It is a very common Experiment, by striking with a Flint against a Steel,
to make certain fiery and shining Sparks to fly out from between those two
compressing Bodies. About eight years since, upon casually reading the
Explication of this odd _Phænomenon_, by the most Ingenious _Des Cartes_, I
had a great desire to be satisfied, what that Substance was that gave such
a shining and bright Light: And to that end I spread a sheet of white
Paper, and on it, observing the place where several of these Sparks seemed
to vanish, I found certain very small, black, but glistering Spots of a
movable Substance, each of which examining with my _Microscope_, I found to
be a small round _Globule_; some of which, as they looked prety small, so
did they from their Surface yield a very bright and strong reflection on
that side which was next the Light; and each look’d almost like a prety
bright Iron-Ball, whose Surface was prety regular, such as is represented
by the Figure A. In this I could perceive the Image of the Window prety
well, or of a Stick, which I moved up and down between the Light and it.
Others I found, which were, as to the bulk of the Ball, prety regularly
round, but the Surface of them, as it was not very smooth, but rough, and
more irregular, so was the reflection from it more faint and confused. Such
were the Surfaces of B. C. D. and E. Some of these I found cleft or
cracked, as C, others quite broken in two and hollow, as D. which seemed to
be half the hollow shell of a Granado, broken irregularly in pieces.
Several others I found of other shapes; but that which is represented by E,
I observed to be a very big Spark of fire, which went out upon one side of
the Flint that I struck fire withall, to which it stuck by the root F, at
the end of which small Stem was fastened-on a _Hemisphere_, or half a
hollow Ball, with the mouth of it open from the stemwards, so that it
looked much like a Funnel, or an old fashioned Bowl without a foot. This
night, making many tryals and observations of this Experiment, I met, among
a multitude of the Globular ones which I had observed, a couple of
Instances, which are very remarkable to the confirmation of my
_Hypothesis_.
And the First was of a pretty big Ball fastened on to the end of a small
sliver of Iron, which _Compositum_ seemed to be nothing else but a long
thin chip of Iron, one of whose ends was melted into a small round Globul;
the other end remaining unmelted and irregular, and perfectly Iron.
The Second Instance was not less remarkable then the First; for I found,
when a Spark went out, nothing but a very small thin long sliver of Iron or
Steel, unmelted at either end. So that it seems, that some of these Sparks
are the slivers or chips of the Iron _vitrified_, Others are only the
slivers melted into Balls without vitrification, And the third kind are
only small slivers of the Iron, made red-hot with the violence of the
stroke given on the Steel by the Flint.
He that shall diligently examine the _Phænomena_ of this Experiment, will,
I doubt not, find cause to believe, that the reason I have heretofore given
of it, is the true and genuine cause of it, namely, That _the Spark,
appearing so bright in the falling, is nothing else but a small piece of
the Steel or Flint, but most commonly of the Steel, which by the violence
of the stroke is at the same time sever’d and heat red-hot, and that
sometimes to such a degree, as to make it melt together into a small
Globule of Steel; and sometimes also is that heat so very intense, as
further to melt it and vitrifie it; but many times the heat is so gentle,
as to be able to make the sliver only red hot, which notwithstanding
falling upon the tinder_ (that is only a very curious small Coal made of
the small threads of Linnen burnt to coals and char’d) _it easily sets it
on fire_. Nor will any part of this _Hypothesis_ seem strange to him that
considers, First, that either hammering, or filing or otherwise violently
rubbing of Steel, will presently make it so hot as to be able to burn ones
fingers. Next, that the whole force of the stroke is exerted upon that
small part where the Flint and Steel first touch: For the Bodies being each
of them so very hard, the puls cannot be far communicated, that is, the
parts of each can yield but very little, and therefore the violence of the
concussion will be _exerted_ on that piece of Steel which is cut off by the
Flint. Thirdly, that the filings or small parts of Steel are very apt, as
it were, to take fire, and are presently red hot, that is, there seems to
be a very _combustible sulphureous_ Body in Iron or Steel, which the Air
very readily preys upon, as soon as the body is a little violently heated.
And this is obvious in the filings of Steel or Iron cast through the flame
of a Candle; for even by that sudden _transitus_ of the small chips of
Iron, they are heat red hot, and that _combustible sulphureous_ Body is
presently prey’d upon and devoured by the _aereal_ incompassing
_Menstruum_, whose office in this Particular I have shewn in the
Explication of Charcole.
