The eye the most important of our organs--Popular description
of it--The eye is the most fertile source of mental
illusions--Disappearance of objects when their images fall
upon the base of the optic nerve--Disappearance of objects
when seen obliquely--Deceptions arising from viewing objects
in a faint light--Luminous figures created by pressure on the
eye, either from external causes or from the fulness of the
blood-vessels--Ocular spectra or accidental colours--Remarkable
effects produced by intense light--Influence of the imagination
in viewing these spectra--Remarkable illusion produced by this
affection of the eye--Duration of impressions of light on the
eye--Thaumatrope--Improvements upon it suggested--Disappearance of
halves of objects or of one of two persons--Insensibility of the eye
to particular colours--Remarkable optical illusion described.
Of all the organs by which we acquire a knowledge of external nature,
the eye is the most remarkable and the most important. By our other
senses the information we obtain is comparatively limited. The touch
and the taste extend no farther than the surface of our own bodies.
The sense of smell is exercised within a very narrow sphere, and that
of recognizing sounds is limited to the distance at which we hear the
bursting of a meteor and the crash of a thunderbolt. But the eye enjoys
a boundless range of observation. It takes cognizance not only of
other worlds belonging to the solar system, but of other systems of
worlds infinitely removed into the immensity of space; and when aided
by the telescope, the invention of human wisdom, it is able to discover
the forms, the phenomena, and the movements of bodies whose distance is
as inexpressible in language as it is inconceivable in thought.
While the human eye has been admired by ordinary observers for the
beauty of its form, the power of its movements, and the variety of its
expression, it has excited the wonder of philosophers by the exquisite
mechanism of its interior, and its singular adaptation to the variety
of purposes which it has to serve. The eyeball is nearly globular,
and is about an inch in diameter. It is formed externally by a tough
opaque membrane called the _sclerotic_ coat, which forms the white of
the eye, with the exception of a small circular portion in front called
the _cornea_. This portion is perfectly transparent, and so tough in
its nature as to afford a powerful resistance to external injury.
Immediately within the cornea, and in contact with it, is the _aqueous_
humour, a clear fluid, which occupies only a small part of the front
of the eye. Within this humour is the iris, a circular membrane, with
a hole in its centre called the _pupil_. The colour of the eye resides
in this membrane, which has the curious property of contracting and
expanding so as to diminish or enlarge the pupil,--an effect which
human ingenuity has not been able even to imitate. Behind the iris is
suspended the _crystalline_ lens, in a fine transparent capsule or bag
of the same form with itself. It is then succeeded by the _vitreous
humour_, which resembles the transparent white of an egg, and fills
up the rest of the eye. Behind the vitreous humour, there is spread
out on the inside of the eyeball a fine delicate membrane, called the
_retina_, which is an expansion of the _optic nerve_, entering the back
of the eye and communicating with the brain.
A perspective view and horizontal section of the left eye, shown in the
annexed figure, will convey a popular idea of its structure. It is,
as it were, a small camera obscura, by means of which the pictures of
external objects are painted on the retina, and, in a way of which we
are ignorant, it conveys the impression of them to the brain.
[Illustration: Fig. 1.]
This wonderful organ may be considered as the sentinel which guards the
pass between the worlds of matter and of spirit, and through which all
their communications are interchanged. The optic nerve is the channel
by which the mind peruses the hand-writing of Nature on the retina,
and through which it transfers to that material tablet its decisions
and its creations. The eye is consequently the principal seat of the
supernatural. When the indications of the marvellous are addressed to
us through the ear, the mind may be startled without being deceived,
and reason may succeed in suggesting some probable source of the
illusion by which we have been alarmed: but when the eye in solitude
sees before it the forms of life, fresh in their colours and vivid in
their outline; when distant or departed friends are suddenly presented
to its view; when visible bodies disappear and reappear without any
intelligible cause; and when it beholds objects, whether real or
imaginary, for whose presence no cause can be assigned, the conviction
of supernatural agency becomes, under ordinary circumstances,
unavoidable.
Hence it is not only an amusing but a useful occupation to acquire a
knowledge of those causes which are capable of producing so strange a
belief, whether it arises from the delusions which the mind practises
upon itself, or from the dexterity and science of others. I shall
therefore proceed to explain those illusions which have their origin in
the eye, whether they are general, or only occasionally exhibited in
particular persons, and under particular circumstances.
There are few persons aware that when they look with one eye, there is
some particular object before them to which they are absolutely blind.
