_Exper._ 7. When I had caused the Rectilinear Sides AF, GM, [in _Fig._
4.] of the Spectrum of Colours made by the Prism to be distinctly
defined, as in the fifth Experiment of the first Part of this Book is
described, there were found in it all the homogeneal Colours in the same
Order and Situation one among another as in the Spectrum of simple
Light, described in the fourth Proposition of that Part. For the Circles
of which the Spectrum of compound Light PT is composed, and which in
the middle Parts of the Spectrum interfere, and are intermix'd with one
another, are not intermix'd in their outmost Parts where they touch
those Rectilinear Sides AF and GM. And therefore, in those Rectilinear
Sides when distinctly defined, there is no new Colour generated by
Refraction. I observed also, that if any where between the two outmost
Circles TMF and PGA a Right Line, as [Greek: gd], was cross to the
Spectrum, so as both Ends to fall perpendicularly upon its Rectilinear
Sides, there appeared one and the same Colour, and degree of Colour from
one End of this Line to the other. I delineated therefore in a Paper the
Perimeter of the Spectrum FAP GMT, and in trying the third Experiment of
the first Part of this Book, I held the Paper so that the Spectrum might
fall upon this delineated Figure, and agree with it exactly, whilst an
Assistant, whose Eyes for distinguishing Colours were more critical than
mine, did by Right Lines [Greek: ab, gd, ez,] &c. drawn cross the
Spectrum, note the Confines of the Colours, that is of the red M[Greek:
ab]F, of the orange [Greek: agdb], of the yellow [Greek: gezd], of the
green [Greek: eethz], of the blue [Greek: eikth], of the indico [Greek:
ilmk], and of the violet [Greek: l]GA[Greek: m]. And this Operation
being divers times repeated both in the same, and in several Papers, I
found that the Observations agreed well enough with one another, and
that the Rectilinear Sides MG and FA were by the said cross Lines
divided after the manner of a Musical Chord. Let GM be produced to X,
that MX may be equal to GM, and conceive GX, [Greek: l]X, [Greek: i]X,
[Greek: e]X, [Greek: e]X, [Greek: g]X, [Greek: a]X, MX, to be in
proportion to one another, as the Numbers, 1, 8/9, 5/6, 3/4, 2/3, 3/5,
9/16, 1/2, and so to represent the Chords of the Key, and of a Tone, a
third Minor, a fourth, a fifth, a sixth Major, a seventh and an eighth
above that Key: And the Intervals M[Greek: a], [Greek: ag], [Greek: ge],
[Greek: ee], [Greek: ei], [Greek: il], and [Greek: l]G, will be the
Spaces which the several Colours (red, orange, yellow, green, blue,
indigo, violet) take up.
[Illustration: FIG. 4.]
[Illustration: FIG. 5.]
Now these Intervals or Spaces subtending the Differences of the
Refractions of the Rays going to the Limits of those Colours, that is,
to the Points M, [Greek: a], [Greek: g], [Greek: e], [Greek: e], [Greek:
i], [Greek: l], G, may without any sensible Error be accounted
proportional to the Differences of the Sines of Refraction of those Rays
having one common Sine of Incidence, and therefore since the common Sine
of Incidence of the most and least refrangible Rays out of Glass into
Air was (by a Method described above) found in proportion to their Sines
of Refraction, as 50 to 77 and 78, divide the Difference between the
Sines of Refraction 77 and 78, as the Line GM is divided by those
Intervals, and you will have 77, 77-1/8, 77-1/5, 77-1/3, 77-1/2, 77-2/3,
77-7/9, 78, the Sines of Refraction of those Rays out of Glass into Air,
their common Sine of Incidence being 50. So then the Sines of the
Incidences of all the red-making Rays out of Glass into Air, were to the
Sines of their Refractions, not greater than 50 to 77, nor less than 50
to 77-1/8, but they varied from one another according to all
intermediate Proportions. And the Sines of the Incidences of the
green-making Rays were to the Sines of their Refractions in all
Proportions from that of 50 to 77-1/3, unto that of 50 to 77-1/2. And
by the like Limits above-mentioned were the Refractions of the Rays
belonging to the rest of the Colours defined, the Sines of the
red-making Rays extending from 77 to 77-1/8, those of the orange-making
from 77-1/8 to 77-1/5, those of the yellow-making from 77-1/5 to 77-1/3,
those of the green-making from 77-1/3 to 77-1/2, those of the
blue-making from 77-1/2 to 77-2/3, those of the indigo-making from
77-2/3 to 77-7/9, and those of the violet from 77-7/9, to 78.
These are the Laws of the Refractions made out of Glass into Air, and
thence by the third Axiom of the first Part of this Book, the Laws of
the Refractions made out of Air into Glass are easily derived.
_Exper._ 8. I found moreover, that when Light goes out of Air through
several contiguous refracting Mediums as through Water and Glass, and
thence goes out again into Air, whether the refracting Superficies be
parallel or inclin'd to one another, that Light as often as by contrary
Refractions 'tis so corrected, that it emergeth in Lines parallel to
those in which it was incident, continues ever after to be white. But if
the emergent Rays be inclined to the incident, the Whiteness of the
emerging Light will by degrees in passing on from the Place of
Emergence, become tinged in its Edges with Colours. This I try'd by
refracting Light with Prisms of Glass placed within a Prismatick Vessel
of Water. Now those Colours argue a diverging and separation of the
heterogeneous Rays from one another by means of their unequal
Refractions, as in what follows will more fully appear. And, on the
contrary, the permanent whiteness argues, that in like Incidences of the
Rays there is no such separation of the emerging Rays, and by
consequence no inequality of their whole Refractions. Whence I seem to
gather the two following Theorems.
1. The Excesses of the Sines of Refraction of several sorts of Rays
above their common Sine of Incidence when the Refractions are made out
of divers denser Mediums immediately into one and the same rarer Medium,
suppose of Air, are to one another in a given Proportion.
2. The Proportion of the Sine of Incidence to the Sine of Refraction of
one and the same sort of Rays out of one Medium into another, is
composed of the Proportion of the Sine of Incidence to the Sine of
Refraction out of the first Medium into any third Medium, and of the
Proportion of the Sine of Incidence to the Sine of Refraction out of
that third Medium into the second Medium.
By the first Theorem the Refractions of the Rays of every sort made out
of any Medium into Air are known by having the Refraction of the Rays of
any one sort. As for instance, if the Refractions of the Rays of every
sort out of Rain-water into Air be desired, let the common Sine of
Incidence out of Glass into Air be subducted from the Sines of
Refraction, and the Excesses will be 27, 27-1/8, 27-1/5, 27-1/3, 27-1/2,
27-2/3, 27-7/9, 28. Suppose now that the Sine of Incidence of the least
refrangible Rays be to their Sine of Refraction out of Rain-water into
Air as 3 to 4, and say as 1 the difference of those Sines is to 3 the
Sine of Incidence, so is 27 the least of the Excesses above-mentioned to
a fourth Number 81; and 81 will be the common Sine of Incidence out of
Rain-water into Air, to which Sine if you add all the above-mentioned
Excesses, you will have the desired Sines of the Refractions 108,
108-1/8, 108-1/5, 108-1/3, 108-1/2, 108-2/3, 108-7/9, 109.
By the latter Theorem the Refraction out of one Medium into another is
gathered as often as you have the Refractions out of them both into any
third Medium. As if the Sine of Incidence of any Ray out of Glass into
Air be to its Sine of Refraction, as 20 to 31, and the Sine of Incidence
of the same Ray out of Air into Water, be to its Sine of Refraction as 4
to 3; the Sine of Incidence of that Ray out of Glass into Water will be
to its Sine of Refraction as 20 to 31 and 4 to 3 jointly, that is, as
the Factum of 20 and 4 to the Factum of 31 and 3, or as 80 to 93.
And these Theorems being admitted into Opticks, there would be scope
enough of handling that Science voluminously after a new manner,[K] not
only by teaching those things which tend to the perfection of Vision,
but also by determining mathematically all kinds of Phaenomena of Colours
which could be produced by Refractions. For to do this, there is nothing
else requisite than to find out the Separations of heterogeneous Rays,
and their various Mixtures and Proportions in every Mixture. By this
way of arguing I invented almost all the Phaenomena described in these
Books, beside some others less necessary to the Argument; and by the
successes I met with in the Trials, I dare promise, that to him who
shall argue truly, and then try all things with good Glasses and
sufficient Circumspection, the expected Event will not be wanting. But
he is first to know what Colours will arise from any others mix'd in any
assigned Proportion.
_PROP._ IV. THEOR. III.
_Colours may be produced by Composition which shall be like to the
Colours of homogeneal Light as to the Appearance of Colour, but not as
to the Immutability of Colour and Constitution of Light. And those
Colours by how much they are more compounded by so much are they less
full and intense, and by too much Composition they maybe diluted and
weaken'd till they cease, and the Mixture becomes white or grey. There
may be also Colours produced by Composition, which are not fully like
any of the Colours of homogeneal Light._
For a Mixture of homogeneal red and yellow compounds an Orange, like in
appearance of Colour to that orange which in the series of unmixed
prismatick Colours lies between them; but the Light of one orange is
homogeneal as to Refrangibility, and that of the other is heterogeneal,
and the Colour of the one, if viewed through a Prism, remains unchanged,
that of the other is changed and resolved into its component Colours red
and yellow. And after the same manner other neighbouring homogeneal
Colours may compound new Colours, like the intermediate homogeneal ones,
as yellow and green, the Colour between them both, and afterwards, if
blue be added, there will be made a green the middle Colour of the three
which enter the Composition. For the yellow and blue on either hand, if
they are equal in quantity they draw the intermediate green equally
towards themselves in Composition, and so keep it as it were in
AEquilibrion, that it verge not more to the yellow on the one hand, and
to the blue on the other, but by their mix'd Actions remain still a
middle Colour. To this mix'd green there may be farther added some red
and violet, and yet the green will not presently cease, but only grow
less full and vivid, and by increasing the red and violet, it will grow
more and more dilute, until by the prevalence of the added Colours it be
overcome and turned into whiteness, or some other Colour. So if to the
Colour of any homogeneal Light, the Sun's white Light composed of all
sorts of Rays be added, that Colour will not vanish or change its
Species, but be diluted, and by adding more and more white it will be
diluted more and more perpetually. Lastly, If red and violet be mingled,
there will be generated according to their various Proportions various
Purples, such as are not like in appearance to the Colour of any
homogeneal Light, and of these Purples mix'd with yellow and blue may be
made other new Colours.
