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Micrographia by Robert Hooke

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There is one _Phaenomenon_ more, which may, if care be used, exhibit to the
beholder, as it has divers times to me, an exceeding pleasant, and not less
instructive Spectacle; And that is, if curiosity and diligence be used, you
may so split this admirable Substance, that you may have pretty large
Plates (in companion of those smaller ones which you may observe in the
Rings) that are perhaps an 1/8 or a 1/6 part of an inch over, each of them
appearing through the _Microscope_ most curiously, intirely, and uniformly
adorned with some one vivid colour: this, if examined with the
_Microscope_, may be plainly perceived to be in all parts of it equally
thick. Two, three, or more of these lying one upon another, exhibit
oftentimes curious compounded colours, which produce such a _Compositum_,
as one would scarce imagine should be the result of such _ingredients_: As
perhaps a _faint yellow_ and a _blew_ may produce a very _deep purple_. But
when anon we come to the more strict examination of these _Phaenomena_, and
to inquire into the causes and reasons of these productions, we shall, I
hope, make it more conceivable how they are produced, and shew them to be
no other then the natural and necessary effects arising from the peculiar
union of concurrent causes.

These _Phaenomena_, being so various, and so truly admirable, it will
certainly be very well worth our inquiry, to examine the causes and reasons
of them, and to consider, whether from these causes demonstratively
evidenced, may not be deduced the true causes of the production of all kind
of Colours. And I the rather now do it, instead of an Appendix or
Digression to this History, then upon the occasion of examining the Colours
in Peacocks, or other Feathers, because this Subject, as it does afford
more variety of particular Colours, so does it afford much better wayes of
examining each circumstance. And this will be made manifest to him that
considers, first, that this laminated body is more simple and regular then
the parts of Peacocks feathers, this consisting only of an indefinite
number of plain and smooth Plates, heaped up, or _incumbent_ on each other.
Next, that the parts of this body are much more manageable, to be divided
or joyned, then the parts of a Peacocks feather, or any other substance
that I know. And thirdly, because that in this we are able from a
colourless body to produce several coloured bodies, affording all the
variety of Colours imaginable: And several others, which the subsequent
Inquiry will make manifest.

To begin therefore, it is manifest from several circumstances, that the
material cause of the _apparition_ of these several Colours, is some
_Lamina_ or Plate of a transparent or pellucid body of a thickness very
determinate and proportioned according to the greater or less refractive
power of the _pellucid_ body. And that this is so, abundance of Instances
and particular Circumstances will make manifest.

As _first_, if you take any small piece of the _Muscovy-glass_, and with a
Needle, or some other convenient Instrument, cleave it oftentimes into
thinner and thinner _Laminae_, you shall find, that till you come to a
determinate thinness of them, they shall all appear transparent and
colourless, but if you continue to split and divide them further, you shall
find at last, that each Plate, after it comes to such a determinate
thickness, shall appear most lovely ting'd or imbued with a determinate
colour. If _further_, by any means you so flaw a pretty thick piece, that
one part does begin to cleave a little from the other, and between those
two there be by any means gotten some pellucid _medium_, those _laminated_
pellucid bodies that fill that space, shall exhibit several Rainbows or
coloured Lines, the colours of which will be disposed and ranged according
to the various thicknesses of the several parts of that Plate. That this is
so, is yet _further_ confirmed by this Experiment.

Take two small pieces of ground and polisht Looking-glass-plate, each about
the bigness of a shilling, take these two dry, and with your fore-fingers
and thumbs press them very hard and close together, and you shall find,
that when they approach each other very near, there will appear several
_Irises_ or coloured Lines, in the same manner almost as in the
_Muscovy-glass_; and you may very easily change any of the Colours of any
part of the interposed body, by pressing the Plates closer and harder
together, or leaving them more lax; that is, a part which appeared coloured
with a red, may be presently ting'd with a yellow, blew, green, purple, or
the like, by altering the appropinquation of the terminating Plates. Now
that air is not necessary to be the interposed body, but that any other
transparent fluid will do much the same, may be tryed by wetting those
approximated Surfaces with Water, or any other transparent Liquor, and
proceeding with it in the same manner as you did with the Air; and you will
find much the like effect, only with this difference, that those comprest
bodies, which differ most, in their refractive quality, from the
compressing bodies, exhibit the most strong and vivid tinctures. Nor is it
necessary, that this _laminated_ and _ting'd_ body should be of a fluid
substance, any other substance, provided it be thin enough and transparent,
doing the same thing: this the _Laminae_ of our _Muscovy-glass_ hint; but
it may be confirm'd by multitudes of other Instances.