And in prosecution of this Experiment, having taken the filings of Iron and
Steel, and with the point of a Knife cast them through the flame of a
Candle, I observed where some conspicuous shining Particles fell, and
looking on them with my _Microscope_, I found them to be nothing else but
such round Globules, as I formerly found the Sparks struck from the Steel
by a stroke to be, only a little bigger; and shaking together all the
filings that had fallen upon the sheet of Paper underneath and observing
them with the _Microscope_, I found a great number of small Globules, such
as the former, though there were also many of the parts that had remained
untoucht and rough filings or chips of Iron. So that, it seems, Iron does
contain a very _combustible sulphureous_ Body, which is, in all likelihood,
one of the causes of this _Phænomenon_, and which may be perhaps very much
concerned in the business of its hardening and tempering: of which somewhat
is said in the Description of _Muscovy-glass_.
So that, these things considered, we need not trouble our selves to find
out what kind of Pores they are, both in the Flint and Steel, that contain
the _Atoms of fire_, nor how those _Atoms_ come to be hindred from running
all out, when a dore or passage in their Pores is made by the concussion:
nor need we trouble our selves to examine by what _Prometheus_ the Element
of Fire comes to be fetcht down from above the Regions of the Air, in what
Cells or Boxes it is kept, and what _Epimetheus_ lets it go: Nor to
consider what it is that causes so great a conflux of the atomical
Particles of Fire, which are said to fly to a flaming Body, like Vultures
or Eagles to a putrifying Carcass, and there to make a very great pudder.
Since we have nothing more difficult in this _Hypothesis_ to conceive,
first, as to the kindling of Tinder, then how a large Iron-bullet, let fall
red or glowing hot upon a heap of Small-coal, should set fire to those that
are next to it first: Nor secondly, is this last more difficult to be
explicated, then that a Body, as Silver for Instance, put into a weak
_Menstruum_, as unrectified _Aqua fortis_ should, when it is put in a great
heat, be there dissolved by it, and not before; which _Hypothesis_ is more
largely explicated in the Description of Charcoal. To conclude, we see by
this Instance, how much Experiments may conduce to the regulating of
_Philosophical notions_. For if the most Acute _Des Cartes_ had applied
himself experimentally to have examined what substance it was that caused
that shining of the falling Sparks struck from a Flint and a Steel, he
would certainly have a little altered his _Hypothesis_, and we should have
found, that his Ingenious Principles would have admitted a very plausible
Explication of this _Phænomenon_; whereas by not examining so far as he
might, he has set down an Explication which Experiment do’s contradict.
But before I leave this Description, I must not forget to take notice of
the Globular form into which each of these is most curiously formed. And
this _Phænomenon_, as I have elsewhere more largely shewn, proceeds from a
propriety which belongs to all kinds of fluid Bodies more or less, and is
caused by the Incongruity of the Ambient and included Fluid, which so acts
and modulates each other, that they acquire, as neer as is possible, a
_spherical_ or _globular_ form, which propriety and several of the
_Phænomena_ that proceed from it, I have more fully explicated in the sixth
Observation.
One Experiment, which does very much illustrate my present Explication, and
is in it self exceeding pretty, I must not pass by: And that is a way of
making small _Globules_ or _Balls_ of Lead, or Tin, as small almost as
these of Iron or Steel, and that exceeding easily and quickly, by turning
the filings or chips of those Metals also into perfectly round _Globules_.
The way, in short, as I received it from the _Learned Physitian Doctor_
I.G. is this;
Reduce the Metal you would thus shape, into exceeding fine filings, the
finer the filings are, the finer will the Balls be: _Stratifie_ these
filings with the fine and well dryed powder of quick Lime in a _Crucible_
proportioned to the quantity you intend to make: When you have thus filled
your _Crucible_, by continual _stratifications_ of the filings and powder,
so that, as neer as may be, no one of the filings may touch another, place
the _Crucible_ in a _gradual fire_, and by degrees let it be brought to a
heat big enough to make all the filings, that are mixt with the quick Lime,
to melt, and no more; for if the fire be too hot, many of these filings
will joyn and run together; whereas if the heat be proportioned, upon
washing the Lime-dust in fair Water, all those small filings of the Metal
will subside to the bottom in a most curious powder, consisting all of
exactly round _Globules_, which, if it be very fine, is very excellent to
make Hour-glasses of.
Now though quick Lime be the powder that this direction makes choice of,
yet I doubt not, but that there may be much more convenient ones found out,
one of which I have made tryal of, and found very effectual; and were it
not for discovering, by the mentioning of it, another Secret, which I am
not free to impart, I should have here inserted it.
* * * * *
Observ. IX. _Of the Colours observable in Muscovy-Glass, and other thin
Bodies_.
