6. That after very great Storms of Wind, when the _Quicksilver_ has been
low, it generally rises again very fast.
7. That the more Northerly places have greater Alterations of the
_Baroscope_, than the more Southerly.
8. That within the _Tropicks_ and near them, those Accounts I have had
from others, and my own Observation at St. _Helena_, make very little or
no Variation of the height of the _Mercury_ in all Weathers.
Now that Theory that can well account for all these appearances, will,
in all probability, approach nearer the true cause of the _Barometers_
Variations, than any thing hitherto afforded; and such an one I am bound
to believe, is that which I here lay down with submission to better
Judgments.
I conceive that the principal Cause of the rise and fall of the
_Mercury_, is from the variable Winds, which are found in the _Temperate
Zones_, and whose great unconstancy here in _England_ is most notorious.
I shall not at present inquire into the Cause of its uncertainty, but
the Matter of Fact being most undoubted, the Legitimate Consequences
thereof must be allow'd me, let it proceed from what it will.
A second Cause is the uncertain Exhalation and Præcipitation of the
Vapours lodging in the Air, whereby it comes to be at one time much more
crowded than at another, and consequently heavier; but this latter in a
great measure depends upon the former. Now from these Principles I shall
endeavour to explicate the several _Phænomena_ of the _Barometer_,
taking them in the same order I laid them down.
1. _Why in calm Weather the Air being inclin'd to Rain, the _Mercury_ is
commonly low?_ I Answer, That the _Mercury_'s being low, inclines it to
Rain; for the Air being light, the Vapours are no longer supported
thereby, being become specifically heavier than the Medium wherein they
floated; so that they descend towards the Earth, and in their fall
meeting with other aqueous Particles, they incorporate together, and
form little drops of Rain; but the _Mercury_'s being at one time lower
than another, is the effect of two contrary Winds blowing from the place
whence the _Barometer_ stands; whereby the Air of that place is carried
both ways from it, and consequently the incumbent Cylinder of Air is
diminished, and accordingly the _Mercury_ sinks; as for Instance, if in
the _German Ocean_ it should blow a Gale of Westerly Wind, and at the
same time an Easterly Wind in the _Irish Sea_; or if in _France_ it
should blow a Southerly Wind, and in _Scotland_ a Northern; it must be
granted me, that That part of the _Atmosphere_ impendent over _England_,
would thereby be exhausted and attenuated, and the _Mercury_ would
subside, and the Vapours which before floated in those parts of the
_Air_ of equal Gravity with themselves, would sink to the Earth.
2. _Why in serene good settled weather the _Mercury_ is generally high?_
To this I Answer, That the greater height of the _Barometer_, is
occasion'd by two contrary Winds blowing towards the place of
Observation, whereby the Air of other places is brought thither and
accumulated; so that the incumbent Cylinder of Air being encreas'd both
in height and weight, the _Mercury_ press'd thereby must needs rise and
stand high, as long as the Winds continue so to blow; and then the Air
being specifically heavier, the Vapours are better kept suspended, so
that they have no inclination to Præcipitate and fall down in Drops,
which is the reason of the serene good Weather, which attends the
greater heights of the _Mercury_.
3. _Why upon very great Winds or Storms, tho' accompanied with no Rain,
the _Mercury_ sinks lowest of all, with relation to the Point of the
Compass upon which the Wind blows?_ This is caus'd by the very rapid
Motion of the Air in these Storms; for the Tract or Region of the Earths
Surface, wherein these Winds rage, not extending all round the Globe,
that stagnant Air which is left behind, as likewise that on the sides,
cannot come in so fast as to supply the Evacuation made by so swift a
Current; so that the Air must necessarily be attenuated, when and where
the said Winds continue to blow, and that more or less, according to
their Violence; add to which, that the _Horizontal_ Motion of the Air
being so quick as it is, may in all probability take off some part of
the perpendicular pressure thereof; and the great Agitation of its
Particles, is the Reason why the Vapours are dissipated, and do not
condense into Drops, so as to form Rain, otherwise the natural
Consequence of the Airs Rarifaction.
4. _Why _cæteris paribus_ the _Mercury_ stands highest upon an Easterly
or North-Easterly Wind?_ This happens because that in the great
_Atlantick Ocean_, on this side the thirty fifth Degree of North
Latitude, the Westerly and South-Westerly Trade-Winds blow almost
always; so that whenever here the Wind comes up at East and North-East,
'tis sure to be checked by a contrary Gale, as soon as it reaches the
Ocean; wherefore, according to what is made out in our second Remark,
the Air must needs be heaped over this Island; and consequently the
_Mercury_ must stand high, as often as these Winds blow. This holds true
in this Country, but is not a general Rule for others, where the Winds
are under different Circumstances; and I have sometimes seen the
_Mercury_ here as low as twenty nine Inches, upon an Easterly Wind, but
then it blows exceeding hard, and so comes to be accounted for by what
was observ'd upon the third Remark.