If we look with the right eye, this point is always about 15° to the
right of the object which we are viewing, or to the right of the axis
of the eye or the point of most distinct vision. If we look with the
left eye, the point is as far to the left. In order to be convinced
of this curious fact, which was discovered by M. Mariotte, place two
coloured wafers upon a sheet of white paper at the distance of three
inches, and look at the left-hand wafer with the right eye at the
distance of about 11 or 12 inches, taking care to keep the eye straight
above the wafer, and the line which joins the eyes parallel to the line
which joins the wafers. When this is done, and the left eye closed, the
right-hand wafer will no longer be visible. The same effect will be
produced if we close the right eye and look with the left eye at the
right-hand wafer. When we examine the retina to discover to what part
of it this insensibility to light belongs, we find that the image of
the invisible wafer has fallen on the base of the optic nerve, or the
place where this nerve enters the eye and expands itself to form the
retina. This point is shown in the preceding figure by a convexity at
the place where the nerve enters the eye.
But though light of ordinary intensity makes no impression upon this
part of the eye, a very strong light does, and even when we use candles
or highly luminous bodies in place of wafers, the body does not wholly
disappear, but leaves behind a faint cloudy light, without, however,
giving anything like an image of the object from which the light
proceeds.
When the objects are _white_ wafers upon a _black_ ground, the white
wafer absolutely disappears, and the space which it covers appears to
be completely black; and as the light which illuminates a landscape
is not much different from that of a white wafer, we should expect,
whether we use one or both eyes,[2] to see a black or a dark spot
upon every landscape, within 15° of the point which most particularly
attracts our notice. The Divine Artificer, however, has not left his
work thus imperfect. Though the base of the optic nerve is insensible
to light that falls directly upon it, yet it has been made susceptible
of receiving luminous impressions from the parts which surround it; and
the consequence of this is, that when the wafer disappears, the spot
which is occupied, in place of being black, has always the same colour
as the ground upon which the wafer is laid, being white when the wafer
is placed upon a white ground, and red when it is placed upon a red
ground. This curious effect may be rudely illustrated by comparing the
retina to a sheet of blotting-paper, and the base of the optic nerve to
a circular portion of it covered with a piece of sponge. If a shower
falls upon the paper, the protected part will not be wetted by the rain
which falls upon the sponge that covers it, but in a few seconds it
will be as effectually wetted by the moisture which it absorbs from the
wet paper with which it is surrounded. In like manner the insensible
spot on the retina is stimulated by a borrowed light, and the apparent
defect is so completely removed, that its existence can be determined
only by the experiment already described.
[2] When both eyes are open, the object whose image falls upon the
insensible spot of the one eye is seen by the other, so that, though
it is not invisible, yet it will only be half as luminous and,
therefore two dark spots ought to be seen.
Of the same character, but far more general in its effects, and
important in its consequences, is another illusion of the eye which
presented itself to me several years ago. When the eye is steadily
occupied in viewing any particular object, or when it takes a fixed
direction while the mind is occupied with any engrossing topic of
speculation or of grief, it suddenly loses sight of, or becomes blind
to, objects seen indirectly, or upon which it is not fully directed.
This takes place whether we use one or both eyes, and the object which
disappears will reappear without any change in the position of the
eye, while other objects will vanish and revive in succession without
any apparent cause. If a sportsman, for example, is watching with
intense interest the motions of one of his dogs, his companion, though
seen with perfect clearness by indirect vision, will vanish, and the
light of the heath or of the sky will close in upon the spot which he
occupied.
In order to witness this illusion, put a little bit of white paper on
a green cloth, and, within three or four inches of it, place a narrow
strip of white paper. At the distance of twelve or eighteen inches, fix
one eye steadily upon the little bit of white paper, and in a short
time a part or even the whole of the strip of paper will vanish as if
it had been removed from the green cloth. It will again reappear, and
again vanish, the effect depending greatly on the steadiness with which
the eye is kept fixed. This illusion takes place when both the eyes are
open, though it is easier to observe it when one of them is closed. The
same thing happens when the object is luminous. When a candle is thus
seen by indirect vision, it never wholly disappears, but it spreads
itself out into a cloudy mass, the centre of which is blue, encircled
with a bright ring of yellow light.
This inability of the eye to preserve a sustained vision of objects
seen obliquely, is curiously compensated by the greater sensibility of
those parts of the eye that have this defect. The eye has the power
of seeing objects with perfect distinctness only when it is directed
straight upon them; that is, all objects seen indirectly are seen
indistinctly: but it is a curious circumstance, that when we wish to
obtain a sight of a very faint star, such as one of the satellites of
Saturn, we can see it most distinctly _by looking away from it_, and
when the eye is turned full upon it it immediately disappears.