And first, we shall find, that even Glass it self may, by the help of a
Lamp, be blown thin enough to produce these _Phaenomena_ of Colours: which
_Phaenomena_ accidentally happening, as I have been attempting to frame
small Glasses with a Lamp, did not a little surprize me at first, having
never heard or seen any thing of it before; though afterwards comparing it
with the _Phaenomena_, I had often observed in those Bubbles which Children
use to make with Soap-water, I did the less wonder; especially when upon
Experiment I found, I was able to produce the same _Phaenomena_ in thin
Bubbles made with any other transparent Substance. Thus have I produced
them with Bubbles of _Pitch_, _Rosin_, _Colophony_, _Turpentine_,
_Solutions_ of several _Gums_, as _Gum-Arabick_ in water; any _glutinous_
Liquor, as _Wort_, _Wine_, _Spirit of Wine_, _Oyl of Turpentine_, _Glare of
Snails,_ &c.

It would be needless to enumerate the several Instances, these being enough
to shew the generality or universality of this propriety. Only I must not
omit, that we have instances also of this kind even in metalline Bodies and
animal; for those several Colours which are observed to follow each other
upon the polisht surface of hardned Steel, when it is by a sufficient
degree of heat gradually tempered or softened, are produced, from nothing
else but a certain thin _Lamina_ of a _vitrum_ or _vitrified_ part of the
Metal, which by that degree of heat, and the concurring action of the
ambient Air, is driven out and fixed on the surface of the Steel.

And this hints to me a very probable (at least, if not the true) cause of
the hardning and tempering of Steel, which has not, I think, been yet
given, nor, that I know of been so much as thought of by any. And that is
this, that the hardness of it arises from a greater proportion of a
vitrified Substance interspersed through the pores of the Steel. And that
the tempering or softning of it arises from the proportionate or smaller
parcels of it left within those pores. This will seem the more probable, if
we consider these Particulars.

First, That the pure parts of Metals are of themselves very _flexible_ and
_tuff_; that is, will indure bending and hammering, and yet retain their
continuity.

Next, That the Parts of all vitrified Substances, as all kinds of Glass,
the _Scoria_ of Metals, &c. are very hard, and also very brittle, being
neither _flexible_ nor _malleable_, but may by hammering or beating be
broken into small parts or powders.

Thirdly, That all Metals (excepting Gold and Silver, which do not so much
with the bare fire, unless assisted by other saline Bodies) do more or less
_vitrifie_ by the strength of fire, that is, are corroded by a saline
Substance, which I elsewhere shew to be the true cause of fire; and are
thereby, as by several other _Menstruums_ converted into _Scoria_; And this
is called, _calcining_ of them, by Chimists. Thus Iron and Copper by
heating and quenching do turn all of them by degrees into _Scoria_, which
are evidently _vitrified_ Substances, and unite with Glass, and are easily
_fusible_; and when cold, very hard, and very brittle.

Fourthly, That most kind of _Vitrifications_ or _Calcinations_ are made by
Salts, uniting and incorporating with the metalline Particles. Nor do I
know any one _calcination_ wherein a _Saline_ body may not, with very great
probability, be said to be an agent or coadjutor.

Fifthly, That Iron is converted into Steel by means of the incorporation of
certain salts, with which it is kept a certain time in the fire.

Sixthly, That any Iron may, in a very little time, be _case hardned_, as
the Trades-men call it, by casing the iron to be hardned with clay, and
putting between the clay and iron a good quantity of a mixture of _Urine_,
_Soot_, _Sea-salt_, and _Horses hoofs_ (all which contein great quantities
of Saline bodies) and then putting the case into a good strong fire, and
keeping it in a considerable degree of heat for a good while, and
afterwards heating, and quenching or cooling it suddenly in cold water.

Seventhly, That all kind of vitrify'd substances, by being suddenly cool'd,
become very hard and brittle. And thence arises the pretty _Phaenomena_ of
the Glass Drops, which I have already further explained in its own place.

Eighthly, That those metals which are not so apt to vitrifie, do not
acquire any hardness by quenching in water, as Silver, Gold, &c.