5. _Why in calm Weather the _Mercury_ generally stands high?_ The cause
hereof is, as I conceive, that it seldom freezes but when the Winds come
out of the Northern and North-Eastern Quarters, or at least unless those
Winds blow at no great distance off; for the Northern Parts of
_Germany_, _Denmark_, _Sweden_, _Norway_, and all that Tract from whence
North-Eastern Winds come, are subject to almost continual Frost all the
Winter; and thereby the lower Air is very much condens'd, and in that
State is brought hitherwards by these Winds, and being accumulated by
the opposition of the Westerly Wind blowing in the Ocean, the _Mercury_
must needs be prest to a more than ordinary height, and as a concurring
Cause, the shrinking of the lower parts of the Air into lesser room by
cold, must needs cause a descent of the upper parts of the Atmosphere,
to reduce the Cavity made by this contraction to an _Æquilibrium_.
6. _Why after very great Storms of Wind, when the _Mercury_ has been
very low, it generally rises again very fast?_ This I have frequently
observed, and once found it risen an Inch and a half in less than six
Hours, after a long continu'd Storm of South-West Wind. This seems to be
occasion'd by the sudden Accession of new Air to supply the great
Evacuation which such continu'd Storms make thereof, in those places
whence they happen (as in the third Remark) and by the Recoile of the
Air, after the force ceases that impelled it; and the Reason why the
_Mercury_ rises so fast, is because the Air being very much rarify'd
beyond its mean density, the Neighbouring Air runs in the more swiftly
to bring it to an _Æquilibration_, as we see Water runs the faster for
having a great declivity.
7. _Why in more Northerly places the Variations of the _Baroscope_ are
greater than in the Southerly?_ The truth of the Matter of Fact is
prov'd from Observations made at _Clermont_ and _Paris_, compar'd with
others, made at _Stockholm_, as may be seen in the Appendix to Mr.
_Pascal_'s Book before-cited. The Reason I conjecture to be, that the
more Northerly Parts have usually greater Storms of Wind than the more
Southerly, whereby the _Mercury_ should sink lower in that Extream; and
then the Northerly Winds bringing the condens'd and ponderous Air from
the Neighbourhood of the Pole, and that again being check'd by a
Southerly Wind, at no great distance, and so heaped, must of necessity
make the _Mercury_ in such case stand higher in the other Extream.
8. And Lastly, _Why near the _Æquinoctial_, as at _Barbadoes_ and St.
_Helena_, there is very little or no Variation of the height of the
Barometer?_ This Remark, above all others, confirms the Hypothesis of
the variable Winds, being the cause of these Variations of the height of
the _Mercury_; for in the Places above-named, there is always an easie
Gale of Wind blowing nearly upon the same Point, _viz._ E. N. E. at
_Barbadoes_, and E. S. E. at St. _Helena_; so that there being no
contrary Currents of the Air, to exhaust or accumulate it, the
Atmosphere continues much in the same State. However, upon Hurricanes,
the most violent of Storms, the _Mercury_ has been observ'd very low,
but this is but for once in two or three Years, and it soon recovers its
settled state of about 29½ Inches. I doubt not but the same thing is
in the East Coast of _Africa_, and in _India_, where the Monsoons or
Trade-Winds are for half the Year one way, and half the Year another;
only it's probable, that there may something worth noting happen, about
the times of the change or shifting of the Winds, which might be
obtain'd, if any Body had the Curiosity to keep the _Barometer_ at our
Factories in _India_.
