PHYSICAL COLOURS.
136.
We give this designation to colours which are produced by certain
material mediums: these mediums, however, have no colour themselves,
and may be either transparent, semi-transparent yet transmitting light,
or altogether opaque. The colours in question are thus produced in the
eye through such external given causes, or are merely reflected to
the eye when by whatever means they are already produced without us.
Although we thus ascribe to them a certain objective character, their
distinctive quality still consists in their being transient, and not to
be arrested.
137.
They are called by former investigators _colores apparentes, fluxi,
fugitivi, phantastici, falsi, variantes_. They are also called
_speciosi_ and _emphatici_, on account of their striking splendour.
They are immediately connected with the physiological colours, and
appear to have but little more reality: for, while in the production
of the physiological colours the eye itself was chiefly efficient, and
we could only perceive the phenomena thus evoked within ourselves,
but not without us, we have now to consider the fact that colours are
produced in the eye by means of colourless objects; that we thus too
have a colourless surface before us which is acted upon as the retina
itself is, and that we can perceive the appearance produced upon it
without us. In such a process, however, every observation will convince
us that we have to do with colours in a progressive and mutable, but
not in a final or complete, state.
138.
Hence, in directing our attention to these physical colours, we find
it quite possible to place an objective phenomenon beside a subjective
one, and often by means of the union of the two successfully to
penetrate farther into the nature of the appearance.
139.
Thus, in the observations by which we become acquainted with the
physical colours, the eye is not to be considered as acting alone; nor
is the light ever to be considered in immediate relation with the eye:
but we direct our attention especially to the various effects produced
by mediums, those mediums being themselves colourless.
140.
Light under these circumstances may be affected by three conditions.
First, when it flashes back from the surface of a medium; in
considering which _catoptrical_ experiments invite our attention.
Secondly, when it passes by the edge of a medium: the phenomena
thus produced were formerly called _perioptical_; we prefer the
term _paroptical_. Thirdly, when it passes through either a merely
light-transmitting or an actually transparent body; thus constituting
a class of appearances on which _dioptrical_ experiments are founded.
We have called a fourth class of physical colours _epoptical_, as the
phenomena exhibit themselves on the colourless surface of bodies under
various conditions, without previous or actual dye (βαφή).--Note K.
141.
In examining these categories with reference to our three leading
divisions, according to which we consider the phenomena of colours in a
physiological, physical, or chemical view, we find that the catoptrical
colours are closely connected with the physiological; the paroptical
are already somewhat more distinct and independent; the dioptrical
exhibit themselves as entirely and strictly physical, and as having
a decidedly objective character; the epoptical, although still only
apparent, may be considered as the transition to the chemical colours.
142.
If we were desirous of prosecuting our investigation strictly in the
order of nature, we ought to proceed according to the classification
which has just been made; but in didactic treatises it is not of
so much consequence to connect as to duly distinguish the various
divisions of a subject, in order that at last, when every single
class and case has been presented to the mind, the whole may be
embraced in one comprehensive view. We therefore turn our attention
forthwith to the dioptrical class, in order at once to give the reader
the full impression of the physical colours, and to exhibit their
characteristics the more strikingly.
IX.
DIOPTRICAL COLOURS.
143.
Colours are called dioptrical when a colourless medium is necessary
to produce them; the medium must be such that light and darkness can
act through it either on the eye or on opposite surfaces. It is thus
required that the medium should be transparent, or at least capable, to
a certain degree, of transmitting light.
144.
According to these conditions we divide the dioptrical phenomena into
two classes, placing in the first those which are produced by means of
imperfectly transparent, yet light-transmitting mediums; and in the
second such as are exhibited when the medium is in the highest degree
transparent.
X.
DIOPTRICAL COLOURS OF THE FIRST CLASS.
145.
Space, if we assume it to be empty, would have the quality of absolute
transparency to our vision. If this space is filled so that the eye
cannot perceive that it is so, there exists a more or less material
transparent medium, which may be of the nature of air and gas, may be
fluid or even solid.
146.
The pure and light-transmitting semi-transparent medium is only an
accumulated form of the transparent medium. It may therefore be
presented to us in three modes.
147.
The extreme degree of this accumulation is white; the simplest,
brightest, first, opaque occupation of space.
148.
Transparency itself, empirically considered, is already the first
degree of the opposite state. The intermediate degrees from this point
to opaque white are infinite.
149.
At whatever point short of opacity we arrest the thickening medium, it
exhibits simple and remarkable phenomena when placed in relation with
light and darkness.
150.
The highest degree of light, such as that of the sun, of phosphorus
burning in oxygen, is dazzling and colourless: so the light of the
fixed stars is for the most part colourless. This light, however, seen
through a medium but very slightly thickened, appears to us yellow.
If the density of such a medium be increased, or if its volume become
greater, we shall see the light gradually assume a yellow-red hue,
which at last deepens to a ruby-colour.--Note L.
151.
If on the other hand darkness is seen through a semi-transparent
medium, which is itself illumined by a light striking on it, a blue
colour appears: this becomes lighter and paler as the density of the
medium is increased, but on the contrary appears darker and deeper the
more transparent the medium becomes: in the least degree of dimness
short of absolute transparence, always supposing a perfectly colourless
medium, this deep blue approaches the most beautiful violet.
152.
If this effect takes place in the eye as here described, and may
thus be pronounced to be subjective, it remains further to convince
ourselves of this by objective phenomena. For a light thus mitigated
and subdued illumines all objects in like manner with a yellow,
yellow-red, or red hue; and, although the effect of darkness through
the non-transparent medium does not exhibit itself so powerfully, yet
the blue sky displays itself in the camera obscura very distinctly on
white paper, as well as every other material colour.
153.
In examining the cases in which this important leading phenomenon
appears, we naturally mention the atmospheric colours first: most of
these may be here introduced in order.
154.
The sun seen through a certain degree of vapour appears with a yellow
disk; the centre is often dazzlingly yellow when the edges are already
red. The orb seen through a thick yellow mist appears ruby-red (as was
the case in 1794, even in the north); the same appearance is still
more decided, owing to the state of the atmosphere, when the scirocco
prevails in southern climates: the clouds generally surrounding the sun
in the latter case are of the same colour, which is reflected again on
all objects.
The red hues of morning and evening are owing to the same cause. The
sun is announced by a red light, in shining through a greater mass
of vapours. The higher he rises, the yellower and brighter the light
becomes.
155.
If the darkness of infinite space is seen through atmospheric vapours
illumined by the day-light, the blue colour appears. On high mountains
the sky appears by day intensely blue, owing to the few thin vapours
that float before the endless dark space: as soon as we descend in the
valleys, the blue becomes lighter; till at last, in certain regions,
and in consequence of increasing vapours, it altogether changes to a
very pale blue.
156.
The mountains, in like manner, appear to us blue; for, as we see them
at so great a distance that we no longer distinguish the local tints,
and as no light reflected from their surface acts on our vision, they
are equivalent to mere dark objects, which, owing to the interposed
vapours, appear blue.
157.
So we find the shadowed parts of nearer objects are blue when the air
is charged with thin vapours.
158.
The snow-mountains, on the other hand, at a great distance, still
appear white, or approaching to a yellowish hue, because they act on
our eyes as brightness seen through atmospheric vapour.
159.
The blue appearance at the lower part of the flame of a candle belongs
to the same class of phenomena. If the flame be held before a white
ground, no blue will be seen, but this colour will immediately appear
if the flame is opposed to a black ground. This phenomenon may be
exhibited most strikingly with a spoonful of lighted spirits of wine.
We may thus consider the lower part of the flame as equivalent to the
vapour which, although infinitely thin, is still apparent before the
dark surface; it is so thin, that one may easily see to read through
it: on the other hand, the point of the flame which conceals objects
from our sight is to be considered as a self-illuminating body.
