Colour ManagementExcerpt from The Complete Works of Count Rumford, digitized by Google.
An Account of Some Experiments on Coloured Shadows
WHILE I was employed in the prosecution of my
experiments on the intensities of light, I was
struck with a very beautiful and what I then consid-
ered as a new appearance. Desirous of comparing the
intensity of the light of a clear sky, by day, with that
of a common wax candle, I darkened my room, and
letting the daylight from the north (coming through a
hole near the top of the window-shutter) fall at an angle
of about 70* upon a sheet of very fine white paper, I
placed a burning wax candle in such a position that
its rays fell upon the same paper, and, as nearly as I
could guess, in the line of reflection of the rays of day-
light from without; when, interposing a cylinder of
wood, about half an inch in diameter, before the centre
of the paper, and at the distance of about two inches
from its surface, I was much surprised to find that the
t^vo shadows projected by the cylinder upon the paper,
instead of being merely shades, without colour as I
expected to find them, the one of them — that which,
corresponding with the beam of daylight, was illumi-
nated by the candle — was yellow; while the other,
corresponding to the light of the candle, — and conse-
quently illuminated by the light of the heavens, — was
of the most beautiful blue that it is possible to imagine.
This appearance, which was not only unexpected, but
was really in itself in the highest degree striking and
beautiful, I found, upon repeated trials and after vary-
ing the experiment in every way I could think of, to be
so perfectly permanent that it is absolutely impossible
to produce two shadows at the same time from the
same body, the one answering to a beam of daylight
and the other to the light of a candle or lamp, without
these shadows being coloured, the one yellow and the
other blue.
The experiment may very easily be made at any
time by day, and almost in any place, and even by a
person not in the least degree versed in experimental
researches. Nothing more is necessary for that pur-
pose than to take a burning candle into a darkened
room in the daytime, and open one of the window-
shutters a little, about half or three quarters of an inch,
for instance ; when, the candle being placed upon a
table or stand, or given to an assistant to hold, in such
a situation that the rays from the candle may meet
those of daylight from without at an angle of about
40°, at the surface of a sheet of white paper, held in
a proper position to receive them, any solid opaque
body, a cylinder, or even a finger held before the
paper at the distance of two or three inches, will pro-
ject two shadows upon the paper, the one blue and
the other yellow.
If the candle be brought nearer to the paper, the
blue shadow will become of a deeper hue, and the
yellow shadow will gradually grow fainter; but, if it be
removed farther off, the yellow shadow will become
of a deeper colour, and the blue shadow will become
fainter; and, the candle remaining stationary in the
same place, the same varieties in the strength of the
tints of the coloured shadows may be produced merely
by opening the window-shutter a little more or less,
and rendering the illumination of the paper by the
light from without stronger or weaker. By either of
these means the coloured shadows may be made to
pass through all the gradations of shade, from the
deepest to the lightest, and vice versa; and .it is not a
little amusing to see shadows thus glowing with all
the brilliancy of the purest and most intense prismatic
colours, then passing suddenly through all the varieties
of shade, — preserving in all the most perfect purity of
tint, — growing stronger and fainter, and vanishing
and returning at command.
With respect to the causes of the colours of these
shadows, there is no doubt but they arise from the
different qualities of the light by which they are illumi-
nated; but how they are produced does not appear
to me so evident* That the shadow corresponding to
the beam of daylight, which is illuminated by the
yellow light of a candle, should be of a yellowish hue,
is not surprising; but why is the shadow correspond-
ing to the light of the candle, and which is illuminated
by no other light than the apparently white light of the
heavens, blue? I at first thought that it might arise
from the blueness of the sky; but finding that the
broad daylight, reflected from the roof of a neighboui-
ing house covered with the whitest new-fallen snow,
produced the same blue colour, and if possible of a still
more beautiful tint, I was obliged to abandon that
opinion.
To ascertain with some degree of precision the real
colour of the light emitted by a candle, I placed a
lighted wax candle, well trimmed, in the open air, at
mid-day, at a time when the ground was deeply cov-
ered with new-fallen snow, and the heavens were over-
spread with white clouds; when the flame of the
candle, far from being white, as it appears to be when
viewed by night, was evidently of a very decided yellow
colour not even approaching to whiteness.