I doubt not but this Doctrine will find some Opposers, and that one
principal Objection will be, that I suppose the Air sometimes to move
from those Parts where it is already evacuated below the _Æquilibrium_,
and sometimes again towards those parts, where it is condens'd and
crouded above the mean State, which may be thought contradictory to the
Laws of Staticks and the Rules of the _Æquilibrium_ of Fluids. But those
that shall consider how, when once an impetus is given to a Fluid Body,
it is capable of mounting above its Level, and checking others that have
a contrary tendency to descent by their own Gravity, will no longer
regard this as a material Obstacle, but will rather conclude, that the
great _Analogy_ there is between the rising and falling of the Water
upon the Flux and Reflux of the Sea, and this of the accumulating and
extenuating the Air, is a great Argument for the Truth of this
Hypothesis: For as the Sea over against the Coast of _Essex_, rises and
swells by the meeting of the two contrary Tides of Flood, (whereof the
one comes from the S. W. along the Channel of _England_, and the other
from the North); and on the contrary sinks below its Level upon the
retreat of the Water both ways in the Tide of Ebb; so it is very
probable that the Air may Ebb and Flow, after the same manner; but by
reason of the diversity of Causes, whereby the Air may be set in moving,
the times of these Fluxes and Refluxes thereof, are purely Casual, and
not reducible to any Rule, as are the Motions of the Sea, depending
wholly upon the regular Course of the Moon.
[Illustration: _Plate 1. pag. 97_]
_A Letter of Mr. _Isaac Newton_, Professor of the Mathematicks in the
University of _Cambridge_; containing his New Theory about _Light_
and _Colours_: Sent by the Author to the Publisher from _Cambridge_,
Feb. 6. 1671/2; in order to be communicated to the _Royal Society_._
_SIR_,
To perform my late promise to you, I shall without further Ceremony
acquaint you, That in the beginning of the Year 1666 (at which time I
apply'd my self to the grinding of Optick-glasses of other Figures than
_Spherical_,) I procur'd me a Triangular Glass-Prism, to try therewith
the celebrated _Phænomena_ of _Colours_. And in order thereto, having
darken'd my Chamber, and made a small hole in my Window-shuts, to let in
a convenient quantity of the Sun's Light, I plac'd my Prism at his
entrance, that it might be thereby refracted to the opposite Wall. It
was at first a very pleasing Divertisement, to view the vivid and
intense Colours produced thereby; but after a while applying my self to
consider them more circumspectly, I became surpriz'd to see them in an
_oblong_ Form; which, according to the received Laws of Rarefraction, I
expected should have been _Circular_.
They were terminated at the sides with streight Lines, but at the ends,
the decay of Light was so gradual, that it was difficult to determine
justly, what was their Figure; yet they seem'd _Semicircular_.
Comparing the length of this colour'd _Spectrum_ with its breadth, I
found it about five times greater; a disproportion so extravagant, that
it excited me to a more than ordinary Curiosity of examining, from
whence it might proceed. I could scarce think, that the various
thickness of the Glass, or the termination with shadow or darkness,
could have any Influence on Light to produce such an effect; yet I
thought it not amiss, first to examine those Circumstances, and so try'd
what would happen by transmitting Light through parts of the Glass of
divers thicknesses, or through holes in the Window of divers bignesses,
or by setting the Prism without, so that the Light might pass through
it, and be refracted before it was terminated by the hole: But I found
none of those Circumstances material. The fashion of the Colours was, in
all these Cases, the same.
Then I suspected, whether by any unevenness in the Glass, or other
contingent Irregularity, these Colours might be thus dilated. And to try
this, I took another Prism like the former, and so plac'd it, that the
Light passing through them both, might be refracted contrary ways, and
so by the latter return'd into that Course, from which the former had
diverted it. For, by this means, I thought the _regular_ effects of the
first Prism would be destroy'd by the second Prism, but the _irregular_
ones more augmented by the multiplicity of Refractions. The Event was,
that the Light, which by the first Prism was diffused into an _oblong_
Form, was, by the second, reduc'd into an _orbicular_ one, with as much
regularity, as when it did not at all pass through them. So that
whatever was the cause of that length, 'twas not any contingent
Irregularity.
I then proceeded to examine more critically, what might be effected by
the difference of the incidence of Rays coming from divers parts of the
Sun; and to that end, measur'd the several Lines and Angles belonging to
the Image. Its distance from the Hole or Prism was twenty two Foot; its
utmost length 13¼ Inches; its breadth 2⅝; the Diameter of the Hole
¼ of an Inch; the Angle, with the Rays, tending towards the middle of
the Image, made with those Lines, in which they would have proceeded
without Refraction, was 44° 56'. And the Vertical Angle of the Prism,
63° 12'. Also the Refractions on both sides the Prism, that is, of the
Incident, and Emergent Rays, were as near, as I could make them, equal,
and consequently about 54° 4'. And the Rays fell perpendicularly upon
the Wall. Now subducting the Diameter of the Hole from the length and
breadth of the Image, there remains 13 Inches the length, and 2⅜ the
breadth, comprehended by those Rays, which passed thro' the Center of
the said Hole, and consequently the Angle of the Hole, which that
breadth subtended, was about 31', answerable to the Sun's Diameter; but
the Angle, which its length subtended, was more than five such
Diameters, namely 2° 49'.