160.
Lastly, smoke is also to be considered as a semi-transparent medium,
which appears to us yellow or reddish before a light ground, but blue
before a dark one.
161.
If we now turn our attention to fluid mediums, we find that water,
deprived in a very slight degree of its transparency, produces the same
effects.
162.
The infusion of the lignum nephriticum (guilandina Linnæi), which
formerly excited so much attention, is only a semi-transparent liquor,
which in dark wooden cups must appear blue, but held towards the sun in
a transparent glass must exhibit a yellow appearance.
163.
A drop of scented water, of spirit varnish, of several metallic
solutions, may be employed to give various degrees of opacity to water
for such experiments. Spirit of soap perhaps answers best.
164.
The bottom of the sea appears to divers of a red colour in bright
sunshine: in this case the water, owing to its depth, acts as a
semi-transparent medium. Under these circumstances, they find the
shadows green, which is the complemental colour.
165.
Among solid mediums the opal attracts our attention first: its colours
are, at least, partly to be explained by the circumstance that it is,
in fact, a semi-transparent medium, through which sometimes light,
sometimes dark, substrata are visible.
166.
For these experiments, however, the opal-glass (vitrum astroides,
girasole) is the most desirable material. It is prepared in various
ways, and its semi-opacity is produced by metallic oxydes. The same
effect is produced also by melting pulverised and calcined bones
together with the glass, on which account it is also known by the name
of _beinglas_; but, prepared in this mode, it easily becomes too opaque.
167.
This glass may be adapted for experiments in various ways: it may
either be made in a very slight degree non-transparent, in which case
the light seen through various layers placed one upon the other may
be deepened from the lightest yellow to the deepest red, or, if made
originally more opaque, it may be employed in thinner or thicker
laminæ. The experiments may be successfully made in both ways: in
order, however, to see the bright blue colour, the glass should neither
be too opaque nor too thick. For, as it is quite natural that darkness
must act weakly through the semi-transparent medium, so this medium, if
too thick, soon approaches whiteness.
168.
Panes of glass throw a yellow light on objects through those parts
where they happen to be semi-opaque, and these same parts appear blue
if we look at a dark object through them.
169.
Smoked glass may be also mentioned here, and is, in like manner, to be
considered as a semi-opaque medium. It exhibits the sun more or less
ruby-coloured; and, although this appearance may be attributed to the
black-brown colour of the soot, we may still convince ourselves that a
semi-transparent medium here acts if we hold such a glass moderately
smoked, and lit by the sun on the unsmoked side, before a dark object,
for we shall then perceive a bluish appearance.
170.
A striking experiment may be made in a dark room with sheets of
parchment. If we fasten a piece of parchment before the opening in the
window-shutter when the sun shines, it will appear nearly white; by
adding a second, a yellowish colour appears, which still increases as
more leaves are added, till at last it changes to red.
171.
A similar effect, owing to the state of the crystalline lens in milky
cataract, has been already adverted to (131).
172.
Having now, in tracing these phenomena, arrived at the effect of a
degree of opacity scarcely capable of transmitting light, we may here
mention a singular appearance which was owing to a momentary state of
this kind.
A portrait of a celebrated theologian had been painted some years
before the circumstance to which we allude, by an artist who was known
to have considerable skill in the management of his materials. The
very reverend individual was represented in a rich velvet dress, which
was not a little admired, and which attracted the eye of the spectator
almost more than the face. The picture, however, from the effect of the
smoke of lamps and dust, had lost much of its original vivacity. It
was, therefore, placed in the hands of a painter, who was to clean it,
and give it a fresh coat of varnish. This person began his operations
by carefully washing the picture with a sponge: no sooner, however,
had he gone over the surface once or twice, and wiped away the first
dirt, than to his amazement the black velvet dress changed suddenly to
a light blue plush, which gave the ecclesiastic a very secular, though
somewhat old-fashioned, appearance. The painter did not venture to go
on with his washing: he could not comprehend how a light blue should be
the ground of the deepest black, still less how he could so suddenly
have removed a glazing colour capable of converting the one tint to the
other.
At all events, he was not a little disconcerted at having spoilt the
picture to such an extent. Nothing to characterize the ecclesiastic
remained but the richly-curled round wig, which made the exchange
of a faded plush for a handsome new velvet dress far from desirable.
Meanwhile, the mischief appeared irreparable, and the good artist,
having turned the picture to the wall, retired to rest with a mind ill
at ease. But what was his joy the next morning, when, on examining the
picture, he beheld the black velvet dress again in its full splendour.
He could not refrain from again wetting a corner, upon which the blue
colour again appeared, and after a time vanished. On hearing of this
phenomenon, I went at once to see the miraculous picture. A wet sponge
was passed over it in my presence, and the change quickly took place. I
saw a somewhat faded, but decidedly light blue plush dress, the folds
under the arm being indicated by some brown strokes.
I explained this appearance to myself by the doctrine of the
semi-opaque medium. The painter, in order to give additional depth
to his black, may have passed some particular varnish over it: on
being washed, this varnish imbibed some moisture, and hence became
semi-opaque, in consequence of which the black underneath immediately
appeared blue. Perhaps those who are practically acquainted with the
effect of varnishes may, through accident or contrivance, arrive at
some means of exhibiting this singular appearance, as an experiment, to
those who are fond of investigating natural phenomena. Notwithstanding
many attempts, I could not myself succeed in re-producing it.
173.
Having now traced the most splendid instances of atmospheric
appearances, as well as other less striking yet sufficiently remarkable
cases, to the leading examples of semi-transparent mediums, we have no
doubt that attentive observers of nature will carry such researches
further, and accustom themselves to trace and explain the various
appearances which present themselves in every-day experience on the
same principle: we may also hope that such investigators will provide
themselves with an adequate apparatus in order to place remarkable
facts before the eyes of others who may be desirous of information.
174.
We venture, once for all, to call the leading appearance in question,
as generally described in the foregoing pages, a primordial and
elementary phenomenon; and we may here be permitted at once to state
what we understand by the term.
175.
The circumstances which come under our notice in ordinary observation
are, for the most part, insulated cases, which, with some attention,
admit of being classed under general leading facts. These again range
themselves under theoretical rubrics which are more comprehensive, and
through which we become better acquainted with certain indispensable
conditions of appearances in detail. From henceforth everything is
gradually arranged under higher rules and laws, which, however, are not
to be made intelligible by words and hypotheses to the understanding
merely, but, at the same time, by real phenomena to the senses. We
call these primordial phenomena, because nothing appreciable by the
senses lies beyond them, on the contrary, they are perfectly fit to be
considered as a fixed point to which we first ascended, step by step,
and from which we may, in like manner, descend to the commonest case
of every-day experience. Such an original phenomenon is that which has
lately engaged our attention. We see on the one side light, brightness;
on the other darkness, obscurity: we bring the semi-transparent medium
between the two, and from these contrasts and this medium the colours
develop themselves, contrasted, in like manner, but soon, through a
reciprocal relation, directly tending again to a point of union.[1]
176.
With this conviction we look upon the mistake that has been committed
in the investigation of this subject to be a very serious one, inasmuch
as a secondary phenomenon has been thus placed higher in order--the
primordial phenomenon has been degraded to an inferior place; nay, the
secondary phenomenon has been placed at the head, a compound effect has
been treated as simple, a simple appearance as compound: owing to this
contradiction, the most capricious complication and perplexity have
been introduced into physical inquiries, the effects of which are still
apparent.
177.
But when even such a primordial phenomenon is arrived at, the evil
still is that we refuse to recognise it as such, that we still aim at
something beyond, although it would become us to confess that we are
arrived at the limits of experimental knowledge. Let the observer of
nature suffer the primordial phenomenon to remain undisturbed in its
beauty; let the philosopher admit it into his department, and he will
find that important elementary facts are a worthier basis for further
operations than insulated cases, opinions, and hypotheses.--Note M.