The flame of an Argand's lamp, exposed at the same
time in the open air, appeared to be of the same yellow
hue. But the most striking manner of showing the
yellow hue of the light emitted by lamps and candles
is by exposing them in the direct rays of a bright
meridian sun. In that situation, the flame of an
Aland's lamp, burning with its greatest brilliancy,
appears in the form of a dead yellow semi-transparent
smoke. How transcendently pure and inconceivably
bright the rays of the sun are, when compared to the
light of any of our artificial illuminators, may be
gathered from the result of this experiment!
It appearing to me very probable that the difference
in the whiteness of the two kinds of light which were
the subjects of the foregoing experiments might, some-
how or other, be the occasion of the different colours of
the shadows, I attempted to produce the same efiFects
by employing two artificial lights of different colours;
and in this I succeeded completely.
In a room previously darkened, the light from two
burning wax candles being made to fall upon the white
paper at a proper angle in order to form two distinct
shadows of the cylinder, these shadows were found not
to be in the least coloured; but upon interposing a
pane of yellow glass, approaching to a faint orange
colour, before one of the candles, one of the shadows
immediately became yellow and the other blue.
When two Argand's lamps were made use of instead
of the candles, the result was the same : the shadows
were constantly and very deeply coloured, the one
yellow approaching to orange, and the other blue
approaching to green. I imagined that the greenish
cast of this blue colour was owing either to the want
of whiteness of the one light, or to the orange hue of
the other, which it acquired from the glass.
When equal panes of the same yellow glass were
interposed before both the lights, the white paper took
an orange hue, but the shadows were to all appearance
without the least tinge of colour ; but two panes of the
yellow glass being afterwards interposed before one of
the lights, while only one pane remained before the
other, the colours of the shadows immediately re-
turned
The results of these experiments having confirmed
my suspicions that the colours of the shadows arose
from the different degrees of whiteness of the two
lights, I now endeavoured, by bringing daylight to be
of the same yellow tinge with candlelight, by the
interposition of sheets of coloured glass, to prevent
the shadows being coloured when daylight and candle-
light were together the subjects of the experiment;
and in this I succeeded. I was even able to reverse
the colours of the shadows, by causing the daylight to
be of a deeper yellow than the candlelight.
In the course of these experiments, I observed that
different shades of yellow, given to the daylight, pro-
duced very different and often quite unexpected
effects : thus one sheet of the yellow glass, interposed
before the beam of daylight, changed the yellow
shadow to a lively violet colour, and the blue shadow
to a light green ; two sheets of the same glass nearly
destroyed the colours of both the shadows ; and three
sheets changed the shadow which was originally yel-
low to blue, and that which was blue to a purplish
yellow colour.
When the beam of daylight was made to pass
through a sheet of blue glass, the colours of the shad-
ows — the yellow as well as the blue — were improved
and rendered in the highest degree clear and brilliant ;
but, when the blue glass was placed before the candle,
the colours of the shadows were very much impaired.
In order to see what would be the consequence of
rendering the candlelight of a still deeper yellow, I
interposed before it a sheet of yellow or rather orange-
coloured glass, when a very unexpected and most
beautiful appearance took place: the colour of the
yellow shadow was changed to orange, — the blue
shadow remained unchanged, — and the whole surface
of the paper not covered by the shadows appeared to be
tinged of a most beautiful violet colour, approaching to
a light crimson or pink, — almost exactly the same hue
as I have often observed the distant snowy mountains
and valleys of the Alps to take about sunset.
Is it not more than probable that this hue is, in both
cases, produced by nearly the same combinations of
coloured light ? In the one case, it is the white snow
illuminated at the same time by the purest light of the
heavens and by the deep yellow rays from the west ;
and in the other, it is the white paper illuminated
by broad daylight and by the rays from a burning
candle, rendered still more yellow by being transmitted
through the yellow glass.
The beautiful violet colour which spreads itself over
the surface of the paper will appear to the greatest
advantage, if the pane of orange-coloured glass be held
in such a manner before the candle that only a part
of the paper — half of it, for instance — be affected by
it, the other half of it remaining white.