Effects still more remarkable are produced in the eye when it views
objects that are difficult to be seen from the small degree of light
with which they happen to be illuminated. The imperfect view which we
obtain of such objects forces us to fix the eye more steadily upon
them; but the more exertion we make to ascertain what they are, the
greater difficulties do we encounter to accomplish our object. The
eye is actually thrown into a state of the most painful agitation,
the object will swell and contract, and partly disappear, and it will
again become visible when the eye has recovered from the delirium into
which it has been thrown. This phenomenon may be most distinctly seen
when the objects in a room are illuminated with the feeble gleam of a
fire almost extinguished; but it may be observed in daylight by the
sportsman when he endeavours to mark upon the monotonous heath the
particular spot where moor-game has alighted. Availing himself of
the slightest difference of tint in the adjacent heath, he keeps his
eye steadily fixed on it as he advances, but whenever the contrast of
illumination is feeble, he will invariably lose sight of his mark,
and if the retina is capable of taking it up, it is only to lose it a
second time.
This illusion is likely to be most efficacious in the dark, when there
is just sufficient light to render white objects faintly visible, and
to persons who are either timid or credulous must prove a frequent
source of alarm. Its influence, too, is greatly aided by another
condition of the eye, into which it is thrown during partial darkness.
The pupil expands nearly to the whole width of the iris, in order to
collect the feeble light which prevails; but it is demonstrable that
in this state the eye cannot accommodate itself to see near objects
distinctly, so that the forms of persons and things actually become
more shadowy and confused when they come within the very distance at
which we count upon obtaining the best view of them. These affections
of the eye are, we are persuaded, very frequent causes of a particular
class of apparitions which are seen at night by the young and the
ignorant. The spectres which are conjured up are always _white_,
because no other colour can be seen, and they are either formed out of
inanimate objects which reflect more light than others around them,
or of animals or human beings whose colour or change of place renders
them more visible in the dark. When the eye dimly descries an inanimate
object whose different parts reflect different degrees of light, its
brighter parts may enable the spectator to keep up a continued view
of it; but the disappearance and reappearance of its fainter parts,
and the change of shape which ensues, will necessarily give it the
semblance of a living form, and if it occupies a position which is
unapproachable, and where animate objects cannot find their way, the
mind will soon transfer to it a supernatural existence. In like manner
a human figure shadowed forth in a feeble twilight may undergo similar
changes, and after being distinctly seen while it is in a situation
favourable for receiving and reflecting light, it may suddenly
disappear in a position fully before, and within the reach of, the
observer’s eye; and if this evanescence takes place in a path or road
where there was no side-way by which the figure could escape, it is
not easy for an ordinary mind to efface the impression which it cannot
fail to receive. Under such circumstances we never think of distrusting
an organ which we have never found to deceive us; and the truth of
the maxim that “seeing is believing” is too universally admitted, and
too deeply rooted in our nature, to admit on any occasion of a single
exception.
In these observations we have supposed that the spectator bears along
with him no fears or prejudices, and is a faithful interpreter of the
phenomena presented to his senses; but if he is himself a believer
in apparitions, and unwilling to receive an ocular demonstration of
their reality, it is not difficult to conceive the picture which will
be drawn when external objects are distorted and caricatured by the
imperfect indications of his senses, and coloured with all the vivid
hues of the imagination.
Another class of ocular deceptions have their origin in a property of
the eye which has been very imperfectly examined. The fine nervous
fabric which constitutes the retina, and which extends to the brain,
has the singular property of being _phosphorescent by pressure_. When
we press the eyeball outwards by applying the point of the finger
between it and the nose, a circle of light will be seen, which Sir
Isaac Newton describes as “a circle of colours like those in the
feather of a peacock’s tail.” He adds, that “if the eye and the
figure remain quiet, these colours vanish in a second of time; but
if the finger be moved with a quavering motion, they appear again.”
In the numerous observations which I have made on these luminous
circles, I have never been able to observe any colour but white, with
the exception of a general red tinge which is seen when the eyelids
are closed, and which is produced by the light which passes through
them. The luminous circles, too, always continue while the pressure
is applied, and they may be produced as readily after the eye has
been long in darkness as when it has been recently exposed to light.
When the pressure is very gently applied, so as to compress the fine
pulpy substance of the retina, light is immediately created when the
eye is in total darkness; and when in this state light is allowed to
fall upon it, the part compressed is more sensible to light than any
other part, and consequently appears more luminous. If we increase the
pressure, the eyeball, being filled with incompressible fluids, will
protrude all round the point of pressure, and consequently the retina
at the protruded part will be _compressed_ by the outward pressure
of the contained fluid, while the retina on each side, namely, under
the point of pressure and beyond the protruded part, will be drawn
towards the protruded part or _dilated_. Hence the part under the
finger which was originally compressed is now _dilated_, the adjacent
parts _compressed_, and the more remote parts immediately without this
_dilated_ also. Now we have observed, that when the eye is, under
these circumstances, exposed to light, there is a bright luminous
circle shading off externally and internally into total darkness.