[1] That is (according to the author's statement 150. 151.) both tend
to red; the yellow deepening to orange as the comparatively dark medium
is thickened before brightness; the blue deepening to violet as the
light medium is thinned before darkness.--T.
[Pg 74]
XI.
DIOPTRICAL COLOURS OF THE SECOND CLASS.--REFRACTION.
178.
Dioptrical colours of both classes are closely connected, as will
presently appear on a little examination. Those of the first class
appeared through semi-transparent mediums, those of the second class
will now appear through transparent mediums. But since every substance,
however transparent, may be already considered to partake of the
opposite quality (as every accumulation of a medium called transparent
proves), so the near affinity of the two classes is sufficiently
manifest.
179.
We will, however, first consider transparent mediums abstractedly as
such, as entirely free from any degree of opacity, and direct our whole
attention to a phenomenon which here presents itself, and which is
known by the name of refraction.
180.
In treating of the physiological colours, we have already had occasion
to vindicate what [Pg 75] were formerly called illusions of sight, as
the active energies of the healthy and duly efficient eye (2), and we
are now again invited to consider similar instances confirming the
constancy of the laws of vision.
181.
Throughout nature, as presented to the senses, everything depends on
the relation which things bear to each other, but especially on the
relation which man, the most important of these, bears to the rest.
Hence the world divides itself into two parts, and the human being
as _subject_, stands opposed to the _object_. Thus the practical
man exhausts himself in the accumulation of facts, the thinker in
speculation; each being called upon to sustain a conflict which admits
of no peace and no decision.
182.
But still the main point always is, whether the relations are truly
seen. As our senses, if healthy, are the surest witnesses of external
relations, so we may be convinced that, in all instances where they
appear to contradict reality, they lay the greater and surer stress
on true relations. Thus a distant object appears to us smaller; and
precisely by this means we are aware of distance. We produced coloured
appearances on colourless objects, through colourless mediums, and at
the same moment our attention was called to the degree of opacity in
the medium.
183.
Thus the different degrees of opacity in so-called transparent mediums,
nay, even other physical and chemical properties belonging to them,
are known to our vision by means of refraction, and invite us to make
further trials in order to penetrate more completely by physical and
chemical means into those secrets which are already opened to our view
on one side.
184.
Objects seen through mediums more or less transparent do not appear
to us in the place which they should occupy according to the laws of
perspective. On this fact the dioptrical colours of the second class
depend.
185.
Those laws of vision which admit of being expressed in mathematical
formulæ are based on the principle that, as light proceeds in straight
lines, it must be possible to draw a straight line from the eye to any
given object in order that it be seen. If, therefore, a case arises in
which the light arrives to us in a bent or broken line, that we see the
object by means of a bent or broken line, we are at once informed that
the medium between the eye and the object is denser, or that it has
assumed this or that foreign nature.
186.
This deviation from the law of right-lined vision is known by the
general term of refraction; and, although we may take it for granted
that our readers are sufficiently acquainted with its effects, yet we
will here once more briefly exhibit it in its objective and subjective
point of view.
187.
Let the sun shine diagonally into an empty cubical vessel, so that
the opposite side be illumined, but not the bottom: let water be
then poured into this vessel, and the direction of the light will
be immediately altered; for a part of the bottom is shone upon. At
the point where the light enters the thicker medium it deviates from
its rectilinear direction, and appears broken: hence the phenomenon
is called the breaking (_brechung_) or refraction. Thus much of the
objective experiment.
188.
We arrive at the subjective fact in the following mode:--Let the eye
be substituted for the sun: let the sight be directed in like manner
[Pg 78] diagonally over one side, so that the opposite inner side be
entirely seen, while no part of the bottom is visible. On pouring in
water the eye will perceive a part of the bottom; and this takes place
without our being aware that we do not see in a straight line; for
the bottom appears to us raised, and hence we give the term elevation
(_hebung_) to the subjective phenomenon. Some points, which are
particularly remarkable with reference to this, will be adverted to
hereafter.
189.
Were we now to express this phenomenon generally, we might here repeat,
in conformity with the view lately taken, that the relation of the
objects is changed or deranged.
190.
But as it is our intention at present to separate the objective from
the subjective appearances, we first express the phenomenon in a
subjective form, and say,--a derangement or displacement of the object
seen, or to be seen, takes place.
191.
But that which is seen without a limiting outline may be thus affected
without our perceiving the change. On the other hand, if what we look
at has a visible termination, we have an evident indication that a
displacement occurs. If, therefore, [Pg 79] we wish to ascertain the
relation or degree of such a displacement, we must chiefly confine
ourselves to the alteration of surfaces with visible boundaries; in
other words, to the displacement of circumscribed objects.
192.
The general effect may take place through parallel mediums, for every
parallel medium displaces the object by bringing it perpendicularly
towards the eye. The apparent change of position is, however, more
observable through mediums that are not parallel.
193.
These latter may be perfectly spherical, or may be employed in the
form of convex or concave lenses. We shall make use of all these as
occasion may require in our experiments. But as they not only displace
the object from its position, but alter it in various ways, we shall,
in most cases, prefer employing mediums with surfaces, not, indeed,
parallel with reference to each other, but still altogether plane,
namely, prisms. These have a triangle for their base, and may, it is
true, be considered as portions of a lens, but they are particularly
available for our experiments, inasmuch as they very perceptibly
displace the object from its position, without producing a remarkable
distortion.
194.
And now, in order to conduct our observations with as much exactness
as possible, and to avoid all confusion and ambiguity, we confine
ourselves at first to
SUBJECTIVE EXPERIMENTS,
in which, namely, the object is seen by the observer through a
refracting medium. As soon as we have treated these in due series, the
objective experiments will follow in similar order.
XII.
REFRACTION WITHOUT THE APPEARANCE OF COLOUR.
195.
Refraction can visibly take place without our perceiving an appearance
of colour. To whatever extent a colourless or uniformly coloured
surface may be altered as to its position by refraction, no colour
consequent upon refraction appears within it, provided it has no
outline or boundary. We may convince ourselves of this in various ways.
196.
Place a glass cube on any larger surface, and look through the glass
perpendicularly or obliquely, the unbroken surface opposite the eye
appears altogether raised, but no colour exhibits itself. If we look at
a pure grey or blue sky or a uniformly white or coloured wall through a
prism, the portion of the surface which the eye thus embraces will be
altogether changed as to its position, without our therefore observing
the smallest appearance of colour.
XIII.
CONDITIONS OF THE APPEARANCE OF COLOUR.
197.
Although in the foregoing experiments we have found all unbroken
surfaces, large or small, colourless, yet at the outlines or
boundaries, where the surface is relieved upon a darker or lighter
object, we observe a coloured appearance.
198.
Outline, as well as surface, is necessary to constitute a figure or
circumscribed object. We therefore express the leading fact thus:
circumscribed objects must be displaced by refraction in order to the
exhibition of an appearance of colour.
199.
We place before us the simplest object, a light disk on a dark ground
(A).[1] A displacement occurs with regard to this object, if we
apparently extend its outline from the centre by magnifying it. This
may be done with any convex glass, and in this case we see a blue edge
(B).
200.
We can, to appearance, contract the circumference of the same light
disk towards the centre by diminishing the object; the edge will then
appear yellow (C). This may be done with a concave glass, which,
however, should not be ground thin like common eye-glasses, but must
have some substance. In order, however, to make this experiment at once
with the convex glass, let a smaller black disk be inserted within
the light disk on a black ground. If we magnify the black disk on a
white ground with a convex glass, the same result takes place as if we
diminished the white disk; for we extend the black outline upon the
white, and we thus perceive the yellow edge together with the blue edge
(D).
201.