_PROP._ V. THEOR. IV.
_Whiteness and all grey Colours between white and black, may be
compounded of Colours, and the whiteness of the Sun's Light is
compounded of all the primary Colours mix'd in a due Proportion._
The PROOF by Experiments.
_Exper._ 9. The Sun shining into a dark Chamber through a little round
hole in the Window-shut, and his Light being there refracted by a Prism
to cast his coloured Image PT [in _Fig._ 5.] upon the opposite Wall: I
held a white Paper V to that image in such manner that it might be
illuminated by the colour'd Light reflected from thence, and yet not
intercept any part of that Light in its passage from the Prism to the
Spectrum. And I found that when the Paper was held nearer to any Colour
than to the rest, it appeared of that Colour to which it approached
nearest; but when it was equally or almost equally distant from all the
Colours, so that it might be equally illuminated by them all it appeared
white. And in this last situation of the Paper, if some Colours were
intercepted, the Paper lost its white Colour, and appeared of the Colour
of the rest of the Light which was not intercepted. So then the Paper
was illuminated with Lights of various Colours, namely, red, yellow,
green, blue and violet, and every part of the Light retained its proper
Colour, until it was incident on the Paper, and became reflected thence
to the Eye; so that if it had been either alone (the rest of the Light
being intercepted) or if it had abounded most, and been predominant in
the Light reflected from the Paper, it would have tinged the Paper with
its own Colour; and yet being mixed with the rest of the Colours in a
due proportion, it made the Paper look white, and therefore by a
Composition with the rest produced that Colour. The several parts of the
coloured Light reflected from the Spectrum, whilst they are propagated
from thence through the Air, do perpetually retain their proper Colours,
because wherever they fall upon the Eyes of any Spectator, they make the
several parts of the Spectrum to appear under their proper Colours. They
retain therefore their proper Colours when they fall upon the Paper V,
and so by the confusion and perfect mixture of those Colours compound
the whiteness of the Light reflected from thence.
_Exper._ 10. Let that Spectrum or solar Image PT [in _Fig._ 6.] fall now
upon the Lens MN above four Inches broad, and about six Feet distant
from the Prism ABC and so figured that it may cause the coloured Light
which divergeth from the Prism to converge and meet again at its Focus
G, about six or eight Feet distant from the Lens, and there to fall
perpendicularly upon a white Paper DE. And if you move this Paper to and
fro, you will perceive that near the Lens, as at _de_, the whole solar
Image (suppose at _pt_) will appear upon it intensely coloured after the
manner above-explained, and that by receding from the Lens those Colours
will perpetually come towards one another, and by mixing more and more
dilute one another continually, until at length the Paper come to the
Focus G, where by a perfect mixture they will wholly vanish and be
converted into whiteness, the whole Light appearing now upon the Paper
like a little white Circle. And afterwards by receding farther from the
Lens, the Rays which before converged will now cross one another in the
Focus G, and diverge from thence, and thereby make the Colours to appear
again, but yet in a contrary order; suppose at [Greek: de], where the
red _t_ is now above which before was below, and the violet _p_ is below
which before was above.
Let us now stop the Paper at the Focus G, where the Light appears
totally white and circular, and let us consider its whiteness. I say,
that this is composed of the converging Colours. For if any of those
Colours be intercepted at the Lens, the whiteness will cease and
degenerate into that Colour which ariseth from the composition of the
other Colours which are not intercepted. And then if the intercepted
Colours be let pass and fall upon that compound Colour, they mix with
it, and by their mixture restore the whiteness. So if the violet, blue
and green be intercepted, the remaining yellow, orange and red will
compound upon the Paper an orange, and then if the intercepted Colours
be let pass, they will fall upon this compounded orange, and together
with it decompound a white. So also if the red and violet be
intercepted, the remaining yellow, green and blue, will compound a green
upon the Paper, and then the red and violet being let pass will fall
upon this green, and together with it decompound a white. And that in
this Composition of white the several Rays do not suffer any Change in
their colorific Qualities by acting upon one another, but are only
mixed, and by a mixture of their Colours produce white, may farther
appear by these Arguments.
[Illustration: FIG. 6.]
If the Paper be placed beyond the Focus G, suppose at [Greek: de], and
then the red Colour at the Lens be alternately intercepted, and let pass
again, the violet Colour on the Paper will not suffer any Change
thereby, as it ought to do if the several sorts of Rays acted upon one
another in the Focus G, where they cross. Neither will the red upon the
Paper be changed by any alternate stopping, and letting pass the violet
which crosseth it.
And if the Paper be placed at the Focus G, and the white round Image at
G be viewed through the Prism HIK, and by the Refraction of that Prism
be translated to the place _rv_, and there appear tinged with various
Colours, namely, the violet at _v_ and red at _r_, and others between,
and then the red Colours at the Lens be often stopp'd and let pass by
turns, the red at _r_ will accordingly disappear, and return as often,
but the violet at _v_ will not thereby suffer any Change. And so by
stopping and letting pass alternately the blue at the Lens, the blue at
_v_ will accordingly disappear and return, without any Change made in
the red at _r_. The red therefore depends on one sort of Rays, and the
blue on another sort, which in the Focus G where they are commix'd, do
not act on one another. And there is the same Reason of the other
Colours.
I considered farther, that when the most refrangible Rays P_p_, and the
least refrangible ones T_t_, are by converging inclined to one another,
the Paper, if held very oblique to those Rays in the Focus G, might
reflect one sort of them more copiously than the other sort, and by that
Means the reflected Light would be tinged in that Focus with the Colour
of the predominant Rays, provided those Rays severally retained their
Colours, or colorific Qualities in the Composition of White made by them
in that Focus. But if they did not retain them in that White, but became
all of them severally endued there with a Disposition to strike the
Sense with the Perception of White, then they could never lose their
Whiteness by such Reflexions. I inclined therefore the Paper to the Rays
very obliquely, as in the second Experiment of this second Part of the
first Book, that the most refrangible Rays, might be more copiously
reflected than the rest, and the Whiteness at Length changed
successively into blue, indigo, and violet. Then I inclined it the
contrary Way, that the least refrangible Rays might be more copious in
the reflected Light than the rest, and the Whiteness turned successively
to yellow, orange, and red.
Lastly, I made an Instrument XY in fashion of a Comb, whose Teeth being
in number sixteen, were about an Inch and a half broad, and the
Intervals of the Teeth about two Inches wide. Then by interposing
successively the Teeth of this Instrument near the Lens, I intercepted
Part of the Colours by the interposed Tooth, whilst the rest of them
went on through the Interval of the Teeth to the Paper DE, and there
painted a round Solar Image. But the Paper I had first placed so, that
the Image might appear white as often as the Comb was taken away; and
then the Comb being as was said interposed, that Whiteness by reason of
the intercepted Part of the Colours at the Lens did always change into
the Colour compounded of those Colours which were not intercepted, and
that Colour was by the Motion of the Comb perpetually varied so, that in
the passing of every Tooth over the Lens all these Colours, red, yellow,
green, blue, and purple, did always succeed one another. I caused
therefore all the Teeth to pass successively over the Lens, and when the
Motion was slow, there appeared a perpetual Succession of the Colours
upon the Paper: But if I so much accelerated the Motion, that the
Colours by reason of their quick Succession could not be distinguished
from one another, the Appearance of the single Colours ceased. There was
no red, no yellow, no green, no blue, nor purple to be seen any longer,
but from a Confusion of them all there arose one uniform white Colour.
Of the Light which now by the Mixture of all the Colours appeared white,
there was no Part really white. One Part was red, another yellow, a
third green, a fourth blue, a fifth purple, and every Part retains its
proper Colour till it strike the Sensorium. If the Impressions follow
one another slowly, so that they may be severally perceived, there is
made a distinct Sensation of all the Colours one after another in a
continual Succession. But if the Impressions follow one another so
quickly, that they cannot be severally perceived, there ariseth out of
them all one common Sensation, which is neither of this Colour alone nor
of that alone, but hath it self indifferently to 'em all, and this is a
Sensation of Whiteness. By the Quickness of the Successions, the
Impressions of the several Colours are confounded in the Sensorium, and
out of that Confusion ariseth a mix'd Sensation. If a burning Coal be
nimbly moved round in a Circle with Gyrations continually repeated, the
whole Circle will appear like Fire; the reason of which is, that the
Sensation of the Coal in the several Places of that Circle remains
impress'd on the Sensorium, until the Coal return again to the same
Place. And so in a quick Consecution of the Colours the Impression of
every Colour remains in the Sensorium, until a Revolution of all the
Colours be compleated, and that first Colour return again. The
Impressions therefore of all the successive Colours are at once in the
Sensorium, and jointly stir up a Sensation of them all; and so it is
manifest by this Experiment, that the commix'd Impressions of all the
Colours do stir up and beget a Sensation of white, that is, that
Whiteness is compounded of all the Colours.