To make these experiments with more convenience,
the paper, which may be about 8 or 10 inches square,
should be pasted or glued down upon a flat piece of
board, furnished with a ball and socket upon the
hinder side of it, and mounted upon a stand ; and the
cylinder should be fastened to a small arm of wood or
of metal, projecting forward from the bottom of the
board for that purpose. A small stand, capable of
being made higher or lower, as the occasion requires,
should likewise be provided for supporting the candle ;
and, if the board with the paper fastened upon it be
surrounded with a broad black frame, the experiments
will be so much the more striking and beautiful. For
still greater convenience, I have added two other
stands, for holding the coloured glass through which
the light is occasionally made to pass, in its way to the
white surface upon which the shadows are projected.
It will be hardly necessary to add that, in order to the
experiments appearing to the greatest advantage, all
light which is not absolutely necessary to the experi-
ment must be carefully excluded.
Having fitted up a little apparatus according to the
above directions, merely for the purpose of prosecuting
these inquiries respecting the coloured shadows, I pro-
ceeded to make a great variety of experiments, — some
with pointed views, and others quite at random, and
merely in hopes of making some accidental discovery
that might lead to a knowledge of the causes of appear-
ances, which still seemed to me to be enveloped in
much obscurity and uncertainty.
Having found that the shadows corresponding to
two like wax candles were coloured, the one blue and
the other yellow, by interposing a sheet of yellow glass
before one of them, I now tried what the effect would
be when blue glass was made use of instead of yellow,
and I found it to be the same : the shadows were still
coloured, the one blue and the other yellow, with this
difference however, that the colours of the shadows
were reversed; that which, with the yellow glass, was
before yellow, being now blue, and that which was blue
being yellow.
I afterwards tried a glass of a bright amethyst
colour, and was surprised to find that the shadows
still continued to be coloured blue and yellow. The
yellow, it is true, had a dirty purple cast ; but the blue,
though a little inclining to green, was nevertheless a
clean, bright, decided colour.
Having no other coloured glass at hand to push
these particular inquiries farther, I now removed the
candles, and opening two holes in the upper parts of
the window-shutters of two neighbouring windows, I
let into the room, from above, two beams of light from
different parts of the heavens ; and, placing the instru-
ment in such a manner that two distinct shadows were
projected by the cylinder upon the paper, I was enter-
tained by a succession of very amusing appearances.
The shadows were tinged with an infinite variety of
the most unexpected and often most beautiful colours,
which continually varying, sometimes slowly and
sometimes with inconceivable rapidity, absolutely fas-
cinated the eyes, and, commanding the most eager
attention, afforded an enjoyment as new as it was
bewitching.
It was a windy day, with flying clouds, and it seemed
as if every cloud that passed brought with it another
complete succession of varying hues and most harmo-
nious tints. If any colour could be said to predominate,.
it was purples; but all the varieties of browns, and
almost all the other colours I ever remembered to have
seen, appeared in their turns, and there were even
colours which seemed to me to be perfectly new.
Reflecting upon the great variety of colours ob-
served in these last experiments, many of which did.
not appear to have the least relation to the apparent
colours of the light by which they were produced, I
began to suspect that the colours of the shadows
might in many cases, notwithstanding their apparent
brilliancy, be merely an optical deception, owing to
contrast or to some effect of the other real and neigh-
bouring colours upon the eye.
To determine this fact by a direct experiment, I
proceeded in the following manner. Having, by mak-
ing use of a flat ruler instead of the cylinder, contrived
to render the shadows much broader, I shut out of the
room every ray of daylight, and prepared to make the
experiment with two Argand's lamps, well trimmed,
and which were both made to burn with the greatest
possible brilliancy ; and having assured myself that the
light they emitted was precisely of the same colour, by
the shadows being perfectly colourless which were pro-
jected upon the white paper, I directed a tube of about
12 inches long and near an inch in diameter, lined
with black paper, against the centre of one of the
broad shadows; and looking through this tube with
one eye, while the other was closed, I kept my atten-
tion fixed upon the shadow, while an assistant repeat-
edly interposed a sheet of yellow glass before the lamp
whose light corresponded to the shadow I observed,
and as often removed it.
The result of the experiment was very striking, and
fully confirmed my suspicions with respect to the fal-
lacy of many of the appearances in the foregoing ex-
periments.