These two appearances, the blue and yellow, exhibit themselves in and
upon the white: they both assume a reddish hue, in proportion as they
mingle with the black.[2]
[Illustration]
202.
In this short statement we have described the primordial phenomena of
all appearance of colour occasioned by refraction. These undoubtedly
may be repeated, varied, and rendered more striking; may be combined,
complicated, confused; but, after all, may be still restored to their
original simplicity.
203.
In examining the process of the experiment just given, we find that
in the one case we have, to appearance, extended the white edge upon
the dark surface; in the other we have extended the dark edge upon
the white surface, supplanting one by the other, pushing one over
the other. We will now endeavour, step by step, to analyse these and
similar cases.
204.
If we cause the white disk to move, in appearance, entirely from its
place, which can be done effectually by prisms, it will be coloured
according to the direction in which it apparently moves, in conformity
with the above laws. If we look at the disk _a_[3] through a prism,
so that it appear moved to _b_, the outer edge will appear blue and
blue-red, according to the law of the figure B (fig. 1), the other
edge being yellow, and yellow-red, according to the law of the figure
C (fig. 1). For in the first case the white figure is, as it were,
extended over the dark boundary, and in the other case the dark
boundary is passed over the white figure. The same happens if the disk
is, to appearance, moved from _a_ to _c_, from _a_ to _d_, and so
throughout the circle.
205.
As it is with the simple effect, so it is with more complicated
appearances. If we look through a horizontal prism (_a b_[4]) at a
white disk placed at some distance behind it at _e_, the disk will
be raised to _f_, and coloured according to the above law. If we
remove this prism, and look through a vertical one (_c d_) at the same
disk, it will appear at _h_, and coloured according to the same law.
If we place the two prisms one upon the other, the disk will appear
displaced diagonally, in conformity with a general law of nature, and
will be coloured as before; that is, according to its movement in the
direction, _e.g._:[5]
206.
If we attentively examine these opposite coloured edges, we find that
they only appear in the direction of the apparent change of place.
A round figure leaves us in some degree uncertain as to this: a
quadrangular figure removes all doubt.
207.
The quadrangular figure _a_,[6] moved in the direction _a b_ or _a d_
exhibits no colour on the sides which are parallel with the direction
in which it moves: on the other hand, if moved in the direction _a
c_, parallel with its diagonal, all the edges of the figure appear
coloured.[7]
208.
Thus, a former position (203) is here confirmed; viz. to produce
colour, an object must be so displaced that the light edges be
apparently carried over a dark surface, the dark edges over a light
surface, the figure over its boundary, the boundary over the figure.
But if the rectilinear boundaries of a figure could be indefinitely
extended by refraction, so that figure and background might only pursue
their course next, but not over each other, no colour would appear, not
even if they were prolonged to infinity.
[1] Plate 2, fig. 1.
[2] The author has omitted the orange and purple in the coloured
diagrams which illustrate these first experiments, from a wish probably
to present the elementary contrast, on which he lays a stress, in
greater simplicity. The reddish tinge would be apparent, as stated
above, where the blue and yellow are in contact with the black.--T.
[3] Plate 2, fig. 2
[4] Plate 2, fig. 4
[5] In this case, according to the author, the refracting medium being
increased in mass, the appearance of colour is increased, and the
displacement is greater.--T.
[6] Plate 2, fig. 3.
[7] Fig. 2, plate 1, contains a variety of forms, which, when viewed
through a prism, are intended to illustrate the statement in this and
the following paragraph.
XIV.
CONDITIONS UNDER WHICH THE APPEARANCE OF COLOUR INCREASES.
209.
We have seen in the foregoing experiments that all appearance of colour
occasioned by refraction depends on the condition that the boundary or
edge be moved in upon the object itself, or the object itself over the
ground, that the figure should be, as it were, carried over itself, or
over the ground. And we shall now find that, by increased displacement
of the object, the appearance of colour exhibits itself in a greater
degree. This takes place in subjective experiments, to which, for the
present, we confine ourselves, under the following conditions.
210.
First, if, in looking through parallel mediums, the eye is directed
more obliquely.
Secondly, if the surfaces of the medium are no longer parallel, but
form a more or less acute angle.
Thirdly, owing to the increased proportion of the medium, whether
parallel mediums be increased in size, or whether the angle be
increased, provided it does not attain a right angle.
Fourthly, owing to the distance of the eye armed with a refracting
medium from the object to be displaced.
Fifthly, owing to a chemical property that may be communicated to the
glass, and which may be afterwards increased in effect.
211.
The greatest change of place, short of considerable distortion of the
object, is produced by means of prisms, and this is the reason why the
appearance of colour can be exhibited most powerfully through glasses
of this form. Yet we will not, in employing them, suffer ourselves to
be dazzled by the splendid appearances they exhibit, but keep the above
well-established, simple principles calmly in view.
212.
The colour which is outside, or foremost, in the apparent change of an
object by refraction, is always the broader, and we will henceforth
call this a _border_: the colour that remains next the outline is the
narrower, and this we will call an _edge_.
213.
If we move a dark boundary towards a light surface, the yellow broader
border is foremost, and the narrower yellow-red edge follows close to
the outline. If we move a light boundary towards a dark surface, the
broader violet border is foremost, and the narrower blue edge follows.
214.
If the object is large, its centre remains uncoloured. Its inner
surface is then to be considered as unlimited (195): it is displaced,
but not otherwise altered: but if the object is so narrow, that under
the above conditions the yellow border can reach the blue edge, the
space between the outlines will be entirely covered with colour. If we
make this experiment with a white stripe on a black ground,[1] the two
extremes will presently meet, and thus produce green. We shall then see
the following series of colours:--
Yellow-red.
Yellow.
Green.
Blue.
Blue-red.
215.
If we place a black band, or stripe, on white paper,[2] the violet
border will spread till it meets the yellow-red edge. In this case the
intermediate black is effaced (as the intermediate white was in the
last experiment), and in its stead a splendid pure red will appear.[3]
The series of colours will now be as follows:--
Blue.
Blue-red.
Red.
Yellow-red.
Yellow.
216.
The yellow and blue, in the first case (214), can by degrees meet so
fully, that the two colours blend entirely in green, and the order will
then be,
Yellow-red.
Green.
Blue-red.
In the second case (215), under similar circumstances, we see only
Blue.
Red.
Yellow.
This appearance is best exhibited by refracting the bars of a window
when they are relieved on a grey sky.[4]
217.
In all this we are never to forget that this appearance is not to be
considered as a complete or final state, but always as a progressive,
increasing, and, in many senses, controllable appearance. Thus we
find that, by the negation of the above five conditions, it gradually
decreases, and at last disappears altogether.
[1] Plate 2, fig. 5, _left_.
[2] Plate 2, fig. 5, _right_.
[3] This pure red, the union of orange and violet, is considered by the
author the maximum of the coloured appearance: he has appropriated the
term _purpur_ to it. See paragraph 703, and _note_.--T.
[4] The bands or stripes in fig. 4, plate 1, when viewed through a
prism, exhibit the colours represented in plate 2, fig. 5.
XV.
EXPLANATION OF THE FOREGOING PHENOMENA.
218.
Before we proceed further, it is incumbent on us to explain the first
tolerably simple phenomenon, and to show its connexion with the
principles first laid down, in order that the observer of nature may
be enabled clearly to comprehend the more complicated appearances that
follow.
219.
In the first place, it is necessary to remember that we have to do
with circumscribed objects. In the act of seeing, generally, it is
the circumscribed visible which chiefly invites our observation; and
in the present instance, in speaking of the appearance of colour, as
occasioned by refraction, the circumscribed visible, the detached
object solely occupies our attention.
220.
For our chromatic exhibitions we can, however, divide objects generally
into _primary_ and _secondary_. The expressions of themselves denote
what we understand by them, but our meaning will be rendered still more
plain by what follows.