And if the Comb be now taken away, that all the Colours may at once pass
from the Lens to the Paper, and be there intermixed, and together
reflected thence to the Spectator's Eyes; their Impressions on the
Sensorium being now more subtilly and perfectly commixed there, ought
much more to stir up a Sensation of Whiteness.
You may instead of the Lens use two Prisms HIK and LMN, which by
refracting the coloured Light the contrary Way to that of the first
Refraction, may make the diverging Rays converge and meet again in G, as
you see represented in the seventh Figure. For where they meet and mix,
they will compose a white Light, as when a Lens is used.
_Exper._ 11. Let the Sun's coloured Image PT [in _Fig._ 8.] fall upon
the Wall of a dark Chamber, as in the third Experiment of the first
Book, and let the same be viewed through a Prism _abc_, held parallel to
the Prism ABC, by whose Refraction that Image was made, and let it now
appear lower than before, suppose in the Place S over-against the red
Colour T. And if you go near to the Image PT, the Spectrum S will appear
oblong and coloured like the Image PT; but if you recede from it, the
Colours of the spectrum S will be contracted more and more, and at
length vanish, that Spectrum S becoming perfectly round and white; and
if you recede yet farther, the Colours will emerge again, but in a
contrary Order. Now that Spectrum S appears white in that Case, when the
Rays of several sorts which converge from the several Parts of the Image
PT, to the Prism _abc_, are so refracted unequally by it, that in their
Passage from the Prism to the Eye they may diverge from one and the same
Point of the Spectrum S, and so fall afterwards upon one and the same
Point in the bottom of the Eye, and there be mingled.
[Illustration: FIG. 7.]
[Illustration: FIG. 8.]
And farther, if the Comb be here made use of, by whose Teeth the Colours
at the Image PT may be successively intercepted; the Spectrum S, when
the Comb is moved slowly, will be perpetually tinged with successive
Colours: But when by accelerating the Motion of the Comb, the Succession
of the Colours is so quick that they cannot be severally seen, that
Spectrum S, by a confused and mix'd Sensation of them all, will appear
white.
_Exper._ 12. The Sun shining through a large Prism ABC [in _Fig._ 9.]
upon a Comb XY, placed immediately behind the Prism, his Light which
passed through the Interstices of the Teeth fell upon a white Paper DE.
The Breadths of the Teeth were equal to their Interstices, and seven
Teeth together with their Interstices took up an Inch in Breadth. Now,
when the Paper was about two or three Inches distant from the Comb, the
Light which passed through its several Interstices painted so many
Ranges of Colours, _kl_, _mn_, _op_, _qr_, &c. which were parallel to
one another, and contiguous, and without any Mixture of white. And these
Ranges of Colours, if the Comb was moved continually up and down with a
reciprocal Motion, ascended and descended in the Paper, and when the
Motion of the Comb was so quick, that the Colours could not be
distinguished from one another, the whole Paper by their Confusion and
Mixture in the Sensorium appeared white.
[Illustration: FIG. 9.]
Let the Comb now rest, and let the Paper be removed farther from the
Prism, and the several Ranges of Colours will be dilated and expanded
into one another more and more, and by mixing their Colours will dilute
one another, and at length, when the distance of the Paper from the Comb
is about a Foot, or a little more (suppose in the Place 2D 2E) they will
so far dilute one another, as to become white.
With any Obstacle, let all the Light be now stopp'd which passes through
any one Interval of the Teeth, so that the Range of Colours which comes
from thence may be taken away, and you will see the Light of the rest of
the Ranges to be expanded into the Place of the Range taken away, and
there to be coloured. Let the intercepted Range pass on as before, and
its Colours falling upon the Colours of the other Ranges, and mixing
with them, will restore the Whiteness.
Let the Paper 2D 2E be now very much inclined to the Rays, so that the
most refrangible Rays may be more copiously reflected than the rest, and
the white Colour of the Paper through the Excess of those Rays will be
changed into blue and violet. Let the Paper be as much inclined the
contrary way, that the least refrangible Rays may be now more copiously
reflected than the rest, and by their Excess the Whiteness will be
changed into yellow and red. The several Rays therefore in that white
Light do retain their colorific Qualities, by which those of any sort,
whenever they become more copious than the rest, do by their Excess and
Predominance cause their proper Colour to appear.
And by the same way of arguing, applied to the third Experiment of this
second Part of the first Book, it may be concluded, that the white
Colour of all refracted Light at its very first Emergence, where it
appears as white as before its Incidence, is compounded of various
Colours.
[Illustration: FIG. 10.]
_Exper._ 13. In the foregoing Experiment the several Intervals of the
Teeth of the Comb do the Office of so many Prisms, every Interval
producing the Phaenomenon of one Prism. Whence instead of those Intervals
using several Prisms, I try'd to compound Whiteness by mixing their
Colours, and did it by using only three Prisms, as also by using only
two as follows. Let two Prisms ABC and _abc_, [in _Fig._ 10.] whose
refracting Angles B and _b_ are equal, be so placed parallel to one
another, that the refracting Angle B of the one may touch the Angle _c_
at the Base of the other, and their Planes CB and _cb_, at which the
Rays emerge, may lie in Directum. Then let the Light trajected through
them fall upon the Paper MN, distant about 8 or 12 Inches from the
Prisms. And the Colours generated by the interior Limits B and _c_ of
the two Prisms, will be mingled at PT, and there compound white. For if
either Prism be taken away, the Colours made by the other will appear in
that Place PT, and when the Prism is restored to its Place again, so
that its Colours may there fall upon the Colours of the other, the
Mixture of them both will restore the Whiteness.
This Experiment succeeds also, as I have tried, when the Angle _b_ of
the lower Prism, is a little greater than the Angle B of the upper, and
between the interior Angles B and _c_, there intercedes some Space B_c_,
as is represented in the Figure, and the refracting Planes BC and _bc_,
are neither in Directum, nor parallel to one another. For there is
nothing more requisite to the Success of this Experiment, than that the
Rays of all sorts may be uniformly mixed upon the Paper in the Place PT.
If the most refrangible Rays coming from the superior Prism take up all
the Space from M to P, the Rays of the same sort which come from the
inferior Prism ought to begin at P, and take up all the rest of the
Space from thence towards N. If the least refrangible Rays coming from
the superior Prism take up the Space MT, the Rays of the same kind which
come from the other Prism ought to begin at T, and take up the
remaining Space TN. If one sort of the Rays which have intermediate
Degrees of Refrangibility, and come from the superior Prism be extended
through the Space MQ, and another sort of those Rays through the Space
MR, and a third sort of them through the Space MS, the same sorts of
Rays coming from the lower Prism, ought to illuminate the remaining
Spaces QN, RN, SN, respectively. And the same is to be understood of all
the other sorts of Rays. For thus the Rays of every sort will be
scattered uniformly and evenly through the whole Space MN, and so being
every where mix'd in the same Proportion, they must every where produce
the same Colour. And therefore, since by this Mixture they produce white
in the Exterior Spaces MP and TN, they must also produce white in the
Interior Space PT. This is the reason of the Composition by which
Whiteness was produced in this Experiment, and by what other way soever
I made the like Composition, the Result was Whiteness.
Lastly, If with the Teeth of a Comb of a due Size, the coloured Lights
of the two Prisms which fall upon the Space PT be alternately
intercepted, that Space PT, when the Motion of the Comb is slow, will
always appear coloured, but by accelerating the Motion of the Comb so
much that the successive Colours cannot be distinguished from one
another, it will appear white.
_Exper._ 14. Hitherto I have produced Whiteness by mixing the Colours of
Prisms. If now the Colours of natural Bodies are to be mingled, let
Water a little thicken'd with Soap be agitated to raise a Froth, and
after that Froth has stood a little, there will appear to one that shall
view it intently various Colours every where in the Surfaces of the
several Bubbles; but to one that shall go so far off, that he cannot
distinguish the Colours from one another, the whole Froth will grow
white with a perfect Whiteness.
_Exper._ 15. Lastly, In attempting to compound a white, by mixing the
coloured Powders which Painters use, I consider'd that all colour'd
Powders do suppress and stop in them a very considerable Part of the
Light by which they are illuminated. For they become colour'd by
reflecting the Light of their own Colours more copiously, and that of
all other Colours more sparingly, and yet they do not reflect the Light
of their own Colours so copiously as white Bodies do. If red Lead, for
instance, and a white Paper, be placed in the red Light of the colour'd
Spectrum made in a dark Chamber by the Refraction of a Prism, as is
described in the third Experiment of the first Part of this Book; the
Paper will appear more lucid than the red Lead, and therefore reflects
the red-making Rays more copiously than red Lead doth. And if they be
held in the Light of any other Colour, the Light reflected by the Paper
will exceed the Light reflected by the red Lead in a much greater
Proportion. And the like happens in Powders of other Colours. And
therefore by mixing such Powders, we are not to expect a strong and
full White, such as is that of Paper, but some dusky obscure one, such
as might arise from a Mixture of Light and Darkness, or from white and
black, that is, a grey, or dun, or russet brown, such as are the Colours
of a Man's Nail, of a Mouse, of Ashes, of ordinary Stones, of Mortar, of
Dust and Dirt in High-ways, and the like. And such a dark white I have
often produced by mixing colour'd Powders. For thus one Part of red
Lead, and five Parts of _Viride AEris_, composed a dun Colour like that
of a Mouse. For these two Colours were severally so compounded of
others, that in both together were a Mixture of all Colours; and there
was less red Lead used than _Viride AEris_, because of the Fulness of its
Colour. Again, one Part of red Lead, and four Parts of blue Bise,
composed a dun Colour verging a little to purple, and by adding to this
a certain Mixture of Orpiment and _Viride AEris_ in a due Proportion, the
Mixture lost its purple Tincture, and became perfectly dun. But the
Experiment succeeded best without Minium thus. To Orpiment I added by
little and little a certain full bright purple, which Painters use,
until the Orpiment ceased to be yellow, and became of a pale red. Then I
diluted that red by adding a little _Viride AEris_, and a little more
blue Bise than _Viride AEris_, until it became of such a grey or pale
white, as verged to no one of the Colours more than to another. For thus
it became of a Colour equal in Whiteness to that of Ashes, or of Wood
newly cut, or of a Man's Skin. The Orpiment reflected more Light than
did any other of the Powders, and therefore conduced more to the
Whiteness of the compounded Colour than they. To assign the Proportions
accurately may be difficult, by reason of the different Goodness of
Powders of the same kind. Accordingly, as the Colour of any Powder is
more or less full and luminous, it ought to be used in a less or greater
Proportion.