We are led, therefore, to the important conclusions, that when the
retina is compressed in total darkness it gives out light; that when
it is compressed when exposed to light, its sensibility to light is
increased; and that when it is _dilated under exposure to light, it
becomes absolutely blind, or insensible to all luminous impressions_.
When the body is in a state of perfect health, this phosphorescence
of the eye shows itself on many occasions. When the eye or the head
receives a sudden blow, a bright flash of light shoots from the
eyeball. In the act of sneezing, gleams of light are emitted from each
eye both during the inhalation of the air, and during its subsequent
protrusion, and in blowing air violently through the nostrils, two
patches of light appear above the axis of the eye and in front of it,
while other two luminous spots unite into one, and appear as it were
about the point of the nose when the eyes are directed to it. When
we turn the eyeball by the action of its own muscles, the retina is
affected at the place where the muscles are inserted, and there may
be seen opposite each eye, and towards the nose, two semicircles of
light, and other two extremely faint towards the temples. At particular
times, when the retina is more phosphorescent than at others, these
semicircles are expanded into complete circles of light.
In a state of indisposition, the phosphorescence of the retina
appears in new and more alarming forms. When the stomach is under a
temporary derangement accompanied with headache, the pressure of the
blood-vessels upon the retina shows itself, in total darkness, by a
faint blue light floating before the eye, varying in its shape, and
passing away at one side. This blue light increases in intensity,
becomes _green_ and then _yellow_, and sometimes rises to _red_, all
these colours being frequently seen at once, or the mass of light
shades off into darkness. When we consider the variety of distinct
forms which in a state of perfect health the imagination can conjure
up when looking into a burning fire, or upon an irregularly shaded
surface,[3] it is easy to conceive how the masses of coloured light
which float before the eye may be moulded by the same power into those
fantastic and natural shapes, which so often haunt the couch of the
invalid, even when the mind retains its energy, and is conscious of the
illusion under which it labours. In other cases, temporary blindness
is produced by pressure upon the optic nerve, or upon the retina; and
under the excitation of fever or delirium, when the physical cause
which produces spectral forms is at its height, there is superadded a
powerful influence of the mind, which imparts a new character to the
phantasms of the senses.
[3] A very curious example of the influence of the imagination in
creating distinct forms out of an irregularly shaded surface, is
mentioned in the life of Peter Heaman, a Swede, who was executed for
piracy and murder at Leith in 1822. We give it in his own words:--
“One remarkable thing was, one day as we mended a sail, it being a
very thin one, after laying it upon deck in folds, I took the tar
brush and tarred it over in the places which I thought needed to be
strengthened. But when we hoisted it up, I was astonished to see that
the tar I had put upon it represented a gallows and a man under it
without a head. The head was lying beside him. He was complete, body,
thighs, legs, arms, and in every shape like a man. Now, I oftentimes
made remarks upon it, and repeated them to the others. I always said
to them all, You may depend upon it that something will happen. I
afterwards took down the sail on a calm day, and sewed a piece of
canvas over the figure to cover it, for I could not bear to have it
always before my eyes.”
In order to complete the history of the illusions which originate in
the eye, it will be necessary to give some account of the phenomena
called _ocular spectra_, or _accidental colours_. If we cut a figure
out of red paper, and, placing it on a sheet of white paper, view it
steadily for some seconds with one or both eyes fixed on a particular
part of it, we shall observe the red colour to become less brilliant.
If we then turn the eye from the red figure upon the white paper,
we shall see a distinct _green_ figure, which is the _spectrum_, or
accidental colour of the _red_ figure. With differently coloured
figures we shall observe differently coloured spectra, as in the
following table:--
COLOUR OF THE COLOUR OF THE
ORIGINAL FIGURES. SPECTRAL FIGURES.
Red, Bluish-green.
Orange, Blue.
Yellow, Indigo.
Green, Reddish-violet.
Blue, Orange-red.
Indigo, Orange-yellow.
Violet, Yellow.
White, Black.
Black, White.
The two last of these experiments, viz., white and black figures, may
be satisfactorily made by using a white medallion on a dark ground, and
a black profile figure. The spectrum of the former will be found to be
black, and that of the latter white.
These ocular spectra often show themselves without any effort on
our part, and even without our knowledge. In a highly painted room,
illuminated by the sun, those parts of the furniture on which the sun
does not directly fall have always the opposite or accidental colour.