221.
Primary objects may be considered firstly as _original_, as images
which are impressed on the eye by things before it, and which assure
us of their reality. To these the secondary images may be opposed
as _derived_ images, which remain in the organ when the object
itself is taken away; those apparent after-images, which have been
circumstantially treated of in the doctrine of physiological colours.
222.
The primary images, again, may be considered as _direct_ images, which,
like the original impressions, are conveyed immediately from the object
to the eye. In contradistinction to these, the secondary images may
be considered as _indirect_, being only conveyed to us, as it were,
at second-hand from a reflecting surface. These are the mirrored, or
catoptrical, images, which in certain cases can also become double
images:
223.
When, namely, the reflecting body is transparent, and has two parallel
surfaces, one behind the other: in such a case, an image may be
reflected to the eye from both surfaces, and thus arise double images,
inasmuch as the upper image does not quite cover the under one: this
may take place in various ways.
Let a playing-card be held before a mirror. We shall at first see the
distinct image of the card, but the edge of the whole card, as well
as that of every spot upon it, will be bounded on one side with a
border, which is the beginning of the second reflection. This effect
varies in different mirrors, according to the different thickness of
the glass, and the accidents of polishing. If a person wearing a white
waistcoat, with the remaining part of his dress dark, stands before
certain mirrors, the border appears very distinctly, and in like manner
the metal buttons on dark cloth exhibit the double reflection very
evidently.
224.
The reader who has made himself acquainted with our former descriptions
of experiments (80) will the more readily follow the present statement.
The window-bars reflected by plates of glass appear double, and
by increased thickness of the glass, and a due adaptation of the
angle of reflection, the two reflections may be entirely separated
from each other. So a vase full of water, with a plane mirror-like
bottom, reflects any object twice, the two reflections being more or
less separated under the same conditions. In these cases it is to be
observed that, where the two reflections cover each other, the perfect
vivid image is reflected, but where they are separated they exhibit
only weak, transparent, and shadowy images.
225.
If we wish to know which is the under and which the upper image, we
have only to take a coloured medium, for then a light object reflected
from the under surface is of the colour of the medium, while that
reflected from the upper surface presents the complemental colour. With
dark objects it is the reverse; hence black and white surfaces may be
here also conveniently employed. How easily the double images assume
and evoke colours will here again be striking.
226.
Thirdly, the primary images may be considered as _principal_ images,
while the secondary can be, as it were, annexed to these as _accessory_
images. Such an accessory image produces a sort of double form; except
that it does not separate itself from the principal object, although it
may be said to be always endeavouring to do so. It is with secondary
images of this last description that we have to do in prismatic
appearances.
227.
A surface without a boundary exhibits no appearance of colour when
refracted (195). Whatever is seen must be circumscribed by an
outline to produce this effect. In other words a figure, an object,
is required; this object undergoes an apparent change of place by
refraction: the change is however not complete, not clean, not sharp;
but incomplete, inasmuch as an accessory image only is produced.
228.
In examining every appearance of nature, but especially in examining
an important and striking one, we should not remain in one spot, we
should not confine ourselves to the insulated fact, nor dwell on it
exclusively, but look round through all nature to see where something
similar, something that has affinity to it, appears: for it is only by
combining analogies that we gradually arrive at a whole which speaks
for itself, and requires no further explanation.
229.
Thus we here call to mind that in certain cases refraction
unquestionably produces double images, as is the case in Iceland spar:
similar double images are also apparent in cases of refraction through
large rock crystals, and in other instances; phenomena which have not
hitherto been sufficiently observed.[1]
230.
But since in the case under consideration (227) the question relates
not to double but to accessory images, we refer to a phenomenon already
adverted to, but not yet thoroughly investigated. We allude to an
earlier experiment, in which it appeared that a sort of conflict took
place in regard to the retina between a light object and its dark
ground, and between a dark object and its light ground (16). The light
object in this case appeared larger, the dark one smaller.
231.
By a more exact observation of this phenomenon we may remark that the
forms are not sharply distinguished from the ground, but that they
appear with a kind of grey, in some degree, coloured edge; in short,
with an accessory image. If, then, objects seen only with the naked
eye produce such effects, what may not take place when a dense medium
is interposed? It is not that alone which presents itself to us in
obvious operation which produces and suffers effects, but likewise all
principles that have a mutual relation only of some sort are efficient
accordingly, and indeed often in a very high degree.
232.
Thus when refraction produces its effect on an object there appears an
accessory image next the object itself: the real form thus refracted
seems even to linger behind, as if resisting the change of place; but
the accessory image seems to advance, and extends itself more or less
in the mode already shown (212-216).
233.
We also remarked (224) that in double images the fainter appear only
half substantial, having a kind of transparent, evanescent character,
just as the fainter shades of double shadows must always appear as
half-shadows. These latter assume colours easily, and produce them
readily (69), the former also (80); and the same takes place in the
instance of accessory images, which, it is true, do not altogether
quit the real object, but still advance or extend from it as
half-substantial images, and hence can appear coloured so quickly and
so powerfully.
234.
That the prismatic appearance is in fact an accessory image we may
convince ourselves in more than one mode. It corresponds exactly with
the form of the object itself. Whether the object be bounded by a
straight line or a curve, indented or waving, the form of the accessory
image corresponds throughout exactly with the form of the object.[2]
235.
Again, not only the form but other qualities of the object are
communicated to the accessory image. If the object is sharply relieved
from its ground, like white on black, the coloured accessory image in
like manner appears in its greatest force. It is vivid, distinct, and
powerful; but it is most especially powerful when a luminous object is
shown on a dark ground, which may be contrived in various ways.
236.
But if the object is but faintly distinguished from the ground, like
grey objects on black or white, or even on each other, the accessory
image is also faint, and, when the original difference of tint or force
is slight, becomes hardly discernible.
237.
The appearances which are observable when coloured objects are relieved
on light, dark, or coloured grounds are, moreover, well worthy of
attention. In this case a union takes place between the apparent colour
of the accessory image and the real colour of the object; a compound
colour is the result, which is either assisted and enhanced by the
accordance, or neutralised by the opposition of its ingredients.
238.
But the common and general characteristic both of the double and
accessory image is semi-transparence. The tendency of a transparent
medium to become only half transparent, or merely light-transmitting,
has been before adverted to (147, 148). Let the reader assume that he
sees within or through such a medium a visionary image, and he will at
once pronounce this latter to be a semi-transparent image.
239.
Thus the colours produced by refraction may be fitly explained by the
doctrine of the semi-transparent mediums. For where dark passes over
light, as the border of the semi-transparent accessory image advances,
yellow appears; and, on the other hand, where a light outline passes
over the dark background, blue appears (150, 151).
240.
The advancing foremost colour is always the broader. Thus the yellow
spreads over the light with a broad border, but the yellow-red appears
as a narrower stripe and is next the dark, according to the doctrine of
augmentation, as an effect of shade.[3]
241.
On the opposite side the condensed blue is next the edge, while the
advancing border, spreading as a thinner veil over the black, produces
the violet colour, precisely on the principles before explained in
treating of semi-transparent mediums, principles which will hereafter
be found equally efficient in many other cases.
242.
Since an analysis like the present requires to be confirmed by ocular
demonstration, we beg every reader to make himself acquainted with the
experiments hitherto adduced, not in a superficial manner, but fairly
and thoroughly. We have not placed arbitrary signs before him instead
of the appearances themselves; no modes of expression are here proposed
for his adoption which may be repeated for ever without the exercise
of thought and without leading any one to think; but we invite him to
examine intelligible appearances, which must be present to the eye and
mind, in order to enable him clearly to trace these appearances to
their origin, and to explain them to himself and to others.
[1] The date of the publication, 1810, is sometimes to be
remembered.--T.