Having made these Observations, I first computed from them the
refractive Power of that Glass, and found it measur'd by the _ratio_ of
the Sines, twenty to thirty one. And then, by that _ratio_, I computed
the Refractions of two Rays flowing from opposite parts of the Sun's
_discus_, so as to differ 31' in their obliquity of Incidence, and found
that the emergent Rays should have comprehended an Angle of about 31',
as they did, before they were incident.
But because this Computation was founded on the Hypothesis of the
proportionality of the Sines of Incidence and Refraction, which, tho' by
my own Experience I could not imagine to be so erroneous as to make that
Angle but 31', which in reality was 2° 49'; yet my Curiosity caus'd me
again to take my Prism. And having plac'd it at my Window, as before, I
observ'd, that by turning it a little about its _Axis_ to and fro, so as
to vary its obliquity to the light, more than an Angle of four or five
Degrees, the Colours were not thereby sensibly translated from their
place on the Wall, and consequently by that Variation of Incidence, the
quantity of Refraction was not sensibly varied. By this Experiment
therefore, as well as by the former Computation, it was evident, that
the difference of the Incidence of Rays, flowing from divers parts of
the Sun could not make them, after decussation, diverge at a sensibly
greater Angle, than that at which they before converged; which being, at
most, but about thirty one or thirty two Minutes, there still remain'd
some other cause to be found out, from whence it could be two Deg. 49
Min.
Then I began to suspect, whether the Rays, after their Trajection
through the Prism, did not move in curve Lines, and according to their
more or less Curvity, tend to divers parts of the Wall. And it increas'd
my suspicion, when I remember'd that I had often seen a Tennis-Ball,
struck with an oblique Racket, describe such a curve Line. For a
Circular as well as a Progressive Motion being communicated to it by
that stroak, its parts on that side, where the Motions conspire, must
press and beat the contiguous Air more violently than on the other, and
there excite a Reluctancy and Reaction of the Air proportionably
greater. And for the same Reason, if the Rays of Light should possibly
be globular Bodies, and by their oblique Passage out of one Medium into
another, acquire a circulating Motion, they ought to feel the greater
resistance from the ambient Æther, on that side, where this Motion
conspires, and thence be continually bowed to the other. But
notwithstanding this plausible ground of suspicion, when I came to
examine it, I could observe no such Curvity in them. And besides (which
was enough for my purpose) I observ'd, that the difference 'twixt the
length of the Image, and Diameter of the Hole, through which the Light
was transmitted, was proportionable to their distance.
The gradual removal of these suspicions, at length led me to the
_Experimentum Crucis_, which was this; I took two Boards, and plac'd one
of them close behind the Prism at the Window, so that the light might
pass through a small hole, made in it for the purpose, and fall on the
other Board, which I plac'd at about twelve Feet distance, having first
made a small hole in it also, for some of that incident Light to pass
through. Then I plac'd another Prism behind this second Board, so that
the Light, trajected through both the Boards, might pass thro' that
also, and be again refracted before it arrived at the Wall. This done, I
took the first Prism in my Hand, and turn'd it to and fro slowly about
its _Axis_, so much as to make the several parts of the Image, cast on
the second Board, successively pass through the hole in it, that I might
observe to what places on the Wall the second Prism would refract them.
And I saw by the Variation of those places, that the Light, tending to
that end of the Image, towards which the Refraction of the first Prism
was made, did, in the second Prism, suffer a Refraction considerably
greater than the Light tending to the other end. And so the true cause
of the length of that Image was detected to be no other, than that
_Light_ consists of _Rays differently refrangible_, which, without any
respect to a difference in their incidence, were, according to their
degrees of Refrangibility, transmitted towards divers parts of the Wall.
