Miscellaneous optical illusions--Conversion of cameos into intaglios,
or elevations into depressions, and the reverse--Explanation of
this class of deceptions--Singular effects of illumination with
light of one simple colour--Lamps for producing homogeneous yellow
light--Methods of increasing the effect of this exhibition--Method
of reading the inscription of coins in the dark--Art of deciphering
the effaced inscription of coins--Explanation of these singular
effects--Apparent motion of the eyes in portraits--Remarkable examples
of this--Apparent motion of the features of a portrait, when the
eyes are made to move--Remarkable experiment of breathing light and
darkness.
In the preceding letter I have given an account of the most important
instruments of Natural Magic which depend on optical principles: but
there still remain several miscellaneous phenomena on which the stamp
of the marvellous is deeply impressed, and the study of which is
pregnant with instruction and amusement.
One of the most curious of these is that false perception in vision
by which we conceive depressions to be elevations, and elevations
depressions, or by which intaglios are converted into cameos, and
cameos into intaglios. This curious fact seems to have been first
observed at one of the early meetings of the Royal Society of London,
when one of the members, in looking at a guinea through a compound
microscope of new construction, was surprised to see the head upon the
coin depressed, while other members could only see it embossed as it
really was.
While using telescopes and compound microscopes, Dr. Gmelin of
Wurtemburg observed the same fact. The protuberant parts of objects
appeared to him depressed, and the depressed parts protuberant: but
what perplexed him extremely, this illusion took place at some times
and not at others, in some experiments and not in others, and appeared
to some eyes and not to others.
After making a great number of experiments, Dr. Gmelin is said to
have constantly observed the following effects: Whenever he viewed
any object rising upon a plane of any colour whatever, provided it
was neither white nor shining, and provided the eye and the optical
tube were directly opposite to it, the elevated parts appeared
depressed, and the depressed parts elevated. This happened when he
was viewing a seal, and as often as he held the tube of the telescope
perpendicularly, and applied it in such a manner that its whole
surface almost covered the last glass of the tube. The same effect was
produced when a compound microscope was used. When the object hung
perpendicularly, from a plane, and the tube was supported horizontally
and directly opposite to it, the illusion also took place, and the
appearance was not altered when the object hung obliquely and even
horizontally. Dr. Gmelin is said to have at last discovered a method
of preventing this illusion, which was by looking, not towards the
centre of the convexity, but at first to the edges of it only, and then
gradually taking in the whole. “But why these things should so happen,
he did not pretend to determine.”
[Illustration: _Fig. 14._]
The best method of observing this deception is to view the engraved
seal of a watch with the eyepiece of an achromatic telescope, or with
a compound microscope, or any combination of lenses which inverts the
objects that are viewed through it.[11] The depression in the seal
will immediately appear an elevation, like the wax impression which
is taken from it; and though we know it to be hollow, and feel its
concavity with the point of our finger, the illusion is so strong
that it continues to appear a protuberance. The cause of this will be
understood from Fig. 14, where S is the window of the apartment, or
the light which illuminates the _hollow_ seal LR, whose shaded side
is of course on the same side L with the light. If we now invert the
seal, with one or more lenses, so that it may look in the opposite
direction, it will appear to the eye as in Fig. 15, with the shaded
side L farthest from the window. But as we know that the window is
still on our left hand, and that the light falls in the direction RL,
and as everybody with its shaded side farthest from the light must
necessarily be convex or protuberant, we immediately believe that the
hollow seal is now a cameo or bas-relief. The proof which the eye thus
receives of the seal being raised, overcomes the evidence of its being
hollow, derived from our actual knowledge, and from the sense of touch.
In this experiment the deception takes place from our knowing the real
direction of the light which falls upon the seal; for if the place of
the window, with respect to the seal, had been inverted as well as the
seal itself, the illusion could not have taken place.
[11] A single convex lens will answer the purpose, provided we hold
the eye six or eight inches behind the image of the seal formed in its
conjugate focus.
[Illustration: _Fig. 15._]
[Illustration: _Fig. 16._]
In order to explain this better, let us suppose the seal LR, Fig. 14,
to be illuminated with a candle S, the place of which we can change at
pleasure. If we invert LR, it will rise into a cameo, as in Fig. 15;
and if we then place another candle S on the other side of it, as in
Fig. 16, the hollow seal will be equally illuminated on all sides, and
it will sink down into a cavity or intaglio. If the two candles do not
illuminate the seal equally, or if any accidental circumstance produces
a belief that the light is wholly or principally on one side, the mind
will entertain a corresponding opinion respecting the state of the
seal, regarding it as a hollow if it believes the light to come wholly
or principally from the right hand, and as a cameo if it believes the
light to come from the left hand.