Moscovy-glass, or _Lapis specularis_, is a Body that seems to have as many
Curiosities in its Fabrick as any common Mineral I have met with: for
first, It is transparent to a great thickness: Next, it is compounded of an
infinite number of thin flakes joyned or generated one upon another so
close & smooth, as with many hundreds of them to make one smooth and thin
Plate of a transparent flexible substance, which with care and diligence
may be slit into pieces so exceedingly thin as to be hardly perceivable by
the eye, and yet even those, which I have thought the thinnest, I have with
a good _Microscope_ found to be made up of many other Plates, yet thinner;
and it is probable, that, were our _Microscopes_ much better, we might much
further discover its divisibility. Nor are these flakes only regular as to
the smoothness of their Surfaces, but thirdly, In many Plates they may be
perceived to be terminated naturally with edges of the figure of a
_Rhomboeid_. This Figure is much more conspicuous in our English talk, much
whereof is found in the Lead Mines, and is commonly called _Spar_, and
_Kauck_, which is of the same kind of substance with the _Selenitis_, but
is seldom found in so large flakes as that is, nor is it altogether so
tuff, but is much more clear and transparent, and much more curiously
shaped, and yet may be cleft and flak’d like the other _Selenitis_. But
fourthly, this stone has a property, which in respect of the _Microscope_,
is more notable, and that is, that it exhibits several appearances of
Colours, both to the naked Eye, but much more conspicuously to the
_Microscope_; for the exhibiting of which, I took a piece of
_Muscovy-glass_, and splitting or cleaving it into thin Plates, I found
that up and down in several parts of them I could plainly perceive several
white specks or flaws, and others diversly coloured with all the Colours of
the _Rainbow_; and with the _Microscope_ I could perceive, that these
Colours were ranged in rings that incompassed the white speck or flaw, and
were round or irregular, according to the shape of the spot which they
terminated; and the position of Colours, in respect of one another, was the
very same as in the _Rainbow_. The consecution of those Colours from the
middle of the spot outward being Blew, Purple, Scarlet, Yellow, Green;
Blew, Purple, Scarlet, and so onwards, sometimes half a score times
repeated, that is, there appeared six, seven, eight, nine or ten several
coloured rings or lines, each incircling the other, in the same manner as I
have often seen a very _vivid Rainbow_ to have four or five several Rings
of Colours, that is, accounting all the Gradations between Red and Blew for
one: But the order of the Colours in these Rings was quite contrary to the
primary or innermost _Rainbow_, and the same with those of the secondary or
outermost Rainbow; these coloured Lines or _Irises_, as I may so call them,
were some of them much brighter then others, and some of them also very
much broader, they being some of them ten, twenty, nay, I believe, neer a
hundred times broader then others; and those usually were broadest which
were neerest the center or middle of the flaw. And oftentimes I found, that
these Colours reacht to the very middle of the flaw, and then there
appeared in the middle a very large spot, for the most part, all of one
colour, which was very vivid, and all the other Colours incompassing it,
gradually ascending, and growing narrower towards the edges, keeping the
same order, as in the _secundary Rainbow_, that is, if the middle were
Blew, the next incompassing it would be a Purple, the third a Red, the
fourth a Yellow, &c. as above; if the middle were a Red, the next without
it would be a Yellow, the third a Green, the fourth a Blew, and so onward.
And this order it alwayes kept whatsoever were the middle Colour.
There was further observable in several other parts of this Body, many
Lines or Threads, each of them of some one peculiar Colour, and those so
exceedingly bright and vivid, that it afforded a very pleasant object
through the _Microscope_. Some of these _threads_ I have observed also to
be pieced or made up of several short lengths of differently coloured
_ends_ (as I may so call them) as a line appearing about two inches long
through the _Microscope_, has been compounded of about half an inch of a
Peach colour, ⅛ of a lovely Grass-green, ¾ of an inch more of a bright
Scarlet, and the rest of the line of a Watchet blew. Others of them were
much otherwise coloured; the variety being almost infinite. Another thing
which is very observable, is, that if you find any place where the colours
are very broad and conspicuous to the naked eye, you may, by pressing that
place with your finger, make the colours change places, and go from one
part to another.
There is one _Phænomenon_ 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 ⅛ or a ⅙ 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 _Phænomena_, 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 _Phænomena_, 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 _Laminæ_, 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 _Laminæ_ 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 _Phænomena_ of Colours: which
_Phænomena_ 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 _Phænomena_, 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 _Phænomena_ 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 Tradesmen 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 _Phænomena_ 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 _Phænomena_ 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 rigidness 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 Laminæ_ on _Steel_, _Lead_,
and other Metals, one surface of which _Laminæ_ is contiguous to the
surface of the Metal, the other to that of the Air.