Now, considering that these grey and dun Colours may be also produced by
mixing Whites and Blacks, and by consequence differ from perfect Whites,
not in Species of Colours, but only in degree of Luminousness, it is
manifest that there is nothing more requisite to make them perfectly
white than to increase their Light sufficiently; and, on the contrary,
if by increasing their Light they can be brought to perfect Whiteness,
it will thence also follow, that they are of the same Species of Colour
with the best Whites, and differ from them only in the Quantity of
Light. And this I tried as follows. I took the third of the
above-mention'd grey Mixtures, (that which was compounded of Orpiment,
Purple, Bise, and _Viride AEris_) and rubbed it thickly upon the Floor of
my Chamber, where the Sun shone upon it through the opened Casement; and
by it, in the shadow, I laid a Piece of white Paper of the same Bigness.
Then going from them to the distance of 12 or 18 Feet, so that I could
not discern the Unevenness of the Surface of the Powder, nor the little
Shadows let fall from the gritty Particles thereof; the Powder appeared
intensely white, so as to transcend even the Paper it self in Whiteness,
especially if the Paper were a little shaded from the Light of the
Clouds, and then the Paper compared with the Powder appeared of such a
grey Colour as the Powder had done before. But by laying the Paper where
the Sun shines through the Glass of the Window, or by shutting the
Window that the Sun might shine through the Glass upon the Powder, and
by such other fit Means of increasing or decreasing the Lights wherewith
the Powder and Paper were illuminated, the Light wherewith the Powder is
illuminated may be made stronger in such a due Proportion than the Light
wherewith the Paper is illuminated, that they shall both appear exactly
alike in Whiteness. For when I was trying this, a Friend coming to visit
me, I stopp'd him at the Door, and before I told him what the Colours
were, or what I was doing; I asked him, Which of the two Whites were the
best, and wherein they differed? And after he had at that distance
viewed them well, he answer'd, that they were both good Whites, and that
he could not say which was best, nor wherein their Colours differed.
Now, if you consider, that this White of the Powder in the Sun-shine was
compounded of the Colours which the component Powders (Orpiment, Purple,
Bise, and _Viride AEris_) have in the same Sun-shine, you must
acknowledge by this Experiment, as well as by the former, that perfect
Whiteness may be compounded of Colours.
From what has been said it is also evident, that the Whiteness of the
Sun's Light is compounded of all the Colours wherewith the several sorts
of Rays whereof that Light consists, when by their several
Refrangibilities they are separated from one another, do tinge Paper or
any other white Body whereon they fall. For those Colours (by _Prop._
II. _Part_ 2.) are unchangeable, and whenever all those Rays with those
their Colours are mix'd again, they reproduce the same white Light as
before.
_PROP._ VI. PROB. II.
_In a mixture of Primary Colours, the Quantity and Quality of each being
given, to know the Colour of the Compound._
[Illustration: FIG. 11.]
With the Center O [in _Fig._ 11.] and Radius OD describe a Circle ADF,
and distinguish its Circumference into seven Parts DE, EF, FG, GA, AB,
BC, CD, proportional to the seven Musical Tones or Intervals of the
eight Sounds, _Sol_, _la_, _fa_, _sol_, _la_, _mi_, _fa_, _sol_,
contained in an eight, that is, proportional to the Number 1/9, 1/16,
1/10, 1/9, 1/16, 1/16, 1/9. Let the first Part DE represent a red
Colour, the second EF orange, the third FG yellow, the fourth CA green,
the fifth AB blue, the sixth BC indigo, and the seventh CD violet. And
conceive that these are all the Colours of uncompounded Light gradually
passing into one another, as they do when made by Prisms; the
Circumference DEFGABCD, representing the whole Series of Colours from
one end of the Sun's colour'd Image to the other, so that from D to E be
all degrees of red, at E the mean Colour between red and orange, from E
to F all degrees of orange, at F the mean between orange and yellow,
from F to G all degrees of yellow, and so on. Let _p_ be the Center of
Gravity of the Arch DE, and _q_, _r_, _s_, _t_, _u_, _x_, the Centers of
Gravity of the Arches EF, FG, GA, AB, BC, and CD respectively, and about
those Centers of Gravity let Circles proportional to the Number of Rays
of each Colour in the given Mixture be describ'd: that is, the Circle
_p_ proportional to the Number of the red-making Rays in the Mixture,
the Circle _q_ proportional to the Number of the orange-making Rays in
the Mixture, and so of the rest. Find the common Center of Gravity of
all those Circles, _p_, _q_, _r_, _s_, _t_, _u_, _x_. Let that Center be
Z; and from the Center of the Circle ADF, through Z to the
Circumference, drawing the Right Line OY, the Place of the Point Y in
the Circumference shall shew the Colour arising from the Composition of
all the Colours in the given Mixture, and the Line OZ shall be
proportional to the Fulness or Intenseness of the Colour, that is, to
its distance from Whiteness. As if Y fall in the middle between F and G,
the compounded Colour shall be the best yellow; if Y verge from the
middle towards F or G, the compound Colour shall accordingly be a
yellow, verging towards orange or green. If Z fall upon the
Circumference, the Colour shall be intense and florid in the highest
Degree; if it fall in the mid-way between the Circumference and Center,
it shall be but half so intense, that is, it shall be such a Colour as
would be made by diluting the intensest yellow with an equal quantity of
whiteness; and if it fall upon the center O, the Colour shall have lost
all its intenseness, and become a white. But it is to be noted, That if
the point Z fall in or near the line OD, the main ingredients being the
red and violet, the Colour compounded shall not be any of the prismatick
Colours, but a purple, inclining to red or violet, accordingly as the
point Z lieth on the side of the line DO towards E or towards C, and in
general the compounded violet is more bright and more fiery than the
uncompounded. Also if only two of the primary Colours which in the
circle are opposite to one another be mixed in an equal proportion, the
point Z shall fall upon the center O, and yet the Colour compounded of
those two shall not be perfectly white, but some faint anonymous Colour.
For I could never yet by mixing only two primary Colours produce a
perfect white. Whether it may be compounded of a mixture of three taken
at equal distances in the circumference I do not know, but of four or
five I do not much question but it may. But these are Curiosities of
little or no moment to the understanding the Phaenomena of Nature. For in
all whites produced by Nature, there uses to be a mixture of all sorts
of Rays, and by consequence a composition of all Colours.
To give an instance of this Rule; suppose a Colour is compounded of
these homogeneal Colours, of violet one part, of indigo one part, of
blue two parts, of green three parts, of yellow five parts, of orange
six parts, and of red ten parts. Proportional to these parts describe
the Circles _x_, _v_, _t_, _s_, _r_, _q_, _p_, respectively, that is, so
that if the Circle _x_ be one, the Circle _v_ may be one, the Circle _t_
two, the Circle _s_ three, and the Circles _r_, _q_ and _p_, five, six
and ten. Then I find Z the common center of gravity of these Circles,
and through Z drawing the Line OY, the Point Y falls upon the
circumference between E and F, something nearer to E than to F, and
thence I conclude, that the Colour compounded of these Ingredients will
be an orange, verging a little more to red than to yellow. Also I find
that OZ is a little less than one half of OY, and thence I conclude,
that this orange hath a little less than half the fulness or intenseness
of an uncompounded orange; that is to say, that it is such an orange as
may be made by mixing an homogeneal orange with a good white in the
proportion of the Line OZ to the Line ZY, this Proportion being not of
the quantities of mixed orange and white Powders, but of the quantities
of the Lights reflected from them.
This Rule I conceive accurate enough for practice, though not
mathematically accurate; and the truth of it may be sufficiently proved
to Sense, by stopping any of the Colours at the Lens in the tenth
Experiment of this Book. For the rest of the Colours which are not
stopp'd, but pass on to the Focus of the Lens, will there compound
either accurately or very nearly such a Colour, as by this Rule ought to
result from their Mixture.
_PROP._ VII. THEOR. V.
_All the Colours in the Universe which are made by Light, and depend not
on the Power of Imagination, are either the Colours of homogeneal
Lights, or compounded of these, and that either accurately or very
nearly, according to the Rule of the foregoing Problem._
For it has been proved (in _Prop. 1. Part 2._) that the changes of
Colours made by Refractions do not arise from any new Modifications of
the Rays impress'd by those Refractions, and by the various Terminations
of Light and Shadow, as has been the constant and general Opinion of
Philosophers. It has also been proved that the several Colours of the
homogeneal Rays do constantly answer to their degrees of Refrangibility,
(_Prop._ 1. _Part_ 1. and _Prop._ 2. _Part_ 2.) and that their degrees
of Refrangibility cannot be changed by Refractions and Reflexions
(_Prop._ 2. _Part_ 1.) and by consequence that those their Colours are
likewise immutable. It has also been proved directly by refracting and
reflecting homogeneal Lights apart, that their Colours cannot be
changed, (_Prop._ 2. _Part_ 2.) It has been proved also, that when the
several sorts of Rays are mixed, and in crossing pass through the same
space, they do not act on one another so as to change each others
colorific qualities. (_Exper._ 10. _Part_ 2.) but by mixing their
Actions in the Sensorium beget a Sensation differing from what either
would do apart, that is a Sensation of a mean Colour between their
proper Colours; and particularly when by the concourse and mixtures of
all sorts of Rays, a white Colour is produced, the white is a mixture of
all the Colours which the Rays would have apart, (_Prop._ 5. _Part_ 2.)