If we use a small telescope to invert the seal, and if we cover up
all the candle but the flame, and arrange the experiment so that the
candle may be inverted along with the image, the seal will still retain
its concavity, because the shadow is still on the same side with the
illuminating body.
If we make the same experiments with the raised impression of the seal
taken upon wax, we shall observe the very same phenomena, the seal
being depressed when it alone is inverted, and retaining its convexity
when the light is inverted along with it.
The illusion, therefore, under our consideration is the result of an
operation of our own minds, whereby we judge of the forms of bodies by
the knowledge we have acquired of light and shadow. Hence the illusion
depends on the accuracy and extent of our knowledge on this subject;
and while some persons are under its influence, others are entirely
insensible to it. When the seal or hollow cavity is not polished, but
ground, and the surface round it of uniform colour and smoothness,
almost every person, whether young or old, learned or ignorant, will be
subject to the illusion; because the youngest and the most careless
observers cannot but know that the shadow of a hollow is always on
the side next the light, and the shadow of a protuberance on the side
opposite to the light; but if the object is the raised impression of a
seal upon wax, I have found that, when inverted, it still seemed raised
to the three youngest of six persons, while the three eldest were
subject to the deception.
[Illustration: _Fig. 17._]
This illusion may be dissipated by a process of reasoning arising from
the introduction of a new circumstance in the experiment. Thus, let RL,
Fig. 17, be the inverted seal, which consequently appears raised, and
let an opaque and unpolished pin, A, be placed on one side of the seal.
Its shadow will be of course opposite the candle as at B. In this case
the seal, which had become a cameo by its inversion, will now sink down
into a cavity by the introduction of the pin and its shadow; for as the
pin and its shadow are inverted, as shown in Fig. 18, while the candle
retains its place, the shadow of the pin falling in the direction AB
is a stronger proof to the eye that the light is coming from the right
hand, than the actual knowledge of the candle being on the left hand,
and therefore the cameo necessarily sinks into a cavity, or the shadow
is now on the same side as the light. This experiment will explain to
us why on some occasions an acute observer will elude the deception,
while every other person is subject to it. Let us suppose that a
particle of dust, or a little bit of wax, capable of giving a shadow,
is adhering to the surface of the seal, an ordinary observer will
take no notice of this, or if he does, he will probably not make it a
subject of consideration, and will therefore see the head on the seal
raised into a cameo; but the attentive observer, noticing the little
protuberance, and observing that its shadow lies to the left of it,
will instantly infer that the light comes in that direction, and will
still see the seal hollow.
[Illustration: _Fig. 18._]
I have already mentioned that in some cases even the sense of touch
does not correct the erroneous perception. We of course feel that the
part of the hollow on which the finger is placed is actually hollow;
but if we look at the other part of the hollow it will still appear
raised.
By using two candles yielding different degrees of light, and thus
giving an uncertainty to the direction of the light, we may weaken the
illusion in any degree we choose, so as to overpower it by touch, or by
a process of reasoning.
I have had occasion to observe a series of analogous phenomena arising
from the same cause, but produced without any instrument for inverting
the object. If AB, for example, is a plate of mother-of-pearl, and LR
a circular or any other cavity (Fig. 19) ground or turned in it, then
if this cavity is illuminated by a candle or a window at S, in place of
there being a shadow of the margin L of the hollow next the light, as
there would have been had the body been opaque, a quantity of bright
refracted light will appear where there would have been a shadow, and
the rest of the cavity will be comparatively obscure, as if it were
in shade. The necessary consequence of this is, that the cavity will
appear as an elevation when seen only by the naked eye, as it is only
an elevated surface that could have its most luminous side at L.
[Illustration: _Fig. 19._]
Similar illusions take place in certain pieces of polished wood,
chalcedony, and mother-of-pearl, where the surface is perfectly
smooth. This arises from there being at that place a knot or growth,
or nodule, of different transparency from the surrounding mass, and
the cause of it will be understood from Fig. 20. Let _m_ _o_ be the
surface of a mahogany table, _m_ A _o_ B a section of the table, and
_m_ _n_ _o_ a section of a knot more transparent than the rest of the
mass. Owing to the transparency of the thin edge at _o_, opposite to
the candle S, the side _o_ is illuminated, while the rest of the knot
is comparatively dark, so that, on the principles already explained,
the spot _m n o_ appears to be a hollow in the table. From this cause
arises the appearance of dimples in certain plates of chalcedony,
called hammered chalcedony, owing to its having the look of being
dimpled with a hammer. The surface on which these cavities are seen is
a section of small spherical aggregations of siliceous matter, which
exhibit the same phenomena as the cavities in wood. Mother-of-pearl
presents the very same phenomena, and it is indeed so common in this
substance, that it is nearly impossible to find a mother-of-pearl
button or counter which seems to have its surface flat, although they
are perfectly so when examined by the touch. Owing to the different
refraction of the incident light by the different growths of the shell
cut in different directions by the artificial surface, like the annual
growth of wood in a dressed plank, the surface has necessarily an
unequal and undulating appearance.