So far from being able to observe any change in the
shadow upon which my eye was fixed, I was not able
even to tell when the yellow glass was before the lamp
and when it was not ; and, though the assistant often
exclaimed at the striking brilliancy and beauty of the
blue colour of the very shadow I was observing, I
could not discover in it the least appearance of any
colour at all. But as soon as I removed my eye from
the tube, and contemplated the shadow with all its
neighbouring accompaniments, — the other shadow
rendered really yellow by the effect of the yellow glass
and the white paper, which had likewise from the
same cause acquired a yellowish hue, — the shadow
in question appeared to me, as it did to my assistant,
of a beautiful blue colour.
I afterwards repeated the same experiment with the
apparently blue shadow produced in the experiment
with daylight and candlelight, and with exactly the
same result.
How far these experiments may enable us to ac-
count for the apparent blue colour of the sky and the
great variety of colours which frequently adorn the
clouds, as also what other useful observations may be
drawn from them, I leave to philosophers, opticians,
and painters to determine. In the mean time I be-
lieve it is a new discovery — at least it is undoubtedly
a very extraordinary fact — that our eyes are not al-
ways to be believed, even with respect to the presence
or absence of colours.
I cannot finish this paper without mentioning one
circumstance, which struck me very forcibly in all
these experiments upon coloured shadows, — and that
is, the most perfect harmony which always appeared to
subsist between the colours — whatever they were — of
the two shadows ; and this harmony seemed to me to
be full as perfect and pleasing when the shadows were
of different tints of brown as when one of them was
blue and the other yellow. In short, the harmony of
these colours was in all cases not only very striking,
but the appearances altogether were quite enchanting ;
and I never found anybody to whom I showed these
experiments whose eyes were not fascinated with them.
It is, however, more than probable that a great part of
the pleasure which these experiments afforded to the
spectators arose from the continual changes of colour,
tint, and shade with which the eye was amused and
the attention kept awake.
We are used to seeing colours fixed and unalterable,
— hard as the solid bodies from which they come,
and just as motionless, — consequently dead^ uninter-
estingy and tiresome to the eye; but in these experi*
ments all is motion, life and beauty.
It appears to me very probable that a further prose-
cution of these experiments upon coloured shadows
may not only lead to a knowledge of the real nature of
the harmony of colours, or the peculiar circumstances
upon which that harmony depends, but that it may
also enable us to construct instruments for producing
that harmony for the entertainment of the eyes, in a
manner similar to that in which the ears are enter-
tained by musical sounds. I know that attempts have
already been made for that purpose ; but, when I con-
sider the means employed, I am not surprised that they
did not succeed. Where the flowing tide, the vary-
ing swell, the crescendo is wanting, colours must ever
remain hard, cold, and inanimate masses.
I am very sorry that my more serious occupations
do not at present permit me to pursue these most
entertaining inquiries. Perhaps at some future period
I may find leisure to resume them.
* I ought to inform the reader that when the above was written I had not
the smallest recollection of what, many years before, I had read concerning
coloured shadows, in Priestley's History of Optics. It may perhaps be thought
(by others, as well as by myself) that it was a fortunate circumstance that I
had forgotten what I had read ; for it left my mind in perfect freedom to pur-
sue, in my own way, the investigation of the causes of the phenomena which
presented themselves to my observation, without my being biassed by the opin-
ions of others, who, before me, had attempted to explain them. Had I recol-
lected what others had done, I should not, most probably, have given myself
the trouble of engaging in the prosecution of these inquiries.
But although at tfu time when this paper was written I had really no remem-
brance whatever of what had been written and published before on this subject,
yet soon after the paper was finished, and some time before it was sent to
England to be laid before the Royal Society, I was, by an accidental circum-
stance, made to recollect what I had so entirely forgotten. Shall I confess
what the motives were which induced me to expose myself to the danger of
being thought ignorant^ or something worse, by suffering my paper to go out of
my hands without alteration ? When the glow of the sudden blush which I
felt on discovering my danger had passed off, and I had taken time to reflect
coolly on all the circumstances of the case, I concluded that it might be useful
to permit my paper to go forth into the world in its original state. I conceived
that it would show, in a very striking manner, if not the advantages which
sometimes result from forgetting what we have read, at least the very great
importance of preserving the mind totally unbiassed by the speculative opinions
of others when we are in search of truth.
An ardent lover of science will not hesita e to expose himself to personal
danger when he perceives that by so doing he has a chance of promoting useful
investigation.