The Rays in that mixture do not lose or alter their several colorific
qualities, but by all their various kinds of Actions mix'd in the
Sensorium, beget a Sensation of a middling Colour between all their
Colours, which is whiteness. For whiteness is a mean between all
Colours, having it self indifferently to them all, so as with equal
facility to be tinged with any of them. A red Powder mixed with a little
blue, or a blue with a little red, doth not presently lose its Colour,
but a white Powder mix'd with any Colour is presently tinged with that
Colour, and is equally capable of being tinged with any Colour whatever.
It has been shewed also, that as the Sun's Light is mix'd of all sorts
of Rays, so its whiteness is a mixture of the Colours of all sorts of
Rays; those Rays having from the beginning their several colorific
qualities as well as their several Refrangibilities, and retaining them
perpetually unchanged notwithstanding any Refractions or Reflexions they
may at any time suffer, and that whenever any sort of the Sun's Rays is
by any means (as by Reflexion in _Exper._ 9, and 10. _Part_ 1. or by
Refraction as happens in all Refractions) separated from the rest, they
then manifest their proper Colours. These things have been prov'd, and
the sum of all this amounts to the Proposition here to be proved. For if
the Sun's Light is mix'd of several sorts of Rays, each of which have
originally their several Refrangibilities and colorific Qualities, and
notwithstanding their Refractions and Reflexions, and their various
Separations or Mixtures, keep those their original Properties
perpetually the same without alteration; then all the Colours in the
World must be such as constantly ought to arise from the original
colorific qualities of the Rays whereof the Lights consist by which
those Colours are seen. And therefore if the reason of any Colour
whatever be required, we have nothing else to do than to consider how
the Rays in the Sun's Light have by Reflexions or Refractions, or other
causes, been parted from one another, or mixed together; or otherwise to
find out what sorts of Rays are in the Light by which that Colour is
made, and in what Proportion; and then by the last Problem to learn the
Colour which ought to arise by mixing those Rays (or their Colours) in
that proportion. I speak here of Colours so far as they arise from
Light. For they appear sometimes by other Causes, as when by the power
of Phantasy we see Colours in a Dream, or a Mad-man sees things before
him which are not there; or when we see Fire by striking the Eye, or see
Colours like the Eye of a Peacock's Feather, by pressing our Eyes in
either corner whilst we look the other way. Where these and such like
Causes interpose not, the Colour always answers to the sort or sorts of
the Rays whereof the Light consists, as I have constantly found in
whatever Phaenomena of Colours I have hitherto been able to examine. I
shall in the following Propositions give instances of this in the
Phaenomena of chiefest note.
_PROP._ VIII. PROB. III.
_By the discovered Properties of Light to explain the Colours made by
Prisms._
Let ABC [in _Fig._ 12.] represent a Prism refracting the Light of the
Sun, which comes into a dark Chamber through a hole F[Greek: ph] almost
as broad as the Prism, and let MN represent a white Paper on which the
refracted Light is cast, and suppose the most refrangible or deepest
violet-making Rays fall upon the Space P[Greek: p], the least
refrangible or deepest red-making Rays upon the Space T[Greek: t], the
middle sort between the indigo-making and blue-making Rays upon the
Space Q[Greek: ch], the middle sort of the green-making Rays upon the
Space R, the middle sort between the yellow-making and orange-making
Rays upon the Space S[Greek: s], and other intermediate sorts upon
intermediate Spaces. For so the Spaces upon which the several sorts
adequately fall will by reason of the different Refrangibility of those
sorts be one lower than another. Now if the Paper MN be so near the
Prism that the Spaces PT and [Greek: pt] do not interfere with one
another, the distance between them T[Greek: p] will be illuminated by
all the sorts of Rays in that proportion to one another which they have
at their very first coming out of the Prism, and consequently be white.
But the Spaces PT and [Greek: pt] on either hand, will not be
illuminated by them all, and therefore will appear coloured. And
particularly at P, where the outmost violet-making Rays fall alone, the
Colour must be the deepest violet. At Q where the violet-making and
indigo-making Rays are mixed, it must be a violet inclining much to
indigo. At R where the violet-making, indigo-making, blue-making, and
one half of the green-making Rays are mixed, their Colours must (by the
construction of the second Problem) compound a middle Colour between
indigo and blue. At S where all the Rays are mixed, except the
red-making and orange-making, their Colours ought by the same Rule to
compound a faint blue, verging more to green than indigo. And in the
progress from S to T, this blue will grow more and more faint and
dilute, till at T, where all the Colours begin to be mixed, it ends in
whiteness.
[Illustration: FIG. 12.]
So again, on the other side of the white at [Greek: t], where the least
refrangible or utmost red-making Rays are alone, the Colour must be the
deepest red. At [Greek: s] the mixture of red and orange will compound a
red inclining to orange. At [Greek: r] the mixture of red, orange,
yellow, and one half of the green must compound a middle Colour between
orange and yellow. At [Greek: ch] the mixture of all Colours but violet
and indigo will compound a faint yellow, verging more to green than to
orange. And this yellow will grow more faint and dilute continually in
its progress from [Greek: ch] to [Greek: p], where by a mixture of all
sorts of Rays it will become white.
These Colours ought to appear were the Sun's Light perfectly white: But
because it inclines to yellow, the Excess of the yellow-making Rays
whereby 'tis tinged with that Colour, being mixed with the faint blue
between S and T, will draw it to a faint green. And so the Colours in
order from P to [Greek: t] ought to be violet, indigo, blue, very faint
green, white, faint yellow, orange, red. Thus it is by the computation:
And they that please to view the Colours made by a Prism will find it so
in Nature.
These are the Colours on both sides the white when the Paper is held
between the Prism and the Point X where the Colours meet, and the
interjacent white vanishes. For if the Paper be held still farther off
from the Prism, the most refrangible and least refrangible Rays will be
wanting in the middle of the Light, and the rest of the Rays which are
found there, will by mixture produce a fuller green than before. Also
the yellow and blue will now become less compounded, and by consequence
more intense than before. And this also agrees with experience.
And if one look through a Prism upon a white Object encompassed with
blackness or darkness, the reason of the Colours arising on the edges is
much the same, as will appear to one that shall a little consider it. If
a black Object be encompassed with a white one, the Colours which appear
through the Prism are to be derived from the Light of the white one,
spreading into the Regions of the black, and therefore they appear in a
contrary order to that, when a white Object is surrounded with black.
And the same is to be understood when an Object is viewed, whose parts
are some of them less luminous than others. For in the borders of the
more and less luminous Parts, Colours ought always by the same
Principles to arise from the Excess of the Light of the more luminous,
and to be of the same kind as if the darker parts were black, but yet to
be more faint and dilute.
What is said of Colours made by Prisms may be easily applied to Colours
made by the Glasses of Telescopes or Microscopes, or by the Humours of
the Eye. For if the Object-glass of a Telescope be thicker on one side
than on the other, or if one half of the Glass, or one half of the Pupil
of the Eye be cover'd with any opake substance; the Object-glass, or
that part of it or of the Eye which is not cover'd, may be consider'd as
a Wedge with crooked Sides, and every Wedge of Glass or other pellucid
Substance has the effect of a Prism in refracting the Light which passes
through it.[L]
How the Colours in the ninth and tenth Experiments of the first Part
arise from the different Reflexibility of Light, is evident by what was
there said. But it is observable in the ninth Experiment, that whilst
the Sun's direct Light is yellow, the Excess of the blue-making Rays in
the reflected beam of Light MN, suffices only to bring that yellow to a
pale white inclining to blue, and not to tinge it with a manifestly blue
Colour. To obtain therefore a better blue, I used instead of the yellow
Light of the Sun the white Light of the Clouds, by varying a little the
Experiment, as follows.
[Illustration: FIG. 13.]
_Exper._ 16 Let HFG [in _Fig._ 13.] represent a Prism in the open Air,
and S the Eye of the Spectator, viewing the Clouds by their Light coming
into the Prism at the Plane Side FIGK, and reflected in it by its Base
HEIG, and thence going out through its Plane Side HEFK to the Eye. And
when the Prism and Eye are conveniently placed, so that the Angles of
Incidence and Reflexion at the Base may be about 40 Degrees, the
Spectator will see a Bow MN of a blue Colour, running from one End of
the Base to the other, with the Concave Side towards him, and the Part
of the Base IMNG beyond this Bow will be brighter than the other Part
EMNH on the other Side of it. This blue Colour MN being made by nothing
else than by Reflexion of a specular Superficies, seems so odd a
Phaenomenon, and so difficult to be explained by the vulgar Hypothesis of
Philosophers, that I could not but think it deserved to be taken Notice
of. Now for understanding the Reason of it, suppose the Plane ABC to cut
the Plane Sides and Base of the Prism perpendicularly. From the Eye to
the Line BC, wherein that Plane cuts the Base, draw the Lines S_p_ and
S_t_, in the Angles S_pc_ 50 degr. 1/9, and S_tc_ 49 degr. 1/28, and the
Point _p_ will be the Limit beyond which none of the most refrangible
Rays can pass through the Base of the Prism, and be refracted, whose
Incidence is such that they may be reflected to the Eye; and the Point
_t_ will be the like Limit for the least refrangible Rays, that is,
beyond which none of them can pass through the Base, whose Incidence is
such that by Reflexion they may come to the Eye. And the Point _r_ taken
in the middle Way between _p_ and _t_, will be the like Limit for the
meanly refrangible Rays. And therefore all the least refrangible Rays
which fall upon the Base beyond _t_, that is, between _t_ and B, and can
come from thence to the Eye, will be reflected thither: But on this side
_t_, that is, between _t_ and _c_, many of these Rays will be
transmitted through the Base. And all the most refrangible Rays which
fall upon the Base beyond _p_, that is, between, _p_ and B, and can by
Reflexion come from thence to the Eye, will be reflected thither, but
every where between _p_ and _c_, many of these Rays will get through the
Base, and be refracted; and the same is to be understood of the meanly
refrangible Rays on either side of the Point _r_. Whence it follows,
that the Base of the Prism must every where between _t_ and B, by a
total Reflexion of all sorts of Rays to the Eye, look white and bright.