These considerations premis'd, will, I suppose, make way for the more easie
reception of this following Explication of the _Phaenomena_ of hardned and
temper'd Steel. That Steel is a substance made out of Iron, by means of a
certain proportionate _Vitrification_ of several parts, which are so
curiously and proportionately mixt with the more tough and unalter'd parts
of the Iron, that when by the great heat of the fire this vitrify'd
substance is melted, and consequently rarify'd, and thereby the pores of
the Iron are more open, if then by means of dipping it in cold water it be
suddenly cold, and the parts hardned, that is, stay'd in that same degree
of _Expansion_ they were in when hot, the parts become very hard and
brittle, and that upon the same account almost as small parcels of glass
quenched in water grow brittle, which we have already explicated. If after
this the piece of Steel be held in some convenient heat, till by degrees
certain colours appear upon the surface of the brightned metal, the very
hard and brittle tone of the metal, by degrees relaxes and becomes much
more tough and soft; namely, the action of the heat does by degrees loosen
the parts of the Steel that were before streached or set _atilt_ as it
were, and stayed open by each other, whereby they become relaxed and set at
liberty, whence some of the more brittle interjacent parts are thrust out
and melted into a thin skin on the surface of the Steel, which from no
colour increases to a deep Purple, and so onward by these _gradations_ or
consecutions, _White, Yellow, Orange, Minium, Scarlet, Purple, Blew,
Watchet,_ &c. and the parts within are more conveniently, and
proportionately mixt; and so they gradually subside into a texture which is
much better proportion'd and closer joyn'd, whence that rigidnesse of parts
ceases, and the parts begin to acquire their former _ductilness_.

Now, that 'tis nothing but the vitrify'd metal that sticks upon the surface
of the colour'd body, is evident from this, that if by any means it be
scraped and rubb'd off, the metal underneath it is white and clear; and if
it be kept longer in the fire, so as to increase to a considerable
thickness, it may, by blows, be beaten off in flakes. This is further
confirm'd by this observable, that that Iron or Steel will keep longer from
rusting which is covered with this vitrify'd case: Thus also Lead will, by
degrees, be all turn'd into a litharge; for that colour which covers the
top being scum'd or shov'd aside, appears to be nothing else but a litharge
or vitrify'd Lead.

This is observable also in some sort, on Brass, Copper, Silver, Gold, Tin,
but is most conspicuous in Lead: all those Colours that cover the surface
of the Metal being nothing else, but a very thin vitrifi'd part of the
heated Metal.

The other Instance we have, is in Animal bodies, as in Pearls, Mother of
Pearl-shels, Oyster-shels, and almost all other kinds of stony shels
whatsoever. This have I also sometimes with pleasure observ'd even in
Muscles and Tendons. Further, if you take any glutinous substance and run
it exceedingly thin upon the surface of a smooth glass or a polisht
metaline body, you shall find the like effects produced: and in general,
wheresoever you meet with a transparent body thin enough, that is
terminated by reflecting bodies of differing refractions from it, there
will be a production of these pleasing and lovely colours.

Nor is it necessary, that the two _terminating_ Bodies should be both of
the same kind, as may appear by the _vitrified Laminae_ on _Steel_, _Lead_,
and other Metals, one surface of which _Laminae_ is contiguous to the
surface of the Metal, the other to that of the Air.

Nor is it necessary, that these colour'd _Laminae_ should be of an even
thickness, that is, should have their edges and middles of equal thickness,
as in a Looking-glass-plate, which circumstance is only requisite to make
the Plate appear all of the same colour; but they may resemble a _Lens_,
that is, have their middles thicker then their edges; or else a _double
concave_, that is, be thinner in the middle then at the edges; in both
which cases there will be various coloured rings or lines, with differing
consecutions or orders of Colours; the order of the first from the middle
outwards being Red, Yellow, Green, Blew, &c. And the latter quite contrary.

But further, it is altogether necessary, that the Plate, in the places
where the Colours appear, should be of a determinate thickness: First, It
must not be more then such a thickness, for when the Plate is increased to
such a thickness, the Colours cease; and besides, I have seen in a thin
piece of _Muscovy-glass_, where the two ends of two Plates, which appearing
both single, exhibited two distinct and differing Colours; but in that
place where they were united, and constituted one double Plate (as I may
call it) they appeared transparent and colourless. Nor, Secondly, may the
Plates be _thinner_ then such a determinate _cize_; for we alwayes find,
that the very outmost Rim of these flaws is terminated in a white and
colourless Ring.

