New Theory About Light and Colour

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 applied myself to the grinding of optic
glasses of other figures than spherical) I procured me a triangular glass prism to try
therewith the celebrated phenomena of colours. And in order thereto having darkened my
chamber and made a small hole in my window-shuts to let in a convenient quantity of the
sun's light, I placed my prism at his entrance that it might be thereby refracted to the
opposite wall. It was at first a very pleasing divertissement to view the vivid and intense
colours produced thereby; but after a while, applying myself to consider them more
circumspectly, I became surprised to see them in an oblong form, which according to the
received laws of refraction I expected should have been circular.

They were terminated at the sides with straight 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
seemed semicircular.

Comparing the length of this coloured 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 tried 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 placed it that the light, passing through them both, might be refracted contrary
ways, and so by the latter returned 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 destroyed 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 reduced 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.

The gradual removal of these suspicions at length led me to the experimentum crucis,
which was this; I took two boards, and placed 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 placed at about 12 feet distance, having first made a small
hole in it also, for some of that incident light to pass through. Then I placed another prism
behind this second board so that the light, targeted through both the boards, might pass
through that also, and be again refracted before it arrived at the wall. This done, I took the
first prism in my hand, and turned 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.

I shall now proceed to acquaint you with another more notable difformity 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 retained its colour, notwithstanding my
utmost endeavours to change it. I have refracted it with prisms and reflected it with bodies
which in daylight were of other colours; I have intercepted it with the coloured film of air
interceding two compressed plates of glass; transmitted it through coloured mediums and
through mediums irradiated with other sorts of rays, and diversely terminated it; and,
yet could never produce any new colour out of it. It would by contracting or 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 changed 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 middling 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 entirely
mixed 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 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, indigo, and an indefinite variety of intermediate
graduations.

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
mixed 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 indigo produce not the intermediate green, nor scarlet and
green the intermediate yellow.

7. But the most surprising 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, mixed 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, entirely 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 endued 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 considered, 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 infrangibility, 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 looked 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 exterior 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 exterior part of the primary and
interior part of the secondary bow.

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 daylight 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 endued with red; and consequently
when illustrated with daylight, 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 entire 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 enlightened.

These things being so it can no longer be 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 entire 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, who ever 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 easy. And I shall not mingle conjectures with certainties.