If the sun shines through a chink in a _red_ window-curtain, its light
will appear _green_, varying as in the above table, with the colour of
the curtain; and if we look at the image of a candle, reflected from
the water in a _blue_ finger-glass, it will appear _yellow_. Whenever,
in short, the eye is affected with one prevailing colour, it sees at
the same time the spectral or accidental colour, just as when a musical
string is vibrating, the ear hears at the same time its fundamental and
its harmonic sounds.
If the prevailing light is _white_ and _very strong_, the spectra which
it produces are no longer black, but of various colours in succession.
If we look at the sun, for example, when near the horizon, or when
reflected from glass or water so as to moderate its brilliancy, and
keep the eye upon it steadily for a few seconds, we shall see, even for
hours afterwards, and whether the eyes are open or shut, a spectrum
of the sun varying in its colours. At first, with the eye open, it is
_brownish-red_ with a _sky-blue_ border, and when the eye is shut,
it is _green_ with a _red_ border. The _red_ becomes more brilliant,
and the _blue_ more vivid, till the impression is gradually worn off;
but even when they become very faint, they may be revived by a gentle
pressure on the eyeball.
Some eyes are more susceptible than others of these spectral
impressions, and Mr. Boyle mentions an individual who continued for
years to see the spectre of the sun when he looked upon bright objects.
This fact appeared to Locke so interesting and inexplicable, that he
consulted Sir Isaac Newton respecting its cause, and drew from him the
following interesting account of a similar effect upon himself:--“The
observation you mention in Mr. Boyle’s book of colours, I once made
upon myself with the hazard of my eyes. The manner was this: I looked
a very little while upon the sun in the looking-glass with my right
eye, and then turned my eyes into a dark corner of my chamber, and
winked, to observe the impression made, and the circles of colours
which encompassed it, and how they decayed by degrees, and at last
vanished. This I repeated a second and a third time. At the third time,
when the phantasm of light and colours about it were almost vanished,
intending my fancy upon them to see their last appearance, I found, to
my amazement, that they began to return, and by little and little to
become as lively and vivid as when I had newly looked upon the sun.
But when I ceased to intend my face upon them, they vanished again.
After this I found that as often as I went into the dark, and intended
my mind upon them, as when a man looks earnestly to see anything which
is difficult to be seen, I could make the phantasm return without
looking any more upon the sun; and the oftener I made it return, the
more easily I could make it return again. And at length, by repeating
this without looking any more upon the sun, I made such an impression
on my eye, that, if I looked upon the clouds, or a book, or any bright
object, I saw upon it a round bright spot of light like the sun, and,
which is still stranger, though I looked upon the sun with my right eye
only, and not with my left, yet my fancy began to make an impression
upon my left eye as well as upon my right. For if I shut my right eye,
and looked upon a book or the clouds with my left eye, I could see the
spectrum of the sun almost as plain as with my right eye, if I did
but intend my fancy a little while upon it: for at first, if I shut
my right eye, and looked with my left, the spectrum of the sun did
not appear till I intended my fancy upon it; but by repeating, this
appeared every time more easily. And now in a few hours’ time I had
brought my eyes to such a pass, that I could look upon no bright object
with either eye but I saw the sun before me, so that I durst neither
write nor read; but to recover the use of my eyes, shut myself up in
my chamber, made dark, for three days together, and used all means
in my power to direct my imagination from the sun. For if I thought
upon him, I presently saw his picture, though I was in the dark. But
by keeping in the dark; and employing my mind about other things, I
began, in three or four days, to have more use of my eyes again; and
by forbearing to look upon bright objects, recovered them pretty well;
though not so well but that, for some months after, the spectrums
of the sun began to return as often as I began to meditate upon the
phenomena, even though I lay in bed at midnight with my curtains drawn.
But now I have been well for many years, though I am apt to think, if
I durst venture my eyes, I could still make the phantasm return by
the power of my fancy. This story I tell you, to let you understand,
that in the observation related by Mr. Boyle, the man’s fancy probably
concurred with the impression made by the sun’s light to produce that
phantasm of the sun which he constantly saw in bright objects.”[4]
[4] See the _Edinburgh Encyclopædia_, Art. ACCIDENTAL COLOURS.
I am not aware of any effects that had the character of supernatural
having been actually produced by the causes above described; but it
is obvious, that if a living figure had been projected against the
strong light which imprinted these durable spectra of the sun, which
might really happen when the solar rays are reflected from water, and
diffused by its ruffled surface, this figure would have necessarily
accompanied all the luminous spectres which the fancy created. Even in
ordinary lights, strange appearances may be produced by even transient
impressions; and if I am not greatly mistaken, the case which I am
about to mention is not only one which may occur, but which actually
happened. A figure dressed in _black_, and mounted upon a _white_
horse, was riding along, exposed to the bright rays of the sun,
which, through a small opening in the clouds, was throwing its light
only upon that part of the landscape. The _black_ figure was projected
against a white cloud, and the white horse shone with particular
brilliancy by its contrast with the dark soil against which it was
seen. A person interested in the arrival of such a stranger had been
for some time following his movements with intense anxiety, but, upon
his disappearance behind a wood, was surprised to observe the spectre
of the mounted stranger in the form of a _white_ rider upon a _black_
steed, and this spectre was seen for some time in the sky, or upon
any pale ground to which the eye was directed. Such an occurrence,
especially if accompanied with a suitable combination of events, might,
even in modern times, have formed a chapter in the history of the
marvellous.