When I understood this, I left off my aforesaid Glass Works; for I saw,
that the perfection of Telescopes was hitherto limited, not so much for
want of Glasses truly figur'd, according to the prescriptions of Optick
Authors (which all Men have hitherto imagin'd), as because that Light it
self is a _Heterogeneous mixture of differently refrangible Rays_. So
that, were a Glass so exactly figur'd, so as to collect any one sort of
Rays into one Point, it could not collect those also into the same
Point, which having the same Incidence upon the same Medium, are apt to
suffer a different Refraction. Nay, I wonder'd, that seeing the
difference of Refrangibility was so great, as I found it, Telescopes
should arrive to that perfection they are now at. For, measuring the
Refractions in one of my Prisms, I found, that, supposing the common
Sine of Incidence upon one of its plains, was forty four Parts, the Sine
of Refraction of the utmost Rays on the red end of the Colours, made out
of the Glass into the Air, would be sixty eight parts, and the Sine of
Refraction of the utmost Rays on the other end, sixty nine parts; so
that the difference is about a twenty fourth or twenty fifth part of the
whole Refraction. And consequently the Object glass of any Telescope
cannot collect all the Rays, which come from one point of an Object, so
as to make them convene at its _Focus_ in less room than in a Circular
space, whose Diameter is the fiftieth part of the Diameter of its
Aperture; which is an irregularity, some hundred of times greater, than
a circularly figur'd _Lens_, of so small a section as the Object-glasses
of long Telescopes are, would cause by the unfitness of its Figure, were
Light _uniform_.
This made me take _Reflections_ into Consideration, and finding them
regular, so that the Angle of Reflection of all sorts of Rays was equal
to their Angle of Incidence; I understood, that by their mediation,
Optick Instruments might be brought to any degree of Perfection
imaginable, provided a _Reflecting_ Substance could be found, which
would polish as finely as Glass, and _reflect_ as much Light as Glass
_transmits_; and the art of communicating to it a _Parabolick_ Figure be
also attain'd. But there seem'd very great Difficulties, and I have
almost thought them insuperable, when I further consider'd, that every
Irregularity in a reflecting Superficies makes the Rays stray five or
six times more out of their due course, than the like Irregularities in
a refracting one; So that a much greater Curiosity would be here
requisite, than in Figuring Glasses for Refraction.
Amidst these Thoughts I was forc'd from _Cambridge_ by the Intervening
Plague, and it was more than two Years before I proceeded further. But
then having thought on a tender way of polishing, proper for Metal,
whereby, as I imagin'd, the Figure also would be corrected to the last;
I began to try what might be effected in this kind, and by degrees so
far perfected an Instrument (in the essential parts of it like that I
sent to _London_,) by which I could discern _Jupiter_'s four
Concomitants, and shew'd them divers times to two others of my
Acquaintance. I could also discern the Moon-like Phase of _Venus_, but
not very distinctly, nor without some niceness in disposing the
Instrument.
From that time I was interrupted, till this last Autumn, when I made the
other. And as that was sensibly better than the first (especially for
Day-Objects,) so I doubt not but they will be still brought to a much
greater perfection by their Endeavours, who, as you inform me, are
taking care about it at _London_.
I have sometimes thought to make a _Microscope_, which in like manner
should have, instead of an Object-glass, a reflecting piece of Metal.
And this I hope they will also take into Consideration: For those
Instruments seem as capable of improvement as _Telescopes_, and perhaps
more, because but one reflective piece of Metal is requisite in them, as
you may perceive in _Plate 3. Fig. 1._ where _AB_ representeth the
Object Metal, _CD_ the Eye-glass, _F_ their common Focus, and _O_ the
other Focus of the Metal, in which the Object is placed.
But to return from this digression, I told you, that Light is not
similar, or homogeneal, but consists of _difform_ Rays, some of which
are more refrangible than others: So that of those, which are alike
incident on the same Medium, some shall be more refracted than others,
and that not by any virtue of the Glass, or other external Cause, but
from a predisposition, which every particular Ray hath to suffer a
particular degree of Refraction.
I shall now proceed to acquaint you with another more notable deformity
in its Rays, wherein the _Origin of Colours_ is unfolded: Concerning
which I shall lay down the _Doctrine_ first, and then, for its
Examination, give you an Instance or two of the _Experiments_, as a
Specimen of the rest.
The Doctrine you will find comprehended and illustrated in the following
Propositions.
1. As the Rays of Light differ in degrees of Refrangibility, so they
also differ in their disposition to exhibit this or that particular
Colour. Colours are not _Qualifications of Light_, derived from
Refractions, or Reflections of natural Bodies, (as 'tis generally
believed) but _Original_ and _connate Properties_, which in divers Rays
are divers. Some Rays are disposed to exhibit a red Colour and no other;
some a yellow and no other, some a green and no other, and so of the
rest. Nor are there only Rays proper and particular to the more eminent
Colours, but even to all their intermediate Gradations.
2. To the same degree of Refrangibility ever belongs the same Colour,
and to the same Colour ever belongs the same degree of Refrangibility.