Nor is it necessary, that these colour’d _Laminæ_ 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 _verticity_ 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 _Energies_ 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 or _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 _vibration_ 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.
But because all transparent _mediums_ are not _Homogeneous_ to one another,
therefore we will next examine how this pulse or motion will be propagated
through differingly transparent _mediums_. And here, according to the most
acute and excellent Philosopher _Des Cartes_, I suppose the sign of the
angle of inclination in the first _medium_ to be to the sign of refraction
in the second, As the density of the first, to the density of the second.
By density, I mean not the density in respect of gravity (with which the
refractions or transparency of _mediums_ hold no proportion) but in respect
onely to the _trajection_ of the Rays of light, in which respect they only
differ in this; that the one propagates the pulse more easily and weakly,
the other more slowly, but more strongly. But as for the pulses themselves,
they will by the refraction acquire another propriety, which we shall now
endeavour to explicate.
We will suppose therefore in the first Figure ACFD to be a physical Ray, or
ABC and DEF to be two Mathematical Rays, _trajected_ from a very remote
point of a luminous body through an _Homogeneous_ transparent _medium_ LLL,
and DA, EB, FC, to be small portions of the orbicular impulses which must
therefore cut the Rays at right angles; these Rays meeting with the plain
surface NO of a _medium_ that yields an easier _transitus_ to the
propagation of light, and falling _obliquely_ on it, they will in the
_medium_ MMM be refracted towards the perpendicular of the surface. And
because this _medium_ is more easily _trajected_ then the former by a
third, therefore the point C of the orbicular pulse FC will be mov’d to H
four spaces in the same time that F the other end of it is mov’d to G three
spaces, therefore the whole refracted pulse GH shall be _oblique_ to the
refracted Rays CHK and GI; and the angle GHC shall be an acute, and so much
the more acute by how much the greater the refraction be, then which
nothing is more evident, for the sign of the inclination is to the sign of
refraction as GF to TC the distance between the point C and the
perpendicular from G on CK, which being as four to three, HC being longer
then GF is longer also then TC, therefore the angle GHC is less than GTC.
So that henceforth the parts of the pulses GH and IK are mov’d ascew, or
cut the Rays at _oblique_ angles.
It is not my business in this place to set down the reasons why this or
that body should impede the Rays more, others less: as why Water should
transmit the Rays more easily, though more weakly than air. Onely thus much
in general I shall hint, that I suppose the _medium_ MMM to have less of
the transparent undulating subtile matter, and that matter to be less
implicated by it, whereas LLL I suppose to contain a greater quantity of
the fluid undulating substance, and this to be more implicated with the
particles of that _medium_.
But to proceed, the same kind of _obliquity_ of the Pulses and Rays will
happen also when the refraction is made out of a more easie into a more
difficult _mediu_; as by the calculations of GQ & CSR which are refracted
from the perpendicular. In both which calculations ’tis _obvious_ to
observe, that always that part of the Ray towards which the refraction is
made has the end of the _orbicular pulse_ precedent to that of the other
side. And always, the oftner the refraction is made the same way, Or the
greater the single refraction is, the more is this unequal progress. So
that having found this odd propriety to be an inseparable concomitant of a
refracted Ray, not streightned by a contrary refraction, we will next
examine the refractions of the Sun-beams, as they are suffer’d onely to
pass through a small passage, _obliquely_ out of a more difficult, into a
more easie _medium_.
Let us suppose therefore ABC in the second Figure to represent a large
_Chemical Glass-body_ about two foot long, filled with very fair Water as
high as AB, and inclin’d in a convenient posture with B towards the Sun:
Let us further suppose the top of it to be cover’d with an _opacous_ body,
all but the hole ab, through which the Sun-beams are suffer’d to pass into
the Water, and are thereby refracted to cdef, against which part, if a
Paper be expanded on the outside, there will appear all the colours of the
Rainbow, that is, there will be generated the two principal colours,
_Scarlet_ and _Blue_, and all the _intermediate_ ones which arise from the
composition and dilutings of these two, that is, cd shall exhibit a
_Scarlet_, which toward d is diluted into a _Yellow_; this is the
refraction of the Ray, ik, which comes from the underside of the Sun; and
the Ray ef shall appear of a deep _Blue_, which is gradually towards e
diluted into a pale _Watchet-blue_. Between d and e the two _diluted_
colours. _Blue_ and _Yellow_ are mixt and compounded into a _Green_; and
this I imagine to be the reason why _Green_ is so acceptable a colour to
the eye, and that either of the two extremes are, if intense, rather a
little offensive, namely, the being plac’d in the middle between the two
extremes, and compounded out of both those, _diluted_ also, or somewhat
qualifi’d, for the _composition_, arising from the mixture of the two
extremes _undiluted_, makes a _Purple_, which though it be a lovely colour,
and pretty acceptable to the eye, yet is it nothing comparable to the
ravishing pleasure with which a curious and well tempered _Green_ affects
the eye. If removing the Paper, the eye be plac’d against cd, it will
perceive the lower side of the Sun (or a Candle at night which is much
better, because it offends not the eye, and is more easily manageable) to
be of a deep _Red_, and if against ef it will perceive the upper part of
the luminous body to be of a deep _Blue_; and these colours will appear
deeper and deeper, according as the Rays from the luminous body fall more
_obliquely_ on the surface of the Water, and thereby suffer a greater
refraction, and the more distinct, the further cdef is removed from the
trajecting hole.