It is a curious circumstance, that when the image of an object is
impressed upon the retina only for a few moments, the picture which
is left is exactly of the same colour with the object. If we look,
for example, at a window at some distance from the eye, and then
transfer the eye quickly to the wall, we shall see it distinctly, but
momentarily, with _light_ panes and _dark_ bars; but in a space of
time incalculably short, this picture is succeeded by the spectral
impression of the window, which will consist of _black_ panes and
_white_ bars. The similar spectrum, or that of the same colour as the
object, is finely seen in the experiment of forming luminous circles by
whirling round a burning stick, in which case the circles are always
red.
In virtue of this property of the eye, an object may be seen in
many places at once; and we may even exhibit at the same instant
the two opposite sides of the same object, or two pictures painted
on the opposite sides of a piece of card. It was found by a French
philosopher, M. D’Arcet, that the impression of light continued on the
retina about the eighth part of a second after the luminous body was
withdrawn, and upon this principle Dr. Paris has constructed the pretty
little instrument called the _Thaumatrope_, or the _Wonder-turner_. It
consists of a number of circular pieces of card, about two or three
inches broad, which may be twirled round with great velocity by the
application of the fore-finger and thumb of each hand to pieces of silk
string attached to opposite points of their circumference. On each
side of the circular piece of card is painted part of a picture, or
a part of a figure, in such a manner that the two parts would form a
group or a whole figure, if we could see both sides at once. Harlequin,
for example, is painted on one side, and Columbine on the other, so
that by twirling round the card the two are seen at the same time in
their usual mode of combination. The body of a Turk is drawn on one
side, and his head on the reverse, and by the rotation of the card the
head is replaced upon his shoulders. The principle of this illusion
may be extended to many other contrivances. Part of a sentence may be
written on one side of a card, and the rest on the reverse. Particular
letters may be given on one side, and others upon the other, or even
halves or parts of each letter may be put upon each side, or all these
contrivances may be combined, so that the sentiment which they express
can be understood only when all the scattered parts are united by the
revolution of the card.
As the revolving card is virtually transparent, so that bodies beyond
it can be seen through it, the power of the illusion might be greatly
extended by introducing into the picture other figures, either animate
or inanimate. The setting sun, for example, might be introduced into a
landscape; part of the flame of a fire might be seen to issue from the
crater of a volcano, and cattle grazing in a field might make part of
the revolutionary landscape. For such purposes, however, the form of
the instrument would require to be completely changed, and the rotation
should be effected round a standing axis by wheels and pinions, and a
screen placed in front of the revolving plane with open compartments
or apertures, through which the principal figures would appear. Had
the principle of this instrument been known to the ancients, it would
doubtless have formed a powerful engine of delusion in their temples,
and might have been more effective than the optical means which they
seem to have employed for producing the apparitions of their gods.
In certain diseased conditions of the eye, effects of a very remarkable
kind are produced. The faculty of seeing objects double is too common
to be noticed as remarkable; and though it may take place with only one
eye, yet, as it generally arises from a transient inability to direct
the axis of both eyes to the same point, it excites little notice.
That state of the eye, however, in which we lose sight of half of
every object at which we look, is more alarming and more likely to be
ascribed to the disappearance of part of the object than to a defect
of sight. Dr. Wollaston, who experienced this defect twice, informs us
that, after taking violent exercise, he “suddenly found that he could
see but half of a man whom he met, and that on attempting to read the
name of JOHNSON over a door, he saw only SON, the commencement of the
name being wholly obliterated from his view.” In this instance, the
part of the object which disappeared was towards his left; but on a
second occurrence of the same affection, the part which disappeared
was towards his right. There are many occasions on which this defect
of the eye might alarm the person who witnessed it for the first time.
At certain distances from the eye one of two persons would necessarily
disappear; and by a slight change of position either in the observer
or the person observed, the person that vanished would reappear, while
the other would disappear in his turn. The circumstances under which
these evanescences would take place could not be supposed to occur to
an ordinary observer, even if he should be aware that the cause had its
origin in himself. When a phenomenon so strange is seen by a person in
perfect health, as it generally is, and who has never had occasion to
distrust the testimony of his senses, he can scarcely refer it to any
other cause than a supernatural one.