The _least Refrangible_ Rays are all disposed to exhibit a _Red_ Colour,
and contrarily those Rays, which are disposed to exhibit a _Red_ Colour,
are all the least Refrangible: So the _most Refrangible_ Rays are all
disposed to exhibit a deep _Violet Colour_, and contrarily those which
are apt to exhibit such a _Violet Colour_, are all the most Refrangible.
And so to all the intermediate Colours in a continued Series belong
intermediate degrees of Refrangibility. And this Analogy 'twixt Colours,
and Refrangibility, is very precise and strict; the Rays always either
exactly agreeing in both, or proportionally disagreeing in both.
3. The Species of Colour, and Degree of Refrangibility proper to any
particular sort of Rays, is not mutable by Refraction, nor by Reflection
from Natural Bodies, nor by any other Cause, that I could yet observe.
When any one sort of Rays hath been well parted from those of other
kinds, it hath afterwards obstinately retain'd its Colour,
notwithstanding my utmost Endeavours to change it. I have refracted it
with Prisms, and reflected it with Bodies, which in Day-light were of
other Colours; I have intercepted it with the colour'd Film of Air
interceding two compressed Plates of Glass; transmitted it through
colour'd Mediums, and through Mediums irradiated with other sorts of
Rays, and diversly terminated it, and yet could never produce any new
Colour out of it. It would by contracting and dilating become more
brisk, or faint, and by the loss of many Rays in some Cases very obscure
and dark; but I could never see it chang'd _in specie_.
4. Yet seeming Transmutations of Colours may be made, where there is any
mixture of divers sorts of Rays. For in such mixtures, the component
Colours appear not, but by their mutual allaying each other, constitute
a midling Colour. And therefore, if by Refraction, or any other of the
aforesaid Causes, the difform Rays, latent in such a mixture, be
separated, there shall emerge Colours different from the colour of the
Composition. Which Colours are not new generated, but only made apparent
by being parted; for if they be again intirely mix'd and blended
together, they will again compose that Colour, which they did before
separation. And for the same reason, Transmutations made by the
convening of divers Colours are not real; for when the difform Rays are
again severed, they will exhibit the very same Colours, which they did
before they entered the Composition; as you see, _Blue_ and _Yellow_
Powders, when finely mixed, appear to the naked Eye _Green_, and yet the
Colours of the component Corpuscles are not thereby really transmuted,
but only blended. For, when viewed with a good Microscope, they still
appear _Blue_ and _Yellow_ interspersedly.
5. There are therefore two sorts of Colours. The one Original and
Simple, the other compounded of these. The Original or Primary Colours
are, _Red_, _Yellow_, _Green_, _Blue_, and a _Violet-purple_, together
with Orange, Indico, and an indefinite variety of intermediate
Gradations.
6. The same Colours in _Specie_ with these primary Ones, may be also
produced by Composition: For, a mixture of _Yellow_ and _Blue_ makes
_Green_; of _Red_ and _Yellow_, makes _Orange_; of _Orange_ and
_Yellowish Green_, makes _Yellow_. And in general, if any two Colours be
mix'd, which in the Series of those, generated by the Prism, are not too
far distant one from another, they by their mutual Alloy compound that
Colour, which in the said Series appeareth in the mid-way between them.
But those, which are situated at too great a distance, do not so.
_Orange_ and _Indico_ produce not the intermediate _Green_, nor
_Scarlet_ and _Green_ the intermediate _Yellow_.
7. But the most surprizing and wonderful Composition was that of
_Whiteness_. There is no one sort of Rays which alone can exhibit this.
'Tis ever compounded, and to its Composition are requisite all the
aforesaid primary Colours, mix'd in a due proportion. I have often with
admiration beheld, that all the Colours of the Prism being made to
converge, and thereby to be again mixed as they were in the light before
it was incident upon the Prism, reproduced light, intirely and perfectly
white, and not at all sensibly differing from a _direct_ light of the
Sun, unless when the Glasses, I used, were not sufficiently clear; for
then they would a little incline it to _their_ Colour.
8. Hence therefore it comes to pass, that _Whiteness_ is the usual
Colour of _Light_; for Light is a confused aggregate of Rays, indued
with all sorts of Colours, as they are promiscuously darted from the
various parts of luminous Bodies. And of such a confused aggregate, as I
said, is generated Whiteness, if there be a due proportion of the
Ingredients; but if any one predominate, the Light must incline to that
Colour; as it happens in the blue Flame of Brimstone, the yellow Flame
of a Candle, and the various Colours of the fixed Stars.