[Illustration: _Fig. 20._]
Among the wonders of science there are perhaps none more surprising
than the effects produced upon coloured objects by illuminating
them with homogeneous light, or light of one colour. The light which
emanates from the sun, and by which all the objects of the material
world are exhibited to us, is composed of three different colours,
_red_, _yellow_, and _blue_, by the mixture of which in different
proportions all the various hues of nature may be produced. These three
colours, when mixed in the proportion in which they occur in the sun’s
rays, compose a purely white light; but if any body on which this white
light falls shall absorb, or stop, or detain within its substance any
part of any one or more of these simple colours, it will appear to the
eye of that colour which arises from the mixture of all the rays which
it does not absorb, or of that colour which white light would have if
deprived of the colours which are absorbed. Scarlet cloth, for example,
absorbs most of the blue rays and many of the yellow, and hence appears
_red_. Yellow cloth absorbs most of the blue and many of the red rays,
and therefore appears yellow; and blue cloth absorbs most of the yellow
and red rays. If we were to illuminate the _scarlet_ cloth with pure
and unmixed _yellow_ light, it would appear _yellow_, because the
scarlet cloth does not absorb all the yellow rays, but reflects some
of them; and if we illuminate _blue_ cloth with yellow light, it will
appear nearly _black_, because it absorbs all the yellow light, and
reflects almost none of it. But whatever be the nature and colour of
the bodies on which the yellow light falls, the light which it reflects
must be yellow, for no other light falls upon them, and those which are
not capable of reflecting yellow light must appear absolutely black,
however brilliant be their colour in the light of day.
[Illustration: _Fig. 21._]
As the methods now discovered of producing yellow light in abundance
were not known to the ancient conjurors, nor even to those of later
times, they have never availed themselves of this valuable resource. It
has been long known that salt thrown into the wick of a flame produces
yellow light, but this light is mixed with blue and green rays, and is,
besides, so small in quantity, that it illuminates objects only that
are in the immediate vicinity of the flame. A method which I have found
capable of producing it in abundance is shown in Fig. 21, where AB is a
lamp, containing at A a large quantity of alcohol and water, or ardent
spirits, which gradually descends into a platina or metallic cup D.
This cup is strongly heated by a spirit-lamp L, inclosed in a dark
lantern, and when the diluted alcohol in D is inflamed, it will burn
with a fierce and powerful yellow flame. If the flame should not be
perfectly yellow, owing to an excess of alcohol, a proportion of salt
thrown into the cup will answer the same purpose as a further dilution
of the alcohol.[12]
[12] See _Edinburgh Transactions_, vol. ix., p. 435.
A monochromatic lamp for producing yellow light may be constructed most
effectually, by employing a portable gas lamp, containing compressed
oil gas. If we allow the gas to escape in a copious stream, and set it
on fire, it will form an explosive mixture with the atmospheric air,
and will no longer burn with a white flame, but will emit a bluish
and reddish light. The force of the issuing gas, or any accidental
current of air, is capable of blowing out this flame, so that it is
necessary to have a contrivance for sustaining it. The method which I
used for this purpose is shown in Fig. 22. A small gas tube _a b c_,
arising from the main burner MN of the gas lamp PQ, terminates above
the burner, and has a short tube _d e_, moveable up and down within it,
so as to be gas-tight. This tube _d e_, closed at _e_, communicates
with the hollow ring _f g_, in the inside of which four apertures are
perforated in such a manner as to throw their jets of gas to the apex
of a cone, of which _f g_ is the base. When we cause the gas to flow
from the burner M, by opening the main cock A, it will rush into the
tube _a b c d_, and issue in small flames at the four holes in the
ring _f g_. The size of these flames is regulated by the cock _b_.
The inflammation, therefore, of the ignited gas will be sustained by
these four subsidiary flames through which it passes, independent of
any agitation of the air, or of the force with which it issues from
the burner. On a projecting arm _e h_, carrying a ring _h_, I fixed a
broad collar, made of coarse cotton wick, which had been previously
soaked in a saturated solution of common salt. When the gas was allowed
to escape at M, with such force as to produce a long and broad column
of an explosive mixture of gas and atmospheric air, the bluish flame
occasioned by the explosion passes through the salted collar, and is
converted by it into a mass of homogeneous yellow light. This collar
will last a long time without any fresh supply of salt, so that the gas
lamp will yield a permanent monochromatic yellow flame, which will
last as long as there is gas in the reservoir. In place of a collar
of cotton wick, a hollow cylinder of sponge, with numerous projecting
tufts, may be used, or a collar may be similarly constructed with
asbestos cloth, and, if thought necessary, it might be supplied with a
saline solution from a capillary fountain.