So that upon the whole, we shall find that the reason of the _Phænomena_
seems to depend upon the _obliquity_ of the _orbicular pulse_, to the Lines
of Radiation, and in particular, that the Ray cd which constitutes the
_Scarlet_ has its inner parts, namely those which are next to the middle of
the luminous body, precedent to the outermost which are contiguous to the
dark and _unradiating_ skie. And that the Ray ef which gives a _Blue_, has
its outward part, namely, that which is contiguous to the dark skie
precedent to the pulse from the innermost, which borders on the bright
_area_ of the luminous body.
We may observe further, that the cause of the _diluting_ of the colours
towards the middle, proceeds partly from the wideness of the hole through
which the Rays pass, whereby the Rays from several parts of the luminous
body, fall upon many of the same parts between c and f as is more manifest
by the Figure: And partly also from the nature of the refraction it self,
for the vividness or strength of the two terminating colours, arising
chiefly as we have seen, from the very great difference that is betwixt the
outsides of those _oblique undulations_ & the dark Rays circumambient, and
that disparity betwixt the _approximate_ Rays, decaying gradually: the
further inward toward the middle of the luminous body they are remov’d, the
more must the colour approach to a white or an undisturbed light.
Upon the calculation of the refraction and reflection from a Ball of Water
or Glass, we have much the same _Phænomena_, namely, an _obliquity_ of the
undulation in the same manner as we have found it here. Which, because it
is very much to our present purpose, and affords such an _Instancia
crucis_, as no one that I know has hitherto taken notice of, I shall
further examine. For it does very plainly and positively distinguish, and
shew, which of the two _Hypotheses_, either the _Cartesian_ or this is to
be followed, by affording a generation of all the colors in the Rainbow,
where according to the _Cartesian Principles_ there should be none at all
generated. And secondly, by affording an instance that does more closely
confine the cause of these _Phænomena_ of colours to this present
_Hypothesis_.
And first, for the _Cartesian_, we have this to object against it, That
whereas he says (_Meteorum Cap. 8. Sect. 5._) _Sed judicabam unicam
(refractione scilicet) ad minimum requiri, & quidem talem ut ejus effectus
aliâ contrariâ (refractione) non destruatur: Nam experientia docet si
superficies _NM_ & _NP_ (nempe refringentes) Parallelæ forent, radios
tantundem per alteram iterum erectos quantum per unam frangerentur, nullos
colores depicturos_; This Principle of his holds true indeed in a prisme
where the refracting surfaces are plain, but is contradicted by the Ball or
Cylinder, whether of Water or Glass, where the refracting surfaces are
Orbicular or Cylindrical. For if we examine the passage of any _Globule_ or
Ray of the primary _Iris_, we shall find it to pass out of the Ball or
Cylinder again, with the same inclination and refraction that it enter’d in
withall, and that that last refraction by means of the _intermediate_
reflection shall be the same as if without any reflection at all the Ray
had been twice refracted by two Parallel surfaces.
And that this is true, not onely in one, but in every Ray that goes to the
constitution of the Primary Iris; nay, in every Ray, that suffers only two
refractions, and one reflection, by the surface of the round body, we shall
presently see most evident, if we repeat the _Cartesian Scheme_, mentioned
in the tenth _Section_ of the eighth _Chapter_ of his _Meteors_, where
EFKNP in the third Figure[9] is one of the Rays of the Primary Iris, twice
refracted at F and N, and once reflected at K by the surface of the
Water-ball. For, first it is evident, that KF and KN are equal, because KN
being the reflected part of KF they have both the same inclination on the
surface K that is the angles FKT, and NKV made by the two Rays and the
Tangent of K are equal, which is evident by the Laws of reflection; whence
it will follow also, that KN has the same inclination on the surface N, or
the Tangent of it XN that the Ray KF has to the surface F, or the Tangent
of it FY, whence it must necessarily follow, that the refractions at F and
N are equal, that is, KFE and KNP are equal. Now, that the surface N is by
the reflection at K made parallel to the surface at F, is evident from the
principles of reflection; for reflection being nothing but an inverting of
the Rays, if we re-invert the Ray KNP, and make the same inclinations below
the line TKV that it has above, it will be most evident, that KH the
inverse of KN will be the continuation of the line FK, and that LHI the
inverse of OX is parallel to FY. And HM the inverse of NP is Parallel to EF
for the angle KHI is equal to KNO which is equal to KFY, and the angle KHM
is equal to KNP which is equal to KFE which was to be prov’d.