9. These things consider'd, the _manner_, how Colours are produced by
the Prism, is evident. For, of the Rays, constituting the incident
Light, since those which differ in Colour proportionally differ in
Refrangibility, _they_ by their unequal Refractions must be severed and
dispersed into an oblong Form, in an orderly succession, from the least
refracted Scarlet to the most refracted Violet. And for the same reason
it is, that Objects, when look'd upon through a Prism, appear coloured.
For the difform Rays, by their unequal Refractions, are made to diverge
towards several parts of the _Retina_, and there express the Images of
things coloured, as in the former case they did the Sun's Image upon a
Wall. And by this inequality of Refractions, they become not only
coloured, but also very confused and indistinct.
10. Why the Colours of the _Rainbow_ appear in falling drops of Rain, is
also from hence evident. For those drops, which refract the Rays,
disposed to appear Purple, in greatest quantity to the Spectator's Eye,
refract the Rays of other sorts so much less, as to make them pass
beside it; and such are the drops on the inside of the _Primary_ Bow,
and on the outside of the _Secondary_ or Exteriour one. So those drops,
which refract in greatest plenty the Rays, apt to appear red, toward the
Spectator's Eye, refract those of other sorts so much more, as to make
them pass beside it; and such are the drops on the Exteriour part of the
_Primary_, and Interiour part of the _Secondary_ Bow.
11. The odd Phænomena of an infusion of _Lignum Nephriticum_,
_Leaf-gold_, _Fragments of colour'd Glass_, and some other transparently
coloured Bodies, appearing in one Position of one Colour, and of another
in another, are on these grounds no longer Riddles. For those are
Substances apt to reflect one sort of Light, and transmit another; as
may be seen in a dark Room, by illuminating them with similar or
uncompounded Light. For then they appear of that Colour only, with which
they are illuminated; but yet in one Position more vivid and luminous
than in another, accordingly as they are disposed more or less to
reflect or transmit the incident Colour.
12. From hence also is manifest the reason of an unexpected Experiment,
which Mr. _Hook_, somewhere in his _Micrography_, relates to have made
with two wedge-like transparent Vessels fill'd, the one with a red, the
other with a blue Liquor; namely, that though they were severally
transparent enough, yet both together became opake: For, if one
transmitted only red, and the other only blue, no Rays could pass
through both.
13. I might add more Instances of this Nature; but I shall conclude with
this general one, that the Colours of all natural Bodies have no other
Origin than this, that they are variously qualified to reflect one sort
of Light in greater plenty than another. And this I have experimented in
a dark Room, by illuminating those Bodies with uncompounded Light of
divers Colours. For by that means any body may be made to appear of any
Colour. They have there no appropriate Colour, but ever appear of the
Colour of the Light cast upon them; but yet with this difference,
that they are most brisk and vivid in the Light of their own
day-light-colour. _Minium_ appeareth there of any Colour indifferently,
with which 'tis illustrated, but yet most luminous in red; and so _Bise_
appeareth indifferently of any Colour with which 'tis illustrated, but
yet most luminous in blue. And therefore _Minium_ reflecteth Rays of any
Colour, but most copiously those endu'd with red, and consequently when
illustrated with day-light, that is, with all sorts of Rays
promiscuously blended, those qualified with red, shall abound most in
the reflected Light, and by their prevalence cause it to appear of that
Colour. And for the same reason _Bise_, reflecting blue most copiously,
shall appear blue by the excess of those Rays in its reflected Light;
and the like of other Bodies. And that this is the intire and adequate
cause of their Colours, is manifest, because they have no power to
change or alter the Colours of any sort of Rays incident apart, but put
on all Colours indifferently, with which they are enlightned.
These things being so, it can be no longer disputed, whether there be
Colours in the dark, nor whether they be the Qualities of the Objects we
see, no nor perhaps, whether Light be a Body. For, since Colours are the
_Qualities_ of Light, having its Rays for their intire and immediate
Subject, how can we think those Rays _Qualities_ also, unless one
Quality may be the Subject of and sustain another; which in effect is to
call it _Substance_? We should not know Bodies for Substances, were it
not for their sensible Qualities; and the principal of those being now
found due to something else, we have as good reason to believe that to
be a Substance also.
Besides, whoever thought any Quality to be a _heterogeneous_ Aggregate,
such as Light is discovered to be? But to determine more absolutely,
what Light is, after what manner refracted, and by what Modes or Actions
it produceth in our Minds the Phantasms of Colours, is not so easie. And
I shall not mingle Conjectures with Certainties.