And every where between _p_ and C, by reason of the Transmission of many
Rays of every sort, look more pale, obscure, and dark. But at _r_, and
in other Places between _p_ and _t_, where all the more refrangible Rays
are reflected to the Eye, and many of the less refrangible are
transmitted, the Excess of the most refrangible in the reflected Light
will tinge that Light with their Colour, which is violet and blue. And
this happens by taking the Line C _prt_ B any where between the Ends of
the Prism HG and EI.
_PROP._ IX. PROB. IV.
_By the discovered Properties of Light to explain the Colours of the
Rain-bow._
[Illustration: FIG. 14.]
This Bow never appears, but where it rains in the Sun-shine, and may be
made artificially by spouting up Water which may break aloft, and
scatter into Drops, and fall down like Rain. For the Sun shining upon
these Drops certainly causes the Bow to appear to a Spectator standing
in a due Position to the Rain and Sun. And hence it is now agreed upon,
that this Bow is made by Refraction of the Sun's Light in drops of
falling Rain. This was understood by some of the Antients, and of late
more fully discover'd and explain'd by the famous _Antonius de Dominis_
Archbishop of _Spalato_, in his book _De Radiis Visus & Lucis_,
published by his Friend _Bartolus_ at _Venice_, in the Year 1611, and
written above 20 Years before. For he teaches there how the interior Bow
is made in round Drops of Rain by two Refractions of the Sun's Light,
and one Reflexion between them, and the exterior by two Refractions, and
two sorts of Reflexions between them in each Drop of Water, and proves
his Explications by Experiments made with a Phial full of Water, and
with Globes of Glass filled with Water, and placed in the Sun to make
the Colours of the two Bows appear in them. The same Explication
_Des-Cartes_ hath pursued in his Meteors, and mended that of the
exterior Bow. But whilst they understood not the true Origin of Colours,
it's necessary to pursue it here a little farther. For understanding
therefore how the Bow is made, let a Drop of Rain, or any other
spherical transparent Body be represented by the Sphere BNFG, [in _Fig._
14.] described with the Center C, and Semi-diameter CN. And let AN be
one of the Sun's Rays incident upon it at N, and thence refracted to F,
where let it either go out of the Sphere by Refraction towards V, or be
reflected to G; and at G let it either go out by Refraction to R, or be
reflected to H; and at H let it go out by Refraction towards S, cutting
the incident Ray in Y. Produce AN and RG, till they meet in X, and upon
AX and NF, let fall the Perpendiculars CD and CE, and produce CD till it
fall upon the Circumference at L. Parallel to the incident Ray AN draw
the Diameter BQ, and let the Sine of Incidence out of Air into Water be
to the Sine of Refraction as I to R. Now, if you suppose the Point of
Incidence N to move from the Point B, continually till it come to L, the
Arch QF will first increase and then decrease, and so will the Angle AXR
which the Rays AN and GR contain; and the Arch QF and Angle AXR will be
biggest when ND is to CN as sqrt(II - RR) to sqrt(3)RR, in which
case NE will be to ND as 2R to I. Also the Angle AYS, which the Rays AN
and HS contain will first decrease, and then increase and grow least
when ND is to CN as sqrt(II - RR) to sqrt(8)RR, in which case NE
will be to ND, as 3R to I. And so the Angle which the next emergent Ray
(that is, the emergent Ray after three Reflexions) contains with the
incident Ray AN will come to its Limit when ND is to CN as sqrt(II -
RR) to sqrt(15)RR, in which case NE will be to ND as 4R to I. And the
Angle which the Ray next after that Emergent, that is, the Ray emergent
after four Reflexions, contains with the Incident, will come to its
Limit, when ND is to CN as sqrt(II - RR) to sqrt(24)RR, in which
case NE will be to ND as 5R to I; and so on infinitely, the Numbers 3,
8, 15, 24, &c. being gather'd by continual Addition of the Terms of the
arithmetical Progression 3, 5, 7, 9, &c. The Truth of all this
Mathematicians will easily examine.[M]
Now it is to be observed, that as when the Sun comes to his Tropicks,
Days increase and decrease but a very little for a great while together;
so when by increasing the distance CD, these Angles come to their
Limits, they vary their quantity but very little for some time together,
and therefore a far greater number of the Rays which fall upon all the
Points N in the Quadrant BL, shall emerge in the Limits of these Angles,
than in any other Inclinations. And farther it is to be observed, that
the Rays which differ in Refrangibility will have different Limits of
their Angles of Emergence, and by consequence according to their
different Degrees of Refrangibility emerge most copiously in different
Angles, and being separated from one another appear each in their proper
Colours. And what those Angles are may be easily gather'd from the
foregoing Theorem by Computation.
For in the least refrangible Rays the Sines I and R (as was found above)
are 108 and 81, and thence by Computation the greatest Angle AXR will be
found 42 Degrees and 2 Minutes, and the least Angle AYS, 50 Degrees and
57 Minutes. And in the most refrangible Rays the Sines I and R are 109
and 81, and thence by Computation the greatest Angle AXR will be found
40 Degrees and 17 Minutes, and the least Angle AYS 54 Degrees and 7
Minutes.
Suppose now that O [in _Fig._ 15.] is the Spectator's Eye, and OP a Line
drawn parallel to the Sun's Rays and let POE, POF, POG, POH, be Angles
of 40 Degr. 17 Min. 42 Degr. 2 Min. 50 Degr. 57 Min. and 54 Degr. 7 Min.
respectively, and these Angles turned about their common Side OP, shall
with their other Sides OE, OF; OG, OH, describe the Verges of two
Rain-bows AF, BE and CHDG. For if E, F, G, H, be drops placed any where
in the conical Superficies described by OE, OF, OG, OH, and be
illuminated by the Sun's Rays SE, SF, SG, SH; the Angle SEO being equal
to the Angle POE, or 40 Degr. 17 Min. shall be the greatest Angle in
which the most refrangible Rays can after one Reflexion be refracted to
the Eye, and therefore all the Drops in the Line OE shall send the most
refrangible Rays most copiously to the Eye, and thereby strike the
Senses with the deepest violet Colour in that Region. And in like
manner the Angle SFO being equal to the Angle POF, or 42 Degr. 2 Min.
shall be the greatest in which the least refrangible Rays after one
Reflexion can emerge out of the Drops, and therefore those Rays shall
come most copiously to the Eye from the Drops in the Line OF, and strike
the Senses with the deepest red Colour in that Region. And by the same
Argument, the Rays which have intermediate Degrees of Refrangibility
shall come most copiously from Drops between E and F, and strike the
Senses with the intermediate Colours, in the Order which their Degrees
of Refrangibility require, that is in the Progress from E to F, or from
the inside of the Bow to the outside in this order, violet, indigo,
blue, green, yellow, orange, red. But the violet, by the mixture of the
white Light of the Clouds, will appear faint and incline to purple.
[Illustration: FIG. 15.]
Again, the Angle SGO being equal to the Angle POG, or 50 Gr. 51 Min.
shall be the least Angle in which the least refrangible Rays can after
two Reflexions emerge out of the Drops, and therefore the least
refrangible Rays shall come most copiously to the Eye from the Drops in
the Line OG, and strike the Sense with the deepest red in that Region.
And the Angle SHO being equal to the Angle POH, or 54 Gr. 7 Min. shall
be the least Angle, in which the most refrangible Rays after two
Reflexions can emerge out of the Drops; and therefore those Rays shall
come most copiously to the Eye from the Drops in the Line OH, and strike
the Senses with the deepest violet in that Region. And by the same
Argument, the Drops in the Regions between G and H shall strike the
Sense with the intermediate Colours in the Order which their Degrees of
Refrangibility require, that is, in the Progress from G to H, or from
the inside of the Bow to the outside in this order, red, orange, yellow,
green, blue, indigo, violet. And since these four Lines OE, OF, OG, OH,
may be situated any where in the above-mention'd conical Superficies;
what is said of the Drops and Colours in these Lines is to be understood
of the Drops and Colours every where in those Superficies.