So that according to the above mentioned _Cartesian_ principles there
should be generated no colour at all in a Ball of Water or Glass by two
refractions and one reflection, which does hold most true indeed, if the
surfaces be plain, as may be experimented with any kind of prisme where the
two refracting surfaces are equally inclin’d to the reflecting; but in this
the _Phænomena_ are quite otherwise.
The cause therefore of the generation of colour must not be what _Des
Cartes_ assigns, namely, a certain _rotation_ of the _Globuli ætherei_,
which are the particles which he supposes to constitute the _Pellucid
medium_, But somewhat else, perhaps what we have lately supposed, and shall
by and by further prosecute and explain.
But, First I shall crave leave to propound some other difficulties of his,
notwithstanding exceedingly ingenious _Hypothesis_, which I plainly confess
to me seem such; and those are,
First, if that light be (as is affirmed, _Diopt._ cap. 1. §. 8.) not so
properly a motion, as an action or propension to motion, I cannot conceive
how the eye can come to be sensible of the _verticity_ of a _Globule_,
which is generated in a drop of Rain, perhaps a mile off from it. For that
_Globule_ is not carry’d to the eye according to his formerly recited
Principle; and if not so, I cannot conceive how it can communicate its
_rotation_, or circular motion to the line of the _Globules_ between the
drop and the eye. It cannot be by means of every ones turning the next
before him; for if so, then onely all the _Globules_ that are in the odd
places must be turned the same way with the first, namely, the 3. 5. 7. 9.
11, &c. but all the _Globules_ interposited between them in the even
places; namely, the 2. 4. 6. 8. 10. &c. must be the quite contrary, whence,
according to the _Cartesian Hypothesis_, there must be no distinct colour
generated, but a confusion. Next, since the _Cartesian Globuli_ are
suppos’d (_Principiorum Philosoph._ Part. 3. §. 86.) to be each of them
continually in motion about their centers, I cannot conceive how the eye is
able to distinguish this new generated motion from their former inherent
one, if I may so call that other wherewith they are mov’d or _turbinated_,
from some other cause than refraction. And thirdly, I cannot conceive how
these motions should not happen sometimes to oppose each other, and then,
in stead of a _rotation_, there would be nothing but a direct motion
generated, and consequently no colour. And fourthly, I cannot conceive, how
by the _Cartesian Hypothesis_ it is possible to give any plausible reason
of the nature of the Colours generated in the thin _laminæ_ of these our
_Microscopical Observations_; for in many of these, the refracting and
reflecting surfaces are parallel to each other, and consequently no
_rotation_ can be generated, nor is there any necessity of a shadow or
termination of the bright Rays, such as is suppos’d (_Chap._ 8. §. 5. _Et
præterea observavi umbram quoque, aut limitationem luminis requiri:_ and
_Chap._ 8. §. 9.) to be necessary to the generation of any distinct
colours; Besides that, here is oftentimes one colour generated without any
of the other appendant ones, which cannot be by the _Cartesian Hypothesis_.
There must be therefore some other propriety of refraction that causes
colour. And upon the examination of the thing, I cannot conceive any one
more general, inseparable, and sufficient, than that which I have before
assign’d. That we may therefore see how exactly our _Hypothesis_ agrees
also with the _Phænomena_ of the refracting round body, whether _Globe_ or
_Cylinder_, we shall next subjoyn our _Calculation_ or _Examen_ of it.
And to this end, we will calculate any two Rays: as for instance;[10] let
EF be a Ray cutting the _Radius_ CD (divided into 20. parts) in G 16. parts
distant from C, and ef another Ray, which cuts the same _Radius_ in g 17.
parts distant, these will be refracted to K and k, and from thence
reflected to N and n, and from thence refracted toward P and p; therefore
the Arch Ff will be 5.d 5′. The Arch FK 106.d 30′. the Arch fk 101.d 2′.
The line FG 6000. and fg 5267. therefore hf. 733. therefore Fc 980, almost.