Further, in this Production of Colours there is no need of a determinate
Light of such a bigness and no more, nor of a determinate position of that
Light, that it should be on this side, and not on that side; nor of a
terminating shadow, as in the Prisme, and Rainbow, or Water-ball: for we
find, that the Light in the open Air, either in or out of the Sun-beams,
and within a Room, either from one or many Windows, produces much the same
effect: only where the Light is brightest, there the Colours are most
_vivid_. So does the light of a Candle, collected by a Glass-ball. And
further, it is all one whatever side of the coloured Rings be towards the
light; for the whole Ring keeps its proper Colours from the middle outwards
in the same order as I before related, without varying at all, upon
changing the position of the light.

But above all it is most observable, that here are all kind of Colours
generated in a _pellucid_ body, where there is properly no such refraction
as _Des Cartes_ supposes his _Globules_ to acquire a _vertuity_ by: For in
the plain and even Plates it is manifest, that the second refraction
(according to _Des Cartes_ his Principles in the _fifth section of the
eighth Chapter of his Meteors_) does regulate and restore the supposed
_turbinated Globules_ unto their former uniform motion. This Experiment
therefore will prove such a one as our _thrice excellent Verulam_ calls
_Experimentum Crucis_, serving as a Guide or Land-mark, by which to direct
our course in the search after the true cause of Colours. Affording us this
particular negative Information, that for the production of Colours there
is not necessary either a great refraction, as in the Prisme; nor Secondly,
a determination of Light and shadow, such as is both in the Prisme and
Glass-ball. Now that we may see likewise what affirmative and positive
Instruction it yields, it will be necessary, to examine it a little more
particularly and strictly; which that we may the better do, it will be
requisite to premise somewhat in general concerning the nature of Light and
Refraction.

And first for Light it seems very manifest, that there is no luminous Body
but has the parts of it in motion more or less.

First, That all kind of _fiery burning Bodies_ have their parts in motion,
I think, will be very easily granted me. That the _spark_ struck from a
Flint and Steel is in a rapid agitation, I have elsewhere made probable.
And that the Parts of _rotten Wood_, _rotten Fish_ and the like, are also
in motion, I think, will as easily be conceded by those, who consider, that
those parts never begin to shine till the Bodies be in a state of
putrefaction; and that is now generally granted by all, to be caused by the
motion of the parts of putrifying bodies. That the _Bononian stone_ shines
no longer then it is either warmed by the Sun-beams, or by the flame of a
Fire or of a Candle, is the general report of those that write of it, and
of others that have seen it. And that heat argues a motion of the internal
parts is (as I said before) generally granted.

But there is one Instance more, which was first shewn to the _Royal
Society_ by Mr. _Clayton_ a worthy Member thereof, which does make this
Assertion more evident then all the rest: And that is, That a _Diamond_
being _rub'd_, _struck_ or _heated_ in the dark, shines for a pretty while
after, so long as that motion, which is imparted by any of those Agents,
remains (in the same manner as a Glass, rubb'd, struck, or (by a means
which I shall elsewhere mention) heated, yields a sound which lasts as long
as the vibrating motion of that _sonorous_ body) several Experiments made
on which Stone, are since published in a Discourse of Colours, by the truly
honourable Mr. _Boyle_. What may be said of those _Ignes fatui_ that appear
in the night, I cannot so well affirm, having never had the opportunity to
examine them my self, nor to be inform'd by any others that had observ'd
them: And the relations of them in Authors are so imperfect, that nothing
can be built on them. But I hope I shall be able in another place to make
it at least very probable, that there is even in those also a Motion which
causes this effect. That the shining of _Sea-water_ proceeds from the same
cause, may be argued from this, That it shines not till either it be beaten
against a Rock, or be some other wayes broken or agitated by Storms, or
Oars, or other _percussing_ bodies. And that the Animal _Energyes_ or
Spirituous _agil_ parts are very active in _Cats eyes_ when they shine,
seems evident enough, because their eyes never shine but when they look
very intensly either to find their prey, or being hunted in a dark room,
when they seek after their adversary, or to find a way to escape. And the
like may be said of the shining _Bellies of Gloworms_; since 'tis evident
they can at pleasure either increase or extinguish that Radiation.

It would be somewhat too long a work for this place _Zetetically_ to
examine, and positively to prove, what particular kind of motion it is that
must be the efficient of Light; for though it be a motion, yet 'tis not
every motion that produces it, since we find there are many bodies very
violently mov'd, which yet afford not such an effect; and there are other
bodies, which to our other senses, seem not mov'd so much, which yet shine.
Thus Water and quick-silver, and most other liquors heated, shine not; and
several hard bodies, as Iron, Silver, Brass, Copper, Wood, &c. though very
often struck with a hammer, shine not presently, though they will all of
them grow exceeding hot; whereas rotten Wood, rotten Fish, Sea water,
Gloworms, &c. have nothing of tangible heat in them, and yet (where there
is no stronger light to affect the Sensory) they shine some of them so
Vividly, that one may make a shift to read by them.