Thus shall there be made two Bows of Colours, an interior and stronger,
by one Reflexion in the Drops, and an exterior and fainter by two; for
the Light becomes fainter by every Reflexion. And their Colours shall
lie in a contrary Order to one another, the red of both Bows bordering
upon the Space GF, which is between the Bows. The Breadth of the
interior Bow EOF measured cross the Colours shall be 1 Degr. 45 Min. and
the Breadth of the exterior GOH shall be 3 Degr. 10 Min. and the
distance between them GOF shall be 8 Gr. 15 Min. the greatest
Semi-diameter of the innermost, that is, the Angle POF being 42 Gr. 2
Min. and the least Semi-diameter of the outermost POG, being 50 Gr. 57
Min. These are the Measures of the Bows, as they would be were the Sun
but a Point; for by the Breadth of his Body, the Breadth of the Bows
will be increased, and their Distance decreased by half a Degree, and so
the breadth of the interior Iris will be 2 Degr. 15 Min. that of the
exterior 3 Degr. 40 Min. their distance 8 Degr. 25 Min. the greatest
Semi-diameter of the interior Bow 42 Degr. 17 Min. and the least of the
exterior 50 Degr. 42 Min. And such are the Dimensions of the Bows in the
Heavens found to be very nearly, when their Colours appear strong and
perfect. For once, by such means as I then had, I measured the greatest
Semi-diameter of the interior Iris about 42 Degrees, and the breadth of
the red, yellow and green in that Iris 63 or 64 Minutes, besides the
outmost faint red obscured by the brightness of the Clouds, for which we
may allow 3 or 4 Minutes more. The breadth of the blue was about 40
Minutes more besides the violet, which was so much obscured by the
brightness of the Clouds, that I could not measure its breadth. But
supposing the breadth of the blue and violet together to equal that of
the red, yellow and green together, the whole breadth of this Iris will
be about 2-1/4 Degrees, as above. The least distance between this Iris
and the exterior Iris was about 8 Degrees and 30 Minutes. The exterior
Iris was broader than the interior, but so faint, especially on the blue
side, that I could not measure its breadth distinctly. At another time
when both Bows appeared more distinct, I measured the breadth of the
interior Iris 2 Gr. 10', and the breadth of the red, yellow and green in
the exterior Iris, was to the breadth of the same Colours in the
interior as 3 to 2.
This Explication of the Rain-bow is yet farther confirmed by the known
Experiment (made by _Antonius de Dominis_ and _Des-Cartes_) of hanging
up any where in the Sun-shine a Glass Globe filled with Water, and
viewing it in such a posture, that the Rays which come from the Globe to
the Eye may contain with the Sun's Rays an Angle of either 42 or 50
Degrees. For if the Angle be about 42 or 43 Degrees, the Spectator
(suppose at O) shall see a full red Colour in that side of the Globe
opposed to the Sun as 'tis represented at F, and if that Angle become
less (suppose by depressing the Globe to E) there will appear other
Colours, yellow, green and blue successive in the same side of the
Globe. But if the Angle be made about 50 Degrees (suppose by lifting up
the Globe to G) there will appear a red Colour in that side of the Globe
towards the Sun, and if the Angle be made greater (suppose by lifting
up the Globe to H) the red will turn successively to the other Colours,
yellow, green and blue. The same thing I have tried, by letting a Globe
rest, and raising or depressing the Eye, or otherwise moving it to make
the Angle of a just magnitude.
I have heard it represented, that if the Light of a Candle be refracted
by a Prism to the Eye; when the blue Colour falls upon the Eye, the
Spectator shall see red in the Prism, and when the red falls upon the
Eye he shall see blue; and if this were certain, the Colours of the
Globe and Rain-bow ought to appear in a contrary order to what we find.
But the Colours of the Candle being very faint, the mistake seems to
arise from the difficulty of discerning what Colours fall on the Eye.
For, on the contrary, I have sometimes had occasion to observe in the
Sun's Light refracted by a Prism, that the Spectator always sees that
Colour in the Prism which falls upon his Eye. And the same I have found
true also in Candle-light. For when the Prism is moved slowly from the
Line which is drawn directly from the Candle to the Eye, the red appears
first in the Prism and then the blue, and therefore each of them is seen
when it falls upon the Eye. For the red passes over the Eye first, and
then the blue.
The Light which comes through drops of Rain by two Refractions without
any Reflexion, ought to appear strongest at the distance of about 26
Degrees from the Sun, and to decay gradually both ways as the distance
from him increases and decreases. And the same is to be understood of
Light transmitted through spherical Hail-stones. And if the Hail be a
little flatted, as it often is, the Light transmitted may grow so strong
at a little less distance than that of 26 Degrees, as to form a Halo
about the Sun or Moon; which Halo, as often as the Hail-stones are duly
figured may be colour'd, and then it must be red within by the least
refrangible Rays, and blue without by the most refrangible ones,
especially if the Hail-stones have opake Globules of Snow in their
center to intercept the Light within the Halo (as _Hugenius_ has
observ'd) and make the inside thereof more distinctly defined than it
would otherwise be. For such Hail-stones, though spherical, by
terminating the Light by the Snow, may make a Halo red within and
colourless without, and darker in the red than without, as Halos used to
be. For of those Rays which pass close by the Snow the Rubriform will be
least refracted, and so come to the Eye in the directest Lines.
The Light which passes through a drop of Rain after two Refractions, and
three or more Reflexions, is scarce strong enough to cause a sensible
Bow; but in those Cylinders of Ice by which _Hugenius_ explains the
_Parhelia_, it may perhaps be sensible.
_PROP._ X. PROB. V.
_By the discovered Properties of Light to explain the permanent Colours
of Natural Bodies._
These Colours arise from hence, that some natural Bodies reflect some
sorts of Rays, others other sorts more copiously than the rest. Minium
reflects the least refrangible or red-making Rays most copiously, and
thence appears red. Violets reflect the most refrangible most copiously,
and thence have their Colour, and so of other Bodies. Every Body
reflects the Rays of its own Colour more copiously than the rest, and
from their excess and predominance in the reflected Light has its
Colour.
_Exper._ 17. For if in the homogeneal Lights obtained by the solution of
the Problem proposed in the fourth Proposition of the first Part of this
Book, you place Bodies of several Colours, you will find, as I have
done, that every Body looks most splendid and luminous in the Light of
its own Colour. Cinnaber in the homogeneal red Light is most
resplendent, in the green Light it is manifestly less resplendent, and
in the blue Light still less. Indigo in the violet blue Light is most
resplendent, and its splendor is gradually diminish'd, as it is removed
thence by degrees through the green and yellow Light to the red. By a
Leek the green Light, and next that the blue and yellow which compound
green, are more strongly reflected than the other Colours red and
violet, and so of the rest. But to make these Experiments the more
manifest, such Bodies ought to be chosen as have the fullest and most
vivid Colours, and two of those Bodies are to be compared together.
Thus, for instance, if Cinnaber and _ultra_-marine blue, or some other
full blue be held together in the red homogeneal Light, they will both
appear red, but the Cinnaber will appear of a strongly luminous and
resplendent red, and the _ultra_-marine blue of a faint obscure and dark
red; and if they be held together in the blue homogeneal Light, they
will both appear blue, but the _ultra_-marine will appear of a strongly
luminous and resplendent blue, and the Cinnaber of a faint and dark
blue. Which puts it out of dispute that the Cinnaber reflects the red
Light much more copiously than the _ultra_-marine doth, and the
_ultra_-marine reflects the blue Light much more copiously than the
Cinnaber doth. The same Experiment may be tried successfully with red
Lead and Indigo, or with any other two colour'd Bodies, if due allowance
be made for the different strength or weakness of their Colour and
Light.
And as the reason of the Colours of natural Bodies is evident by these
Experiments, so it is farther confirmed and put past dispute by the two
first Experiments of the first Part, whereby 'twas proved in such Bodies
that the reflected Lights which differ in Colours do differ also in
degrees of Refrangibility. For thence it's certain, that some Bodies
reflect the more refrangible, others the less refrangible Rays more
copiously.
And that this is not only a true reason of these Colours, but even the
only reason, may appear farther from this Consideration, that the Colour
of homogeneal Light cannot be changed by the Reflexion of natural
Bodies.
For if Bodies by Reflexion cannot in the least change the Colour of any
one sort of Rays, they cannot appear colour'd by any other means than by
reflecting those which either are of their own Colour, or which by
mixture must produce it.
But in trying Experiments of this kind care must be had that the Light
be sufficiently homogeneal. For if Bodies be illuminated by the ordinary
prismatick Colours, they will appear neither of their own Day-light
Colours, nor of the Colour of the Light cast on them, but of some middle
Colour between both, as I have found by Experience. Thus red Lead (for
instance) illuminated with the ordinary prismatick green will not appear
either red or green, but orange or yellow, or between yellow and green,
accordingly as the green Light by which 'tis illuminated is more or less
compounded. For because red Lead appears red when illuminated with white
Light, wherein all sorts of Rays are equally mix'd, and in the green
Light all sorts of Rays are not equally mix'd, the Excess of the
yellow-making, green-making and blue-making Rays in the incident green
Light, will cause those Rays to abound so much in the reflected Light,
as to draw the Colour from red towards their Colour. And because the red
Lead reflects the red-making Rays most copiously in proportion to their
number, and next after them the orange-making and yellow-making Rays;
these Rays in the reflected Light will be more in proportion to the
Light than they were in the incident green Light, and thereby will draw
the reflected Light from green towards their Colour. And therefore the
red Lead will appear neither red nor green, but of a Colour between
both.
In transparently colour'd Liquors 'tis observable, that their Colour
uses to vary with their thickness. Thus, for instance, a red Liquor in a
conical Glass held between the Light and the Eye, looks of a pale and
dilute yellow at the bottom where 'tis thin, and a little higher where
'tis thicker grows orange, and where 'tis still thicker becomes red, and
where 'tis thickest the red is deepest and darkest. For it is to be
conceiv'd that such a Liquor stops the indigo-making and violet-making
Rays most easily, the blue-making Rays more difficultly, the
green-making Rays still more difficultly, and the red-making most
difficultly: And that if the thickness of the Liquor be only so much as
suffices to stop a competent number of the violet-making and
indigo-making Rays, without diminishing much the number of the rest, the
rest must (by _Prop._ 6. _Part_ 2.) compound a pale yellow. But if the
Liquor be so much thicker as to stop also a great number of the
blue-making Rays, and some of the green-making, the rest must compound
an orange; and where it is so thick as to stop also a great number of
the green-making and a considerable number of the yellow-making, the
rest must begin to compound a red, and this red must grow deeper and
darker as the yellow-making and orange-making Rays are more and more
stopp'd by increasing the thickness of the Liquor, so that few Rays
besides the red-making can get through.