The line FK 16024. and fk 15436. therefore Nd 196. and no 147 almost, the
line Nn 1019 the Arch Nn 5.d 51′. therefore the Angle Nno is 34.d 43′.
therefore the Angle Non is 139.d 56′. which is almost 50.d more than a
right Angle.
It is evident therefore by this _Hypothesis_, that at the same time that ef
touches f. EF is arrived at c. And by that time efkn is got to n, EFKN is
got to d and when it touches N, the pulse of the other Ray is got to o. and
no farther, which is very short of the place it should have arriv’d to, to
make the Ray np to cut the _orbicular pulse_ No at right Angles: therefore
the Angle Nop is an acute Angle, but the quite contrary of this will
happen, if 17. and 18. be calculated in stead of 16. and 17. both which
does most exactly agree with the _Phænomena_: For if the Sun, or a Candle
(which is better) be placed about Ee, and the eye about Pp, the Rays EFef
at 16. and 17. will paint the side of the luminous object toward np _Blue_,
and towards NP _Red_. But the quite contrary will happen when EF is 17. and
ef 18. for then towards NP shall be a _Blue_, and towards np a _Red_,
exactly according to the calculation. And there appears the _Blue_ of the
Rainbow, where the two _Blue_ sides of the two Images unite, and there the
_Red_ where the two _Red_ sides unite, that is, where the two Images are
just disappearing; which is, when the Rays EF and NP produc’d till they
meet, make an Angle of about 41. and an half; the like union is there of
the two Images in the Production of the _Secundary Iris_, and the same
causes, as upon calculation may appear; onely with this difference, that it
is somewhat more faint, by reason of the duplicate reflection, which does
always weaken the impulse the oftner it is repeated.
Now, though the second refraction made at Nn be convenient, that is, do
make the Rays glance the more, yet is it not altogether requisite; for it
is plain from the calculation, that the pulse dn is sufficiently _oblique_
to the Rays KN and kn, as wel as the pulse fc is _oblique_ to the Rays FK &
fk. And therefore if a piece of very fine Paper be held close against Nn
and the eye look on it either through the Ball as from D, or from the other
side, as from B. there shall appear a Rainbow, or colour’d line painted on
it with the part toward X appearing _Red_, towards O, _Blue_; the same also
shall happen, if the Paper be placed about Kk, for towards T shall appear a
_Red_, and towards V a _Blue_, which does exactly agree with this my
_Hypothesis_, as upon the calculation of the progress of the pulse will
most easily appear.
Nor do these two observations of the colours appearing to the eye about p
differing from what they appear on the Paper at N contradict each other;
but rather confirm and exactly agree with one another, as will be evident
to him that examines the reasons set down by the ingenious. _Des Cartes_ in
the 12. _Sect._ of the 8. _Chapter of his Meteors_, where he gives the true
reason why the colours appear of a quite contrary order to the eye, to what
they appear’d on the Paper if the eye be plac’d in steed of the Paper: And
as in the Prisme, so also in the Water-drop, or Globe the _Phænomena_, and
reason are much the same.
Having therefore shewn that there is such a propriety in the _prisme_ and
water _Globule_ whereby the pulse is made _oblique_ to the progressive, and
that so much the more, by how much greater the refraction is, I shall in
the next place consider, how this conduces to the production of colours,
and what kind of impression it makes upon the bottom of the eye; and to
this end it will be requisite to examine this _Hypothesis_ a little more
particularly.
First therefore, if we consider the manner of the progress of the pulse, it
will seem rational to conclude, that that part or end of the pulse which
precedes the other, must necessarily be somwhat more _obtunded_, or
_impeded_ by the resistance of the transparent _medium_, than the other
part or end of it which is subsequent, whose way is, as it were, prepared
by the other; especially if the adjacent _medium_ be not in the same manner
enlightned or agitated. And therefore (in the fourth _Figure_ of the sixth
_Iconism_) the Ray AAAHB will have its side HH more deadened by the
resistance of the dark or quiet _medium_ PPP, Whence there will be a kind
of deadness superinduc’d on the side HHH, which will continually increase
from B, and strike deeper and deeper into the Ray by the line BR; Whence
all the parts of the triangle, RBHO will be of a dead _Blue_ colour, and so
much the deeper, by how much the nearer they lie to the line BHH, which is
most deaded or impeded, and so much the more _dilute_, by how much the
nearer it approaches the line BR. Next on the other side of the Ray AAN,
the end A of the pulse AH will be promoted, or made stronger, having its
passage already prepar’d as ’twere by the other parts preceding, and so its
impression wil be stronger; And because of its _obliquity_ to the Ray,
there will be propagated a kind of faint motion into QQ the adjacent dark
or quiet _medium_, which faint motion will spread further and further into
QQ as the Ray is propagated further and further from A, namely, as far as
the line MA, whence all the triangle MAN will be ting’d with a _Red_, and
that _Red_ will be the deeper the nearer it approaches the line MA, and the
_paler_ or _yellower_ the nearer it is the line NA. And if the Ray be
continued, so that the lines AN and BR (which are the bounds of the _Red_
and _Blue diluted_) do meet and cross each other, there will be beyond that
intersection generated all kinds of _Greens_.