Reviewing what I have written, I see the Discourse it self will lead to
divers Experiments sufficient for its Examination; and therefore I shall
not trouble you farther, than to describe one of those, which I have
already insinuated.
In a darkned Room, make a hole in the shut of a Window, whose Diameter
may conveniently be about a third part of an Inch, to admit a convenient
quantity of the Sun's Light. And there place a clear and colourless
Prism, to refract the entring Light towards the farther part of the
Room; which, as I said, will thereby be diffused into an oblong coloured
Image. Then place a _Lens_ of about three Foot Radius (suppose a broad
Object-glass of a three Foot Telescope,) at the distance of about four
or five Foot from thence, through which all those Colours may at once be
transmitted, and made by its Refraction to convene at a farther distance
of about ten or twelve Feet. If at that distance you intercept this
Light with a Sheet of white Paper, you will see the Colours converted
into whiteness again by being mingled. But it is requisite, that the
_Prism_ and _Lens_ be placed steady, and that the Paper, on which the
Colours are cast, be moved to and fro; for, by such motion, you will not
only find at what distance the whiteness is most perfect, but also see
how the Colours gradually convene, and vanish into whiteness; and
afterwards, having crossed one another in that place where they compound
whiteness, are again dissipated and severed, and in an inverted order
retain the same Colours, which they had before they entred the
Composition. You may also see, that, if any of the Colours at the _Lens_
be intercepted, the whiteness will be changed into the other Colours.
And therefore, that the Composition of whiteness be perfect, care must
be taken that none of the Colours fall besides the _Lens_.
In the annexed Design, _Tab. 3. Fig. 2._ of this Experiment, _ABC_
expresseth the Prism set end-wise to sight, close by the hole _F_ of the
Window _EG_. Its vertical Angle _ABC_ may conveniently be about 60
Degrees: _MN_ designeth the _Lens_. Its breadth 2½ or 3 Inches. _SF_
one of the streight Lines, in which difform Rays may be conceived to
flow successively from the Sun. _FP_, and _FR_ two of those Rays
unequally refracted, which the _Lens_ makes to converge towards _Q_, and
after decussation to diverge again. And _HI_ the Paper, at divers
distances, on which the Colours are projected, which in _Q_ constitute
_Whiteness_, but are _Red_ and _Yellow_ in _R_, _r_, and ρ, and _Blue_
and _Purple_ in _P_, _p_, and π.
If you proceed further to try the impossibility of changing any
uncompounded Colour (which I have asserted in the third and thirteenth
Propositions,) 'tis requisite that the Room may be very dark, lest any
scattering light, mixing with the Colour, disturb and allay it, and
render it compound, contrary to the design of the Experiment. 'Tis also
requisite, that there be a perfecter separation of the Colours, than,
after the manner above described, can be made by the Refraction of one
single Prism; and how to make such farther separations, will scarce be
difficult to them, that consider the discovered Laws of Refractions. But
if trial shall be made with Colours not throughly separated, there must
be allowed changes proportionable to the mixture. Thus if compound
Yellow Light fall upon blue _Bise_, the Bise will not appear perfectly
yellow, but rather green, because there are in the yellow mixture many
Rays indued with green, and green being less remote from the usual blue
Colour of Bise than yellow, is the more copiously reflected by it.
In like manner, if any one of the Prismatick Colours, suppose red, be
intercepted, on design to try the asserted impossibility of reproducing
that Colour out of the others which are pretermitted; 'tis necessary,
either that the Colours be very well parted before the red be
intercepted; or that, together with the red, the neighbouring Colours,
into which any red is secretly dispersed, (that is, the yellow, and
perhaps green too) be intercepted; or else, that allowance be made for
the emerging of so much red out of the yellow green, as may possibly
have been diffused, and scatteringly blended in those Colours. And if
these things be observed, the new Production of red, or any intercepted
Colour, will be found impossible.
This, I conceive, is enough for an Introduction to Experiments of this
kind; which if any of the _Royal Society_ shall be so curious as to
prosecute, I should be very glad to be informed with what success: That,
if any thing seem to be defective, or to thwart this Relation, I may
have an opportunity of giving farther Direction about it, or of
acknowledging my Errors, if I have committed any.
_Since the Publication of this Theory, some Misunderstandings happening
between a _French_ Philosopher at _Paris_ and Mr. _Newton_, he has
endeavour'd to explain himself a little further in these Things,
according to the following Method._
_DEFINITIONS._
1. I call that Light Homogeneal, Similar, or Uniform, whose Rays are
equally refrangible.