Of this kind is an Experiment lately related to me by Mr. _Halley_, who,
in diving deep into the Sea in a diving Vessel, found in a clear
Sun-shine Day, that when he was sunk many Fathoms deep into the Water
the upper part of his Hand on which the Sun shone directly through the
Water and through a small Glass Window in the Vessel appeared of a red
Colour, like that of a Damask Rose, and the Water below and the under
part of his Hand illuminated by Light reflected from the Water below
look'd green. For thence it may be gather'd, that the Sea-Water reflects
back the violet and blue-making Rays most easily, and lets the
red-making Rays pass most freely and copiously to great Depths. For
thereby the Sun's direct Light at all great Depths, by reason of the
predominating red-making Rays, must appear red; and the greater the
Depth is, the fuller and intenser must that red be. And at such Depths
as the violet-making Rays scarce penetrate unto, the blue-making,
green-making, and yellow-making Rays being reflected from below more
copiously than the red-making ones, must compound a green.
Now, if there be two Liquors of full Colours, suppose a red and blue,
and both of them so thick as suffices to make their Colours sufficiently
full; though either Liquor be sufficiently transparent apart, yet will
you not be able to see through both together. For, if only the
red-making Rays pass through one Liquor, and only the blue-making
through the other, no Rays can pass through both. This Mr. _Hook_ tried
casually with Glass Wedges filled with red and blue Liquors, and was
surprized at the unexpected Event, the reason of it being then unknown;
which makes me trust the more to his Experiment, though I have not tried
it my self. But he that would repeat it, must take care the Liquors be
of very good and full Colours.
Now, whilst Bodies become coloured by reflecting or transmitting this or
that sort of Rays more copiously than the rest, it is to be conceived
that they stop and stifle in themselves the Rays which they do not
reflect or transmit. For, if Gold be foliated and held between your Eye
and the Light, the Light looks of a greenish blue, and therefore massy
Gold lets into its Body the blue-making Rays to be reflected to and fro
within it till they be stopp'd and stifled, whilst it reflects the
yellow-making outwards, and thereby looks yellow. And much after the
same manner that Leaf Gold is yellow by reflected, and blue by
transmitted Light, and massy Gold is yellow in all Positions of the Eye;
there are some Liquors, as the Tincture of _Lignum Nephriticum_, and
some sorts of Glass which transmit one sort of Light most copiously, and
reflect another sort, and thereby look of several Colours, according to
the Position of the Eye to the Light. But, if these Liquors or Glasses
were so thick and massy that no Light could get through them, I question
not but they would like all other opake Bodies appear of one and the
same Colour in all Positions of the Eye, though this I cannot yet affirm
by Experience. For all colour'd Bodies, so far as my Observation
reaches, may be seen through if made sufficiently thin, and therefore
are in some measure transparent, and differ only in degrees of
Transparency from tinged transparent Liquors; these Liquors, as well as
those Bodies, by a sufficient Thickness becoming opake. A transparent
Body which looks of any Colour by transmitted Light, may also look of
the same Colour by reflected Light, the Light of that Colour being
reflected by the farther Surface of the Body, or by the Air beyond it.
And then the reflected Colour will be diminished, and perhaps cease, by
making the Body very thick, and pitching it on the backside to diminish
the Reflexion of its farther Surface, so that the Light reflected from
the tinging Particles may predominate. In such Cases, the Colour of the
reflected Light will be apt to vary from that of the Light transmitted.
But whence it is that tinged Bodies and Liquors reflect some sort of
Rays, and intromit or transmit other sorts, shall be said in the next
Book. In this Proposition I content my self to have put it past dispute,
that Bodies have such Properties, and thence appear colour'd.
_PROP._ XI. PROB. VI.
_By mixing colour'd Lights to compound a beam of Light of the same
Colour and Nature with a beam of the Sun's direct Light, and therein to
experience the Truth of the foregoing Propositions._
[Illustration: FIG. 16.]
Let ABC _abc_ [in _Fig._ 16.] represent a Prism, by which the Sun's
Light let into a dark Chamber through the Hole F, may be refracted
towards the Lens MN, and paint upon it at _p_, _q_, _r_, _s_, and _t_,
the usual Colours violet, blue, green, yellow, and red, and let the
diverging Rays by the Refraction of this Lens converge again towards X,
and there, by the mixture of all those their Colours, compound a white
according to what was shewn above. Then let another Prism DEG _deg_,
parallel to the former, be placed at X, to refract that white Light
upwards towards Y. Let the refracting Angles of the Prisms, and their
distances from the Lens be equal, so that the Rays which converged from
the Lens towards X, and without Refraction, would there have crossed and
diverged again, may by the Refraction of the second Prism be reduced
into Parallelism and diverge no more. For then those Rays will recompose
a beam of white Light XY. If the refracting Angle of either Prism be the
bigger, that Prism must be so much the nearer to the Lens. You will know
when the Prisms and the Lens are well set together, by observing if the
beam of Light XY, which comes out of the second Prism be perfectly white
to the very edges of the Light, and at all distances from the Prism
continue perfectly and totally white like a beam of the Sun's Light. For
till this happens, the Position of the Prisms and Lens to one another
must be corrected; and then if by the help of a long beam of Wood, as is
represented in the Figure, or by a Tube, or some other such Instrument,
made for that Purpose, they be made fast in that Situation, you may try
all the same Experiments in this compounded beam of Light XY, which have
been made in the Sun's direct Light. For this compounded beam of Light
has the same appearance, and is endow'd with all the same Properties
with a direct beam of the Sun's Light, so far as my Observation reaches.
And in trying Experiments in this beam you may by stopping any of the
Colours, _p_, _q_, _r_, _s_, and _t_, at the Lens, see how the Colours
produced in the Experiments are no other than those which the Rays had
at the Lens before they entered the Composition of this Beam: And by
consequence, that they arise not from any new Modifications of the Light
by Refractions and Reflexions, but from the various Separations and
Mixtures of the Rays originally endow'd with their colour-making
Qualities.
So, for instance, having with a Lens 4-1/4 Inches broad, and two Prisms
on either hand 6-1/4 Feet distant from the Lens, made such a beam of
compounded Light; to examine the reason of the Colours made by Prisms, I
refracted this compounded beam of Light XY with another Prism HIK _kh_,
and thereby cast the usual Prismatick Colours PQRST upon the Paper LV
placed behind. And then by stopping any of the Colours _p_, _q_, _r_,
_s_, _t_, at the Lens, I found that the same Colour would vanish at the
Paper. So if the Purple _p_ was stopp'd at the Lens, the Purple P upon
the Paper would vanish, and the rest of the Colours would remain
unalter'd, unless perhaps the blue, so far as some purple latent in it
at the Lens might be separated from it by the following Refractions. And
so by intercepting the green upon the Lens, the green R upon the Paper
would vanish, and so of the rest; which plainly shews, that as the white
beam of Light XY was compounded of several Lights variously colour'd at
the Lens, so the Colours which afterwards emerge out of it by new
Refractions are no other than those of which its Whiteness was
compounded. The Refraction of the Prism HIK _kh_ generates the Colours
PQRST upon the Paper, not by changing the colorific Qualities of the
Rays, but by separating the Rays which had the very same colorific
Qualities before they enter'd the Composition of the refracted beam of
white Light XY. For otherwise the Rays which were of one Colour at the
Lens might be of another upon the Paper, contrary to what we find.
So again, to examine the reason of the Colours of natural Bodies, I
placed such Bodies in the Beam of Light XY, and found that they all
appeared there of those their own Colours which they have in Day-light,
and that those Colours depend upon the Rays which had the same Colours
at the Lens before they enter'd the Composition of that beam. Thus, for
instance, Cinnaber illuminated by this beam appears of the same red
Colour as in Day-light; and if at the Lens you intercept the
green-making and blue-making Rays, its redness will become more full and
lively: But if you there intercept the red-making Rays, it will not any
longer appear red, but become yellow or green, or of some other Colour,
according to the sorts of Rays which you do not intercept. So Gold in
this Light XY appears of the same yellow Colour as in Day-light, but by
intercepting at the Lens a due Quantity of the yellow-making Rays it
will appear white like Silver (as I have tried) which shews that its
yellowness arises from the Excess of the intercepted Rays tinging that
Whiteness with their Colour when they are let pass. So the Infusion of
_Lignum Nephriticum_ (as I have also tried) when held in this beam of
Light XY, looks blue by the reflected Part of the Light, and red by the
transmitted Part of it, as when 'tis view'd in Day-light; but if you
intercept the blue at the Lens the Infusion will lose its reflected blue
Colour, whilst its transmitted red remains perfect, and by the loss of
some blue-making Rays, wherewith it was allay'd, becomes more intense
and full. And, on the contrary, if the red and orange-making Rays be
intercepted at the Lens, the Infusion will lose its transmitted red,
whilst its blue will remain and become more full and perfect. Which
shews, that the Infusion does not tinge the Rays with blue and red, but
only transmits those most copiously which were red-making before, and
reflects those most copiously which were blue-making before. And after
the same manner may the Reasons of other Phaenomena be examined, by
trying them in this artificial beam of Light XY.