Among the affections of the eye which not only deceive the person who
is subject to them, but those also who witness their operation, may be
enumerated the insensibility of the eye to particular colours. This
defect is not accompanied with any imperfection of vision, or connected
with any disease either of a local or a general nature, and it has
hitherto been observed in persons who possess a strong and a sharp
sight. Mr. Huddart has described the case of one Harris, a shoemaker
at Maryport in Cumberland, who was subject to this defect in a very
remarkable degree. He seems to have been insensible to every colour,
and to have been capable of recognizing only the two opposite tints
of _black_ and _white_. “His first suspicion of this defect arose
when he was about four years old. Having by accident found in the
street a child’s stocking, he carried it to a neighbouring house to
inquire for the owner. He observed the people call it a _red stocking_,
though he did not understand why they gave it that denomination, as he
himself thought it completely described by being called a stocking.
The circumstance, however, remained in his memory, and, with other
subsequent observations, led him to the knowledge of his defect. He
observed also, that when young, other children could discern cherries
on a tree by some pretended difference of colour, though he could only
distinguish them from the leaves by their difference of size and shape.
He observed also, that by means of this difference of colour, they
could see the cherries at a greater distance than he could, though he
could see other objects at as great a distance as they, that is, where
the sight was not assisted by the colour.” Harris had two brothers,
whose perception of colours was nearly as defective as his own. One of
these, whom Mr. Huddart examined, constantly mistook _light green_ for
_yellow_, and _orange_ for _grass green_.
Mr. Scott has described, in the Philosophical Transactions, his own
defect in perceiving colours. He states that he does not know any
_green_ in the world; that a _pink_ colour and a _pale blue_ are
perfectly alike; that he has often thought a _full red_ and a _full
green_ a good match; that he is sometimes baffled in distinguishing
a _full purple_ from a _deep blue_, but that he knows light, dark,
and middle _yellows_, and all degrees of _blue_ except _sky-blue_. “I
married my daughter to a genteel, worthy man, a few years ago; the
day before the marriage, he came to my house dressed in a new suit of
fine cloth clothes. I was much displeased that he should come, as I
supposed, in _black_, and said that he should go back to change his
colour. But my daughter said, No, no; the colour is very genteel; that
it was my eyes that deceived me. He was a gentleman of the law, in a
fine, rich, claret-coloured dress, which is as much black to my eyes as
any black that ever was dyed.” Mr. Scott’s father, his maternal uncle,
one of his sisters, and her two sons, had all the same imperfection.
Dr. Nichol has recorded a case where a naval officer purchased a
_blue_ uniform coat and waistcoat with _red_ breeches to match the
blue, and Mr. Harvey describes the case of a tailor at Plymouth, who
on one occasion repaired an article of dress with _crimson_ in place
of _black_ silk, and on another patched the elbow of a _blue_ coat
with a piece of _crimson_ cloth. It deserves to be remarked that our
celebrated countrymen, the late Mr. Dugald Stewart, Mr. Dalton, and
Mr. Troughton, have a similar difficulty in distinguishing colours.
Mr. Stewart discovered this defect when one of his family was admiring
the beauty of a Siberian crab-apple, which he could not distinguish
from the leaves but by its form and size. Mr. Dalton cannot distinguish
_blue_ from pink, and the solar spectrum consists only of two colours,
_yellow_ and _blue_. Mr. Troughton regards _red ruddy pinks_, and
brilliant _oranges_, as _yellows_, and _greens_ as _blues_, so that he
is capable only of appreciating _blue_ and _yellow_ colours.
In all those cases which have been carefully studied, at least in
three of them, in which I have had the advantage of making personal
observations, namely, those of Mr. Troughton, Mr. Dalton, and Mr.
Liston, the eye is capable of seeing the whole of the prismatic
spectrum, the red space appearing to be yellow. If the red space
consisted of homogeneous or simple red rays, we should be led to infer
that the eyes in question were not insensible to red light, but were
merely incapable of discriminating between the impressions of red and
yellow light. I have lately shown, however, that the prismatic spectrum
consists of three equal and coincident spectra of _red_, _yellow_, and
_blue_ light, and consequently, that much yellow and a small portion
of blue light exist in the red space; and hence it follows, that those
eyes which see only two colours, viz. _yellow_ and _blue_, in the
spectrum, are really insensible to the red light of the spectrum, and
see only the yellow with the small portion of blue with which the red
is mixed. The faintness of the yellow light which is thus seen in the
red space, confirms the opinion that the retina has not appreciated the
influence of the simple red rays.
