-lead_ made
of a more faint colour by grinding; for the smallest particles of these
which I have view'd with my greatest Magnifying-Glass, if they be well
enlightned, appear very deeply ting'd with their peculiar colours; nor,
though I have magnified and enlightned the particles exceedingly, could I
in many of them, perceive them to be transparent, or to be whole particles,
but the smallest specks that I could find among well ground _Vermilion_ and
_Red-lead_, seem'd to be a Red mass, compounded of a multitude of less and
less motes, which sticking together, compos'd a bulk, not one thousand
thousandth part of the smallest visible sand or mote.

And this I find generally in most _Metalline_ colours, that though they
consist of parts so exceedingly small, yet are they very deeply ting'd,
they being so ponderous, and having such a multitude of terrestrial
particles throng'd into a little room; so that 'tis difficult to find any
particle transparent or resembling a pretious stone, though not impossible;
for I have observ'd divers such shining and resplendent colours intermixt
with the particles of _Cinnaber_, both natural and artificial, before it
hath been ground and broken or flaw'd into _Vermilion_: As I have also in
_Orpiment_, _Red-lead_, and _Bise_, which makes me suppose, that those
_metalline_ colours are by grinding, not onely broken and separated
actually into smaller pieces, but that they are also flaw'd and brused,
whence they, for the most part, become _opacous_, like flaw'd Crystal or
Glass, &c. But for _Smalts_ and _verditures_, I have been able with a
_Microscope_ to perceive their particles very many of them transparent.

Now, that the others also may be transparent, though they do not appear so
to the _Microscope_, may be made probable by this Experiment: that if you
take _ammel_ that is almost _opacous_, and grind it very well on a
_Porphyry_, or _Serpentine_, the small particles will by reason of their
flaws, appear perfectly _opacous_; and that 'tis the flaws that produce
this _opacousness_, may be argued from this, that particles of the same
_Ammel_ much thicker if unflaw'd will appear somewhat transparent even to
the eye; and from this also, that the most transparent and clear Crystal,
if heated in the fire, and then suddenly quenched, so that it be all over
flaw'd, will appear _opacous_ and white.

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