Now, these being the proprieties of every single refracted Ray of light, it
will be easie enough to consider what must be the result of very many such
Rays collateral: As if we suppose infinite such Rays _interjacent_ between
AKSB and ANOB, which are the terminating: For in this case the Ray AKSB
will have its _Red_ triangle intire, as lying next to the dark or quiet
_medium_, but the other side of it BS will have no _Blue_, because the
_medium adjacent_ to it SBO, is mov’d or enlightned, and consequently that
light does destroy the colour. So likewise will the Ray ANOB lose its
_Red_, because the _adjacent medium_ is mov’d or enlightned, but the other
side of the Ray that is _adjacent_ to the dark, namely, AHO will preserve
its _Blue_ entire, and these Rays must be so far produc’d as till AN and BR
cut each other, before there will be any _Green_ produc’d. From these
Proprieties well consider’d, may be deduc’d the reasons of all the
_Phænomena_ of the _prisme_, and of the _Globules_ or drops of Water which
conduce to the production of the Rainbow.
Next for the impression they make on the _Retina_, we will further examine
this _Hypothesis_: Suppose therefore ABCDEF, in the fifth _Figure_, to
represent the Ball of the eye: on the _Cornea_ of which ABC two Rays GACH
and KCAI (which are the terminating Rays of a luminous body) falling, are
by the refraction thereof collected or _converg’d_ into two points at the
bottom of the eye. Now, because these terminating Rays, and all the
_intermediate_ ones which come from any part of the luminous body, are
suppos’d by some sufficient refraction before they enter the eye, to have
their pulses made _oblique_ to their progression, and consequently each Ray
to have potentially _superinduc’d_ two proprieties, or colours, viz. a
_Red_ on the one side, and a _Blue_ on the other, which notwithstanding are
never actually manifest, but when this or that Ray has the one or the other
side of it bordering on a dark or unmov’d _medium_, therefore as soon as
these Rays are entred into the eye and so have one side of each of them
bordering on a dark part of the humours of the eye, they will each of them
actually exhibit some colour; therefore ADC the production of GACH will
exhibit a _Blue_, because the side CD is _adjacent_ to the dark _medium_
CQDC, but nothing of a _Red_, because its side AD is _adjacent_ to the
enlightned _medium_ ADFA: And all the Rays that from the points of the
luminous body are collected on the parts of the _Retina_ between D and F
shall have their _Blue_ so much the more _diluted_ by how much the farther
these points of collection are distant from D towards F; and the Ray AFC
the production of KCAI, will exhibit a _Red_, because the side AF is
adjacent to the dark or quiet _medium_ of the eye APFA, but nothing of a
_Blue_, because its side CF is _adjacent_ to the enlightned _medium_ CFDC,
and all the Rays from the intermediate parts of the luminous body that are
collected between F and D shall have their _Red_ so much the more diluted,
by how much the farther they are distant from F towards D.
Now, because by the refraction in the _Cornea_, and some other parts of the
eye, the sides of each Ray, which before were almost parallel, are made to
_converge_ and meet in a point at the bottom of the eye, therefore that
side of the _pulse_ which preceded before these refractions, shall first
touch the _Retina_, and the other side last. And therefore according as
this or that side, or end of the pulse shall be impeded, accordingly will
the _impressions_ on the _Retina_ be varied; therefore by the Ray GACH
refracted by the _Cornea_ to D there shall be on that point a stroke or
impression confus’d, whose weakest end, namely, that by the line CD shall
precede, and the stronger, namely, that by the line AD shall follow. And by
the Ray KCAI refracted to F, there shall be on that part a confus’d stroke
or impression, whose strongest part, namely, that by the line CF shal
precede, and whose weakest or impeded, namely, that by the line AF shall
follow, and all the intermediate points between F and D will receive
impressions from the _converg’d_ Rays so much the more like the impressions
on F and D by how much the nearer they approach that or this.
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 _Phænomena_ 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 scum 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 _Phænomena_ 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 _Phænomena_ 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.
* * * * *