_PART II._
_PROP._ I. THEOR. I.
_The Phaenomena of Colours in refracted or reflected Light are not caused
by new Modifications of the Light variously impress'd, according to the
various Terminations of the Light and Shadow_.
The PROOF by Experiments.
_Exper._ 1. For if the Sun shine into a very dark Chamber through an
oblong hole F, [in _Fig._ 1.] whose breadth is the sixth or eighth part
of an Inch, or something less; and his beam FH do afterwards pass first
through a very large Prism ABC, distant about 20 Feet from the hole, and
parallel to it, and then (with its white part) through an oblong hole H,
whose breadth is about the fortieth or sixtieth part of an Inch, and
which is made in a black opake Body GI, and placed at the distance of
two or three Feet from the Prism, in a parallel Situation both to the
Prism and to the former hole, and if this white Light thus transmitted
through the hole H, fall afterwards upon a white Paper _pt_, placed
after that hole H, at the distance of three or four Feet from it, and
there paint the usual Colours of the Prism, suppose red at _t_, yellow
at _s_, green at _r_, blue at _q_, and violet at _p_; you may with an
Iron Wire, or any such like slender opake Body, whose breadth is about
the tenth part of an Inch, by intercepting the Rays at _k_, _l_, _m_,
_n_ or _o_, take away any one of the Colours at _t_, _s_, _r_, _q_ or
_p_, whilst the other Colours remain upon the Paper as before; or with
an Obstacle something bigger you may take away any two, or three, or
four Colours together, the rest remaining: So that any one of the
Colours as well as violet may become outmost in the Confine of the
Shadow towards _p_, and any one of them as well as red may become
outmost in the Confine of the Shadow towards _t_, and any one of them
may also border upon the Shadow made within the Colours by the Obstacle
R intercepting some intermediate part of the Light; and, lastly, any one
of them by being left alone, may border upon the Shadow on either hand.
All the Colours have themselves indifferently to any Confines of Shadow,
and therefore the differences of these Colours from one another, do not
arise from the different Confines of Shadow, whereby Light is variously
modified, as has hitherto been the Opinion of Philosophers. In trying
these things 'tis to be observed, that by how much the holes F and H are
narrower, and the Intervals between them and the Prism greater, and the
Chamber darker, by so much the better doth the Experiment succeed;
provided the Light be not so far diminished, but that the Colours at
_pt_ be sufficiently visible. To procure a Prism of solid Glass large
enough for this Experiment will be difficult, and therefore a prismatick
Vessel must be made of polish'd Glass Plates cemented together, and
filled with salt Water or clear Oil.
[Illustration: FIG. 1.]
_Exper._ 2. The Sun's Light let into a dark Chamber through the round
hole F, [in _Fig._ 2.] half an Inch wide, passed first through the Prism
ABC placed at the hole, and then through a Lens PT something more than
four Inches broad, and about eight Feet distant from the Prism, and
thence converged to O the Focus of the Lens distant from it about three
Feet, and there fell upon a white Paper DE. If that Paper was
perpendicular to that Light incident upon it, as 'tis represented in the
posture DE, all the Colours upon it at O appeared white. But if the
Paper being turned about an Axis parallel to the Prism, became very much
inclined to the Light, as 'tis represented in the Positions _de_ and
_[Greek: de]_; the same Light in the one case appeared yellow and red,
in the other blue. Here one and the same part of the Light in one and
the same place, according to the various Inclinations of the Paper,
appeared in one case white, in another yellow or red, in a third blue,
whilst the Confine of Light and shadow, and the Refractions of the Prism
in all these cases remained the same.
[Illustration: FIG. 2.]
[Illustration: FIG. 3.]
_Exper._ 3. Such another Experiment may be more easily tried as follows.
Let a broad beam of the Sun's Light coming into a dark Chamber through a
hole in the Window-shut be refracted by a large Prism ABC, [in _Fig._
3.] whose refracting Angle C is more than 60 Degrees, and so soon as it
comes out of the Prism, let it fall upon the white Paper DE glewed upon
a stiff Plane; and this Light, when the Paper is perpendicular to it, as
'tis represented in DE, will appear perfectly white upon the Paper; but
when the Paper is very much inclin'd to it in such a manner as to keep
always parallel to the Axis of the Prism, the whiteness of the whole
Light upon the Paper will according to the inclination of the Paper this
way or that way, change either into yellow and red, as in the posture
_de_, or into blue and violet, as in the posture [Greek: de]. And if the
Light before it fall upon the Paper be twice refracted the same way by
two parallel Prisms, these Colours will become the more conspicuous.