If one of the two travellers who, in the fable of the chameleon, are
made to quarrel about the colour of that singular animal, had happened
to possess this defect of sight, they would have encountered at every
step of their journey, new grounds of dissension, without the chance
of finding an umpire who could pronounce a satisfactory decision.
Under certain circumstances, indeed, the arbiter might set aside the
opinions of both the disputants, and render it necessary to appeal to
some higher authority,
---- to beg he’d tell them if he knew
Whether the thing was _red_ or _blue_.
[Illustration: _Fig. 2._]
In the course of writing the preceding observations an ocular illusion
occurred to myself of so extraordinary a nature, that I am convinced
it never was seen before, and I think it far from probable that it
will ever be seen again. Upon directing my eyes to the candles that
were standing before me, I was surprised to observe, apparently among
my hair, and nearly straight above my head, and far without the range
of vision, a distinct image of one of the candles inclined about 45°
to the horizon, as shown at A in Fig. 2. The image was as distinct and
perfect as if it had been formed by reflection from a piece of mirror
glass, though of course much less brilliant, and the position of the
image proved that it must be formed by reflection from a perfectly flat
and highly polished surface. But where such a surface could be placed,
and how, even if it were fixed, it could reflect the image of the
candle up through my head, were difficulties not a little perplexing.
Thinking that it might be something lodged in the eyebrow, I covered
it up from the light, but the image still retained its place. I then
examined the eyelashes with as little success, and was driven to the
extreme supposition that a crystallization was taking place in some
part of the aqueous humour of the eye, and that the image was formed by
the reflection of the light of the candle from one of the crystalline
faces. In this state of uncertainty, and, I may add, of anxiety, for
this last supposition was by no means an agreeable one, I set myself
down to examine the phenomenon experimentally. I found that the image
varied its place by the motion of the head and of the eyeball, which
proved that it was either attached to the eyeball or occupied a place
where it was affected by that motion. Upon inclining the candle at
different angles, the image suffered corresponding variations of
position. In order to determine the exact place of the reflecting
substance, I now took an opaque circular body and held it between
the eye and the candle till it eclipsed the mysterious image. By
bringing the body nearer and nearer the eyeball till its shadow became
sufficiently distinct to be seen, it was easy to determine the locality
of the reflector, because the shadow of the opaque body must fall
upon it whenever the image of the candle was eclipsed. In this way I
ascertained that the reflecting body was in the upper eyelash; and I
found, that, in consequence of being disturbed, it had twice changed
its inclination, so as to represent a vertical candle in the horizontal
position B, and afterwards in the inverted position C. Still, however,
I sought for it in vain, and even with the aid of a magnifier I
could not discover it. At last, however, Mrs. B., who possesses the
perfect vision of short-sighted persons, discovered, after repeated
examinations, between two eyelashes, a minute speck, which, upon being
removed with great difficulty, turned out to be a chip of red wax not
above the hundredth part of an inch in diameter, and having its surface
so perfectly flat and so highly polished that I could see in it the
same image of the candle, by placing it extremely near the eye. This
chip of wax had no doubt received its flatness and its polish from the
surface of a seal, and had started into my eye when breaking the seal
of a letter.
That this reflecting substance was the cause of the image of the
candle, cannot admit of a doubt; but the wonder still remains how the
images which it formed occupied so mysterious a place as to be seen
without the range of vision, and apparently through the head. In order
to explain this, let _m_ _n_, Fig. 2, be a lateral view of the eye. The
chip of wax was placed at _m_ at the root of the eyelashes, and being
nearly in contact with the outer surface of the cornea, the light of
the candle, which it reflected, passed very obliquely through the pupil
and fell upon the retina somewhere to the left of _n_, very near where
the retina terminates; but a ray thus falling obliquely on the retina
is seen, in virtue of the law of visible direction already explained,
in a line _n_ C perpendicular to the retina at the point near _n_,
where the ray fell. Hence the candle was necessarily seen through the
head as it were of the observer, and without the range of ordinary
vision. The comparative brightness of the reflected image still
surprises me; but even this, if the image really was brighter, may
be explained by the fact, that it was formed on a part of the retina
upon which light had never before fallen, and which may therefore
be supposed to be more sensible, than the parts of the membrane in
constant use, to luminous impressions.
Independent of its interest as an example of the marvellous in vision,
the preceding fact may be considered as a proof that the retina retains
its power to its very termination near the ciliary processes, and that
the law of visible direction holds true even without the range of
ordinary vision. It is therefore possible that a reflecting surface
favourably placed on the outside of the eye, or that a reflecting
surface in the inside of the eye, may cause a luminous image to fall
nearly on the extreme margin of the retina, the consequence of which
would be, that it would be seen in the back of the head, half way
between a vertical and a horizontal line.