[2] The forms in fig. 2, plate 1, when seen through a prism, are
again intended to exemplify this. In the plates to the original work
curvilinear figures are added, but the circles, fig. 1, in the same
plate, may answer the same end.--T.
[3] The author has before observed that colour is a degree of darkness,
and he here means that increase of darkness, produced by transparent
mediums, is, to a certain extent, increase of colour.--T.
XVI.
DECREASE OF THE APPEARANCE OF COLOUR.
243.
We need only take the five conditions (210) under which the appearance
of colour increases in the contrary order, to produce the contrary or
decreasing state; it may be as well, however, briefly to describe and
review the corresponding modifications which are presented to the eye.
244.
At the highest point of complete junction of the opposite edges, the
colours appear as follows (216):--
Yellow-red. Blue.
Green. Red.
Blue-red. Yellow.
245.
Where the junction is less complete, the appearance is as follows (214,
215):--
Yellow-red. Blue.
Yellow. Blue-red.
Green. Red.
Blue. Yellow-red.
Blue-red. Yellow.
Here, therefore, the surface still appears completely coloured, but
neither series is to be considered as an elementary series, always
developing itself in the same manner and in the same degrees; on the
contrary, they can and should be resolved into their elements; and, in
doing this, we become better acquainted with their nature and character.
246.
These elements then are (199, 200, 201)--
Yellow-red. Blue.
Yellow. Blue-red.
White. Black.
Blue. Yellow-red.
Blue-red. Yellow.
Here the surface itself, the original object, which has been hitherto
completely covered, and as it were lost, again appears in the centre of
the colours, asserts its right, and enables us fully to recognise the
secondary nature of the accessory images which exhibit themselves as
"edges" and "borders."--Note N.
247.
We can make these edges and borders as narrow as we please; nay, we
can still have refraction in reserve after having done away with all
appearance of colour at the boundary of the object.
Having now sufficiently investigated the exhibition of colour in this
phenomenon, we repeat that we cannot admit it to be an elementary
phenomenon. On the contrary, we have traced it to an antecedent and
a simpler one; we have derived it, in connexion with the theory of
secondary images, from the primordial phenomenon of light and darkness,
as affected or acted upon by semi-transparent mediums. Thus prepared,
we proceed to describe the appearances which refraction produces on
grey and coloured objects, and this will complete the section of
subjective phenomena.
XVII.
GREY OBJECTS DISPLACED BY REFRACTION.
248.
Hitherto we have confined our attention to black and white objects
relieved on respectively opposite grounds, as seen through the prism,
because the coloured edges and borders are most clearly displayed in
such cases. We now repeat these experiments with grey objects, and
again find similar results.
249.
As we called black the equivalent of darkness, and white the
representative of light (18), so we now venture to say that grey
represents half-shadow, which partakes more or less of light and
darkness, and thus stands between the two. We invite the reader to call
to mind the following facts as bearing on our present view.
250.
Grey objects appear lighter on a black than on a white ground (33);
they appear as a light on a black ground, and larger; as a dark on the
white ground, and smaller. (16.)
251.
The darker the grey the more it appears as a faint light on black, as a
strong dark on white, and _vice versâ_; hence the accessory images of
dark-grey on black are faint, on white strong: so the accessory images
of light-grey on white are faint, on black strong.
252.
Grey on black, seen through the prism, will exhibit the same
appearances as white on black; the edges are coloured according to the
same law, only the borders appear fainter. If we relieve grey on white,
we have the same edges and borders which would be produced if we saw
black on white through the prism.--Note O.
253.
Various shades of grey placed next each other in gradation will exhibit
at their edges, either blue and violet only, or red and yellow only,
according as the darker grey is placed over or under.
254.
A series of such shades of grey placed horizontally next each other
will be coloured conformably to the same law according as the whole
series is relieved, on a black or white ground above or below.
255.
The observer may see the phenomena exhibited by the prism at one
glance, by enlarging the plate intended to illustrate this section.[1]
256.
It is of great importance duly to examine and consider another
experiment in which a grey object is placed partly on a black and
partly on a white surface, so that the line of division passes
vertically through the object.
257.
The colours will appear on this grey object in conformity with the
usual law, but according to the opposite relation of the light to the
dark, and will be contrasted in a line. For as the grey is as a light
to the black, so it exhibits the red and yellow above the blue and
violet below: again, as the grey is as a dark to the white, the blue
and violet appear above the red and yellow below. This experiment will
be found of great importance with reference to the next chapter.
[1] It has been thought unnecessary to give all the examples in the
plate alluded to, but the leading instance referred to in the next
paragraph will be found in plate 3, fig. 1. The grey square when seen
through a prism will exhibit the effects described in par. 257.--T.
XVIII.
COLOURED OBJECTS DISPLACED BY REFRACTION.
258.
An unlimited coloured surface exhibits no prismatic colour in addition
to its own hue, thus not at all differing from a black, white, or
grey surface. To produce the appearance of colour, light and dark
boundaries must act on it either accidentally or by contrivance. Hence
experiments and observations on coloured surfaces, as seen through the
prism, can only be made when such surfaces are separated by an outline
from another differently tinted surface, in short when _circumscribed
objects_ are coloured.
259.
All colours, whatever they may be, correspond so far with grey, that
they appear darker than white and lighter than black. This shade-like
quality of colour (σκιέρον) has been already alluded to (69), and will
become more and more evident. If then we begin by placing coloured
objects on black and white surfaces, and examine them through the
prism, we shall again have all that we have seen exhibited with grey
surfaces.
[Illustration]
260.
If we displace a coloured object by refraction, there appears, as
in the case of colourless objects and according to the same laws,
an accessory image. This accessory image retains, as far as colour
is concerned, its usual nature, and acts on one side as a blue and
blue-red, on the opposite side as a yellow and yellow-red. Hence the
apparent colour of the edge and border will be either homogeneous
with the real colour of the object, or not so. In the first case the
apparent image identifies itself with the real one, and appears to
increase it, while, in the second case, the real image may be vitiated,
rendered indistinct, and reduced in size by the apparent image. We
proceed to review the cases in which these effects are most strikingly
exhibited.
261.
If we take a coloured drawing enlarged from the plate, which
illustrates this experiment[1], and examine the red and blue squares
placed next each other on a black ground, through the prism as usual,
we shall find that as both colours are lighter than the ground,
similarly coloured edges and borders will appear above and below, at
the outlines of both, only they will not appear equally distinct to the
eye.
262.
Red is proportionally much lighter on black than blue is. The colours
of the edges will therefore appear stronger on the red than on the
blue, which here acts as a dark-grey, but little different from black.
(251.)
263.
The extreme red edge will identify itself with the vermilion colour
of the square, which will thus appear a little elongated in this
direction; while the yellow border immediately underneath it only gives
the red surface a more brilliant appearance, and is not distinguished
without attentive observation.
264.
On the other hand the red edge and yellow border are heterogeneous
with the blue square; a dull red appears at the edge, and a dull green
mingles with the figure, and thus the blue square seems, at a hasty
glance, to be comparatively diminished on this side.
265.
At the lower outline of the two squares a blue edge and a violet border
will appear, and will produce the contrary effect; for the blue edge,
which is heterogeneous with the warm red surface, will vitiate it
and produce a neutral colour, so that the red on this side appears
comparatively reduced and driven upwards, and the violet border on the
black is scarcely perceptible.
266.
On the other hand, the blue apparent edge will identify itself with the
blue square, and not only not reduce, but extend it. The blue edge and
even the violet border next it have the apparent effect of increasing
the surface, and elongating it in that direction.
267.
The effect of homogeneous and heterogeneous edges, as I have now
minutely described it, is so powerful and singular that the two squares
at the first glance seem pushed out of their relative horizontal
position and moved in opposite directions, the red upwards, the blue
downwards. But no one who is accustomed to observe experiments in a
certain succession, and respectively to connect and trace them, will
suffer himself to be deceived by such an unreal effect.