It would be too long, I say, here to insert the discursive progress by
which I inquir'd after the proprieties of the motion of Light, and
therefore I shall only add the result.

And, First, I found it ought to be exceeding _quick_, such as those motions
of _fermentation_ and _putrefaction_, whereby, certainly, the parts are
exceeding nimbly and violently mov'd; and that, because we find those
motions are able more minutely to shatter and divide the body, then the
most violent heats _menstruums_ we yet know. And that fire is nothing else
but such a _dissolution_ of the Burning body, made by the most _universal
menstruum_ of all _sulphureous bodies_, namely, the Air, we shall in an
other place of this Tractate endeavour to make probable. And that, in all
extreamly hot shining bodies, there is a very quick motion that causes
Light, as well as a more robust that causes Heat, may be argued from the
celerity wherewith the bodyes are dissolv'd.

Next, it must be a _Vibrative motion_. And for this the newly mention'd
_Diamond_ affords us a good argument; since if the motion of the parts did
not return, the Diamond must after many rubbings decay and be wasted: but
we have no reason to suspect the latter, especially if we consider the
exceeding difficulty that is found in cutting or wearing away a Diamond.
And a Circular motion of the parts is much more improbable, since, if that
were granted, and they be suppos'd irregular and Angular parts, I see not
how the parts of the Diamond should hold so firmly together, or remain in
the same sensible dimensions, which yet they do. Next, if they be
_Globular_, and mov'd only with a _turbinated_ motion, I know not any cause
that can impress that motion upon the _pellucid medium_, which yet is done.
Thirdly, any other _irregular_ motion of the parts one amongst another,
must necessarily make the body of a fluid consistence, from which it is far
enough. It must therefore be a _Vibrating_ motion.

And Thirdly, That it is a very _short-vibrating motion_, I think the
instances drawn from the shining of Diamonds will also make probable. For a
Diamond being the hardest body we yet know in the World, and consequently
the least apt to yield or bend, must consequently also have its
_vibrations_ exceeding short.

And these, I think, are the three principal proprieties of a motion,
requisite to produce the effect call'd Light in the Object.

The next thing we are to consider, is the way or manner of the _trajection_
of this motion through the interpos'd pellucid body to the eye: And here it
will be easily granted,

First, That it must be a body _susceptible_ and _impartible_ of this motion
that will deserve the name of a Transparent. And next, that the parts of
such a body must be _Homogeneous_, or of the same kind. Thirdly, that the
constitution and motion of the parts must be such, that the appulse of the
luminous body may be communicated or propagated through it to the greatest
imaginable distance in the least imaginable time, though I see no reason to
affirm, that it must be in an instant: For I know not any one Experiment or
observation that does prove it. And, whereas it may be objected, That we
see the Sun risen at the very instant when it is above the sensible
Horizon, and that we see a Star hidden by the body of the Moon at the same
instant, when the Star, the Moon, and our Eye are all in the same line; and
the like Observations, or rather suppositions, may be urg'd. I have this to
answer, That I can as easily deny as they affirm; for I would fain know by
what means any one can be assured any more of the Affirmative, then I of
the Negative. If indeed the propagation were very slow, 'tis possible
something might be discovered by Eclypses of the Moon; but though we should
grant the progress of the light from the Earth to the Moon, and from the
Moon back to the Earth again to be full two Minutes in performing, I know
not any possible means to discover it; nay, there may be some instances
perhaps of Horizontal Eclypses that may seem very much to favour this
supposition of the slower progression of Light then most imagine. And the
like may be said of the Eclypses of the Sun, &c. But of this only by the
by. Fourthly, That the motion is propagated every way through an
_Homogeneous medium_ by _direct_ or _straight_ lines extended every way
like Rays from the center of a Sphere. Fifthly, in an _Homogeneous medium_
this motion is propagated every way with _equal velocity_, whence
necessarily every _pulse_ or _vitration_ of the luminous body will generate
a Sphere, which will continually increase, and grow bigger, just after the
same manner (though indefinitely swifter) as the waves or rings on the
surface of the water do swell into bigger and bigger circles about a point
of it, where, by the sinking of a Stone the motion was begun, whence it
necessarily follows, that all the parts of these Spheres undulated through
an _Homogeneous medium_ cut the Rays at right angles.

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