Here all the middle parts of the broad beam of white Light which fell
upon the Paper, did without any Confine of Shadow to modify it, become
colour'd all over with one uniform Colour, the Colour being always the
same in the middle of the Paper as at the edges, and this Colour changed
according to the various Obliquity of the reflecting Paper, without any
change in the Refractions or Shadow, or in the Light which fell upon the
Paper. And therefore these Colours are to be derived from some other
Cause than the new Modifications of Light by Refractions and Shadows.
If it be asked, what then is their Cause? I answer, That the Paper in
the posture _de_, being more oblique to the more refrangible Rays than
to the less refrangible ones, is more strongly illuminated by the latter
than by the former, and therefore the less refrangible Rays are
predominant in the reflected Light. And where-ever they are predominant
in any Light, they tinge it with red or yellow, as may in some measure
appear by the first Proposition of the first Part of this Book, and will
more fully appear hereafter. And the contrary happens in the posture of
the Paper [Greek: de], the more refrangible Rays being then predominant
which always tinge Light with blues and violets.
_Exper._ 4. The Colours of Bubbles with which Children play are various,
and change their Situation variously, without any respect to any Confine
or Shadow. If such a Bubble be cover'd with a concave Glass, to keep it
from being agitated by any Wind or Motion of the Air, the Colours will
slowly and regularly change their situation, even whilst the Eye and the
Bubble, and all Bodies which emit any Light, or cast any Shadow, remain
unmoved. And therefore their Colours arise from some regular Cause which
depends not on any Confine of Shadow. What this Cause is will be shewed
in the next Book.
To these Experiments may be added the tenth Experiment of the first Part
of this first Book, where the Sun's Light in a dark Room being
trajected through the parallel Superficies of two Prisms tied together
in the form of a Parallelopipede, became totally of one uniform yellow
or red Colour, at its emerging out of the Prisms. Here, in the
production of these Colours, the Confine of Shadow can have nothing to
do. For the Light changes from white to yellow, orange and red
successively, without any alteration of the Confine of Shadow: And at
both edges of the emerging Light where the contrary Confines of Shadow
ought to produce different Effects, the Colour is one and the same,
whether it be white, yellow, orange or red: And in the middle of the
emerging Light, where there is no Confine of Shadow at all, the Colour
is the very same as at the edges, the whole Light at its very first
Emergence being of one uniform Colour, whether white, yellow, orange or
red, and going on thence perpetually without any change of Colour, such
as the Confine of Shadow is vulgarly supposed to work in refracted Light
after its Emergence. Neither can these Colours arise from any new
Modifications of the Light by Refractions, because they change
successively from white to yellow, orange and red, while the Refractions
remain the same, and also because the Refractions are made contrary ways
by parallel Superficies which destroy one another's Effects. They arise
not therefore from any Modifications of Light made by Refractions and
Shadows, but have some other Cause. What that Cause is we shewed above
in this tenth Experiment, and need not here repeat it.
There is yet another material Circumstance of this Experiment. For this
emerging Light being by a third Prism HIK [in _Fig._ 22. _Part_ I.][I]
refracted towards the Paper PT, and there painting the usual Colours of
the Prism, red, yellow, green, blue, violet: If these Colours arose from
the Refractions of that Prism modifying the Light, they would not be in
the Light before its Incidence on that Prism. And yet in that Experiment
we found, that when by turning the two first Prisms about their common
Axis all the Colours were made to vanish but the red; the Light which
makes that red being left alone, appeared of the very same red Colour
before its Incidence on the third Prism. And in general we find by other
Experiments, that when the Rays which differ in Refrangibility are
separated from one another, and any one Sort of them is considered
apart, the Colour of the Light which they compose cannot be changed by
any Refraction or Reflexion whatever, as it ought to be were Colours
nothing else than Modifications of Light caused by Refractions, and
Reflexions, and Shadows. This Unchangeableness of Colour I am now to
describe in the following Proposition.
_PROP._ II. THEOR. II.