268.
A just impression with regard to this important phenomenon will,
however, much depend on some nice and even troublesome conditions,
which are necessary to produce the illusion in question. Paper should
be tinged with vermilion or the best minium for the red square, and
with deep indigo for the blue square. The blue and red prismatic edges
will then unite imperceptibly with the real surfaces where they are
respectively homogeneous; where they are not, they vitiate the colours
of the squares without producing a very distinct middle tint. The real
red should not incline too much to yellow, otherwise the apparent deep
red edge above will be too distinct; at the same time it should be
somewhat yellow, otherwise the transition to the yellow border will be
too observable. The blue must not be light, otherwise the red edge will
be visible, and the yellow border will produce a too decided green,
while the violet border underneath would not give us the impression of
being part of an elongated light blue square.
269.
All this will be treated more circumstantially hereafter, when we speak
of the apparatus intended to facilitate the experiments connected with
this part of our subject.[2] Every inquirer should prepare the figures
himself, in order fairly to exhibit this specimen of ocular deception,
and at the same time to convince himself that the coloured edges, even
in this case, cannot escape accurate examination.
270.
Meanwhile various other combinations, as exhibited in the plate, are
fully calculated to remove all doubt on this point in the mind of every
attentive observer.
271.
If, for instance, we look at a white square, next the blue one, on a
black ground, the prismatic hues of the opposite edges of the white,
which here occupies the place of the red in the former experiment, will
exhibit themselves in their utmost force. The red edge extends itself
above the level of the blue almost in a greater degree than was the
case with the red square itself in the former experiment. The lower
blue edge, again, is visible in its full force next the white, while,
on the other hand, it cannot be distinguished next the blue square. The
violet border underneath is also much more apparent on the white than
on the blue.
272.
If the observer now compares these double squares, carefully prepared
and arranged one above the other, the red with the white, the two blue
squares together, the blue with the red, the blue with the white, he
will clearly perceive the relations of these surfaces to their coloured
edges and borders.
273.
The edges and their relations to the coloured surfaces appear still
more striking if we look at the coloured squares and a black square
on a white ground; for in this case the illusion before mentioned
ceases altogether, and the effect of the edges is as visible as in
any case that has come under our observation. Let the blue and red
squares be first examined through the prism. In both the blue edge now
appears above; this edge, homogeneous with the blue surface, unites
with it, and appears to extend it upwards, only the blue edge, owing
to its lightness, is somewhat too distinct in its upper portion; the
violet border underneath it is also sufficiently evident on the blue.
The apparent blue edge is, on the other hand, heterogeneous with the
red square; it is neutralised by contrast, and is scarcely visible;
meanwhile the violet border, uniting with the real red, produces a hue
resembling that of the peach-blossom.
274.
If thus, owing to the above causes, the upper outlines of these
squares do not appear level with each other, the correspondence of the
under outlines is the more observable; for since both colours, the red
and the blue, are darks compared with the white (as in the former case
they were light compared with the black), the red edge with its yellow
border appears very distinctly under both. It exhibits itself under the
warm red surface in its full force, and under the dark blue nearly as
it appears under the black: as may be seen if we compare the edges and
borders of the figures placed one above the other on the white ground.
275.
In order to present these experiments with the greatest variety and
perspicuity, squares of various colours are so arranged[3] that the
boundary of the black and white passes through them vertically.
According to the laws now known to us, especially in their application
to coloured objects, we shall find the squares as usual doubly coloured
at each edge; each square will appear to be split in two, and to be
elongated upwards or downwards. We may here call to mind the experiment
with the grey figure seen in like manner on the line of division
between black and white (257).[4]
276.
A phenomenon was before exhibited, even to illusion, in the instance of
a red and blue square on a black ground; in the present experiment the
elongation upwards and downwards of two differently coloured figures
is apparent in the two halves of one and the same figure of one and
the same colour. Thus we are still referred to the coloured edges and
borders, and to the effects of their homogeneous and heterogeneous
relations with respect to the real colours of the objects.
277.
I leave it to observers themselves to compare the various gradations
of coloured squares, placed half on black half on white, only inviting
their attention to the apparent alteration which takes place in
contrary directions; for red and yellow appear elongated upwards if
on a black ground, downwards if on a white; blue, downwards if on a
black ground, upwards if on a white. All which, however, is quite in
accordance with the diffusely detailed examples above given.
278.
Let the observer now turn the figures so that the before-mentioned
squares placed on the line of division between black and white may
be in a horizontal series; the black above, the white underneath. On
looking at these squares through the prism, he will observe that the
red square gains by the addition of two red edges; on more accurate
examination he will observe the yellow border on the red figure, and
the lower yellow border upon the white will be perfectly apparent.
279.
The upper red edge on the blue square is on the other hand hardly
visible; the yellow border next it produces a dull green by mingling
with the figure; the lower red edge and the yellow border are displayed
in lively colours.
280.
After observing that the red figure in these cases appears to gain by
an addition on both sides, while the dark blue, on one side at least,
loses something; we shall see the contrary effect produced by turning
the same figures upside down, so that the white ground be above, the
black below.
281.
For as the homogeneous edges and borders now appear above and below
the blue square, this appears elongated, and a portion of the surface
itself seems even more brilliantly coloured: it is only by attentive
observation that we can distinguish the edges and borders from the
colour of the figure itself.
282.
The yellow and red squares, on the other hand, are comparatively
reduced by the heterogeneous edges in this position of the figures,
and their colours are, to a certain extent, vitiated. The blue edge
in both is almost invisible. The violet border appears as a beautiful
peach-blossom hue on the red, as a very pale colour of the same kind on
the yellow; both the lower edges are green; dull on the red, vivid on
the yellow; the violet border is but faintly perceptible under the red,
but is more apparent under the yellow.
283.
Every inquirer should make it a point to be thoroughly acquainted with
all the appearances here adduced, and not consider it irksome to follow
out a single phenomenon through so many modifying circumstances. These
experiments, it is true, may be multiplied to infinity by differently
coloured figures, upon and between differently coloured grounds. Under
all such circumstances, however, it will be evident to every attentive
observer that coloured squares only appear relatively altered, or
elongated, or reduced by the prism, because an addition of homogeneous
or heterogeneous edges produces an illusion. The inquirer will now
be enabled to do away with this illusion if he has the patience to
go through the experiments one after the other, always comparing the
effects together, and satisfying himself of their correspondence.
Experiments with coloured objects might have been contrived in various
ways: why they have been exhibited precisely in the above mode, and
with so much minuteness, will be seen hereafter. The phenomena,
although formerly not unknown, were much misunderstood; and it was
necessary to investigate them thoroughly to render some portions of our
intended historical view clearer.
284.
In conclusion, we will mention a contrivance by means of which our
scientific readers may be enabled to see these appearances distinctly
at one view, and even in their greatest splendour. Cut in a piece of
pasteboard five perfectly similar square openings of about an inch,
next each other, exactly in a horizontal line: behind these openings
place five coloured glasses in the natural order, orange, yellow,
green, blue, violet. Let the series thus adjusted be fastened in an
opening of the camera obscura, so that the bright sky may be seen
through the squares, or that the sun may shine on them; they will thus
appear very powerfully coloured. Let the spectator now examine them
through the prism, and observe the appearances, already familiar by
the foregoing experiments, with coloured objects, namely, the partly
assisting, partly neutralising effects of the edges and borders, and
the consequent apparent elongation or reduction of the coloured squares
with reference to the horizontal line. The results witnessed by the
observer in this case, entirely correspond with those in the cases
before analysed; we do not, therefore, go through them again in detail,
especially as we shall find frequent occasions hereafter to return to
the subject.--Note P.
[1] Plate 3, fig. 1. The author always recommends making the
experiments on an increased scale, in order to see the prismatic
effects distinctly.
[2] Neither the description of the apparatus nor the recapitulation
of the whole theory, so often alluded to by the author, were ever
given.--T.
[3] Plate 3. fig. 1.
[4] The grey square is introduced in the same plate, fig. 1, above the
coloured squares.
XIX.
ACHROMATISM AND HYPERCHROMATISM.
285.
Formerly when much that is regular and constant in nature was
considered as mere aberration and accident, the colours arising from
refraction were but little attended to, and were looked upon as an
appearance attributable to particular local circumstances.
286.
But after it had been assumed that this appearance of colour
accompanies refraction at all times, it was natural that it should
be considered as intimately and exclusively connected with that
phenomenon; the belief obtaining that the measure of the coloured
appearance was in proportion to the measure of the refraction, and that
they must advance _pari passu_ with each other.
287.
If, again, philosophers ascribed the phenomenon of a stronger or weaker
refraction, not indeed wholly, but in some degree, to the different
density of the medium, (as purer atmospheric air, air charged with
vapours, water, glass, according to their increasing density, increase
the so-called refraction, or displacement of the object;) so they
could hardly doubt that the appearance of colour must increase in the
same proportion; and hence took it for granted, in combining different
mediums which were to counteract refraction, that as long as refraction
existed, the appearance of colour must take place, and that as soon as
the colour disappeared, the refraction also must cease.
288.
Afterwards it was, however, discovered that this relation which was
assumed to correspond, was, in fact, dissimilar; that two mediums can
refract an object with equal power, and yet produce very dissimilar
coloured borders.
289.
It was found that, in addition to the physical principle to which
refraction was ascribed, a chemical one was also to be taken into the
account. We propose to pursue this subject hereafter, in the chemical
division of our inquiry, and we shall have to describe the particulars
of this important discovery in our history of the doctrine of colours.
What follows may suffice for the present.
290.
In mediums of similar or nearly similar refracting power, we find
the remarkable circumstance that a greater and lesser appearance of
colour can be produced by a chemical treatment; the greater effect is
owing, namely, to acids, the lesser to alkalis. If metallic oxydes are
introduced into a common mass of glass, the coloured appearance through
such glasses becomes greatly increased without any perceptible change
of refracting power. That the lesser effect, again, is produced by
alkalis, may be easily supposed.
291.
Those kinds of glass which were first employed after the discovery,
are called flint and crown glass; the first produces the stronger, the
second the fainter appearance of colour.
292.
We shall make use of both these denominations as technical terms in our
present statement, and assume that the refractive power of both is
the same, but that flint-glass produces the coloured appearance more
strongly by one-third than the crown-glass. The diagram (Plate 3, fig.
2,) may serve in illustration.
293.
A black surface is here divided into compartments for more convenient
demonstration: let the spectator imagine five white squares between the
parallel lines _a, b,_ and _c, d_. The square No. 1, is presented to
the naked eye unmoved from its place.
294.
But let the square No. 2, seen through a crown-glass prism _g_, be
supposed to be displaced by refraction three compartments, exhibiting
the coloured borders to a certain extent; again, let the square No. 3,
seen through a flint glass prism _h_, in like manner be moved downwards
three compartments, when it will exhibit the coloured borders by about
a third wider than No. 2.
295.
Again, let us suppose that the square No. 4, has, like No. 2, been
moved downwards three compartments by a prism of crown-glass, and that
then by an oppositely placed prism _h_, of flint-glass, it has been
again raised to its former situation, where it now stands.
296.
Here, it is true, the refraction is done away with by the opposition of
the two; but as the prism _h_, in displacing the square by refraction
through three compartments, produces coloured borders wider by a
third than those produced by the prism _g_, so, notwithstanding the
refraction is neutralised, there must be an excess of coloured border
remaining. (The position of this colour, as usual, depends on the
direction of the apparent motion (204) communicated to the square by
the prism _h_, and, consequently, it is the reverse of the appearance
in the two squares 2 and 3, which have been moved in an opposite
direction.) This excess of colour we have called Hyperchromatism, and
from this the achromatic state may be immediately arrived at.
297.
For assuming that it was the square No. 5 which was removed three
compartments from its first supposed place, like No. 2, by a prism of
crown-glass _g_, it would only be necessary to reduce the angle of a
prism of flint-glass _h_, and to connect it, reversed, to the prism
_g_, in order to raise the square No. 5 two degrees or compartments;
by which means the Hyperchromatism of the first case would cease, the
figure would not quite return to its first position, and yet be already
colourless. The prolonged lines of the united prisms, under No. 5, show
that a single complete prism remains: again, we have only to suppose
the lines curved, and an object-glass presents itself. Such is the
principle of the achromatic telescopes.
298.
For these experiments, a small prism composed of three different
prisms, as prepared in England, is extremely well adapted. It is to be
hoped our own opticians will in future enable every friend of science
to provide himself with this necessary instrument.
XX.
ADVANTAGES OF SUBJECTIVE EXPERIMENTS.--TRANSITION TO THE OBJECTIVE.
299.
We have presented the appearances of colour as exhibited by refraction,
first, by means of subjective experiments; and we have so far arrived
at a definite result, that we have been enabled to deduce the phenomena
in question from the doctrine of semi-transparent mediums and double
images.
300.
In statements which have reference to nature, everything depends on
ocular inspection, and these experiments are the more satisfactory as
they may be easily and conveniently made. Every amateur can procure
his apparatus without much trouble or cost, and if he is a tolerable
adept in pasteboard contrivances, he may even prepare a great part of
his machinery himself. A few plain surfaces, on which black, white,
grey, and coloured objects may be exhibited alternately on a light and
dark ground, are all that is necessary. The spectator fixes them before
him, examines the appearances at the edge of the figures conveniently,
and as long as he pleases; he retires to a greater distance, again
approaches, and accurately observes the progressive states of the
phenomena.
301.
Besides this, the appearances may be observed with sufficient exactness
through small prisms, which need not be of the purest glass. The other
desirable requisites in these glass instruments will, however, be
pointed out in the section which treats of the apparatus.[1]
302.
A great advantage in these experiments, again, is, that they can be
made at any hour of the day in any room, whatever aspect it may have.
We have no need to wait for sunshine, which in general is not very
propitious to northern observers.
[1] This description of the apparatus was never given.
OBJECTIVE EXPERIMENTS.
303.
The objective experiments, on the contrary, necessarily require the
sun-light which, even when it is to be had, may not always have the
most desirable relation with the apparatus placed opposite to it.
Sometimes the sun is too high, sometimes too low, and withal only a
short time in the meridian of the best situated room. It changes its
direction during the observation, the observer is forced to alter
his own position and that of his apparatus, in consequence of which
the experiments in many cases become uncertain. If the sun shines
through the prism it exhibits all inequalities, lines, and bubbles
in the glass, and thus the appearance is rendered confused, dim, and
discoloured.
304.
Yet both kinds of experiments must be investigated with equal accuracy.
They appear to be opposed to each other, and yet are always parallel.
What one order of experiments exhibits the other exhibits likewise,
and yet each has its peculiar capabilities, by means of which certain
effects of nature are made known to us in more than one way.
305.
In the next place there are important phenomena which may be exhibited
by the union of subjective and objective experiments. The latter
experiments again have this advantage, that we can in most cases
represent them by diagrams, and present to view the component relations
of the phenomena. In proceeding, therefore, to describe the objective
experiments, we shall so arrange them that they may always correspond
with the analogous subjective examples; for this reason, too, we annex
to the number of each paragraph the number of the former corresponding
one. But we set out by observing generally that the reader must consult
the plates, that the scientific investigator must be familiar with the
apparatus in order that the twin-phenomena in one mode or the other may
be placed before them.