_All homogeneal Light has its proper Colour answering to its Degree of
Refrangibility, and that Colour cannot be changed by Reflexions and
Refractions._
In the Experiments of the fourth Proposition of the first Part of this
first Book, when I had separated the heterogeneous Rays from one
another, the Spectrum _pt_ formed by the separated Rays, did in the
Progress from its End _p_, on which the most refrangible Rays fell, unto
its other End _t_, on which the least refrangible Rays fell, appear
tinged with this Series of Colours, violet, indigo, blue, green, yellow,
orange, red, together with all their intermediate Degrees in a continual
Succession perpetually varying. So that there appeared as many Degrees
of Colours, as there were sorts of Rays differing in Refrangibility.
_Exper._ 5. Now, that these Colours could not be changed by Refraction,
I knew by refracting with a Prism sometimes one very little Part of this
Light, sometimes another very little Part, as is described in the
twelfth Experiment of the first Part of this Book. For by this
Refraction the Colour of the Light was never changed in the least. If
any Part of the red Light was refracted, it remained totally of the same
red Colour as before. No orange, no yellow, no green or blue, no other
new Colour was produced by that Refraction. Neither did the Colour any
ways change by repeated Refractions, but continued always the same red
entirely as at first. The like Constancy and Immutability I found also
in the blue, green, and other Colours. So also, if I looked through a
Prism upon any Body illuminated with any part of this homogeneal Light,
as in the fourteenth Experiment of the first Part of this Book is
described; I could not perceive any new Colour generated this way. All
Bodies illuminated with compound Light appear through Prisms confused,
(as was said above) and tinged with various new Colours, but those
illuminated with homogeneal Light appeared through Prisms neither less
distinct, nor otherwise colour'd, than when viewed with the naked Eyes.
Their Colours were not in the least changed by the Refraction of the
interposed Prism. I speak here of a sensible Change of Colour: For the
Light which I here call homogeneal, being not absolutely homogeneal,
there ought to arise some little Change of Colour from its
Heterogeneity. But, if that Heterogeneity was so little as it might be
made by the said Experiments of the fourth Proposition, that Change was
not sensible, and therefore in Experiments, where Sense is Judge, ought
to be accounted none at all.
_Exper._ 6. And as these Colours were not changeable by Refractions, so
neither were they by Reflexions. For all white, grey, red, yellow,
green, blue, violet Bodies, as Paper, Ashes, red Lead, Orpiment, Indico
Bise, Gold, Silver, Copper, Grass, blue Flowers, Violets, Bubbles of
Water tinged with various Colours, Peacock's Feathers, the Tincture of
_Lignum Nephriticum_, and such-like, in red homogeneal Light appeared
totally red, in blue Light totally blue, in green Light totally green,
and so of other Colours. In the homogeneal Light of any Colour they all
appeared totally of that same Colour, with this only Difference, that
some of them reflected that Light more strongly, others more faintly. I
never yet found any Body, which by reflecting homogeneal Light could
sensibly change its Colour.
From all which it is manifest, that if the Sun's Light consisted of but
one sort of Rays, there would be but one Colour in the whole World, nor
would it be possible to produce any new Colour by Reflexions and
Refractions, and by consequence that the variety of Colours depends upon
the Composition of Light.
_DEFINITION._
The homogeneal Light and Rays which appear red, or rather make Objects
appear so, I call Rubrifick or Red-making; those which make Objects
appear yellow, green, blue, and violet, I call Yellow-making,
Green-making, Blue-making, Violet-making, and so of the rest. And if at
any time I speak of Light and Rays as coloured or endued with Colours, I
would be understood to speak not philosophically and properly, but
grossly, and accordingly to such Conceptions as vulgar People in seeing
all these Experiments would be apt to frame. For the Rays to speak
properly are not coloured. In them there is nothing else than a certain
Power and Disposition to stir up a Sensation of this or that Colour.
For as Sound in a Bell or musical String, or other sounding Body, is
nothing but a trembling Motion, and in the Air nothing but that Motion
propagated from the Object, and in the Sensorium 'tis a Sense of that
Motion under the Form of Sound; so Colours in the Object are nothing but
a Disposition to reflect this or that sort of Rays more copiously than
the rest; in the Rays they are nothing but their Dispositions to
propagate this or that Motion into the Sensorium, and in the Sensorium
they are Sensations of those Motions under the Forms of Colours.
_PROP._ III. PROB. I.
_To define the Refrangibility of the several sorts of homogeneal Light
answering to the several Colours._