[Illustration: _Fig. 22._]
Having thus obtained the means of illuminating any apartment with
yellow light, let the exhibition be made in a room with furniture of
various bright colours, with oil or water-coloured paintings on the
wall. The party which is to witness the experiment should be dressed
in a diversity of the gayest colours; and the brightest-coloured
flowers and highly-coloured drawings should be placed on the tables.
The room being at first lighted with ordinary lights, the bright and
gay colours of everything that it contains will be finely displayed.
If the white lights are now suddenly extinguished, and the yellow
lamps lighted, the most appalling metamorphosis will be exhibited. The
astonished individuals will no longer be able to recognize each other.
All the furniture in the room, and all the objects which it contains,
will exhibit only one colour. The flowers will lose their hues. The
paintings and drawings will appear as if they were executed in China
ink; and the gayest dresses, the brightest scarlets, the purest lilacs,
the richest blues, and the most vivid greens, will all be converted
into one monotonous yellow. The complexions of the parties, too, will
suffer a corresponding change. One pallid, death-like yellow,
---- like the unnatural hue
Which autumn paints upon the perished leaf,
will envelope the young and the old, and the sallow faces will alone
escape from the metamorphosis. Each individual derives merriment from
the cadaverous appearance of his neighbour, without being sensible that
he is himself one of the ghostly assemblage.
If, in the midst of the astonishment which is thus created, the white
lights are restored at one end of the room, while the yellow lights are
taken to the other end, one side of the dress of every person, namely,
that next the white light, will be restored to its original colours,
while the other side will retain its yellow hue. One cheek will appear
in a state of health and colour, while the other retains the paleness
of death; and, as the individuals change their position, they will
exhibit the most extraordinary transformations of colour.
If, when all the lights are yellow, beams of white light are
transmitted through a number of holes, like those in a sieve, each
luminous spot will restore the colour of the dress or furniture upon
which it falls, and the nankeen family will appear all mottled over
with every variety of tint. If a magic lantern is employed to throw
upon the walls or upon the dresses of the company luminous figures of
flowers or animals, the dresses will be painted with these figures
in the real colour of the dress itself. Those alone who appeared in
yellow, and with yellow complexions, will, to a great degree, escape
all these singular changes.
If red and blue light could be produced with the same facility and in
the same abundance as yellow light, the illumination of the apartment
with these lights in succession would add to the variety and wonder of
the exhibition. The red light might perhaps be procured in sufficient
quantity from the nitrate and other salts of strontian; but it would
be difficult to obtain a blue flame of sufficient intensity for the
suitable illumination of a large room. Brilliant white lights, however,
might be used, having for screens glass troughs containing a mass
one or two inches thick of a solution of the ammoniacal carbonate of
copper. This solution absorbs all the rays of the spectrum but the
blue, and the intensity of the blue light thus produced would increase
in the same proportion as the white light employed.
Amongst the numerous experiments with which science astonishes and
sometimes even strikes terror into the ignorant, there is none more
calculated to produce this effect than that of displaying to the eye in
absolute darkness the legend or inscription upon a coin. To do this,
take a silver coin (I have always used an old one), and after polishing
the surface as much as possible, make the parts of it which are raised
rough by the action of an acid, the parts not raised, or those which
are to be rendered darkest, retaining their polish. If the coin thus
prepared is placed upon a mass of red-hot iron, and removed into a dark
room, the inscription upon it will become less luminous than the rest,
so that it may be distinctly read by the spectator. The mass of red-hot
iron should be concealed from the observer’s eye, both for the purpose
of rendering the eye fitter for observing the effect, and of removing
all doubt that the inscription is really read in the dark, that is,
without receiving any light, direct or reflected, from any other body.
If, in place of polishing the depressed parts and roughening its raised
parts, we make the raised parts polished and roughen the depressed
parts, the inscription will now be less luminous than the depressed
parts, and we shall still be able to read it, from its being as it were
written in black letters on a white ground. The first time I made this
experiment, without being aware of what would be the result, I used
a French shilling of Louis XV., and I was not a little surprised to
observe upon its surface, in black letters, the inscription BENEDICTUM
SIT NOMEN DEI.
The most surprising form of this experiment is when we use a coin
from which the inscription has been either wholly obliterated, or
obliterated in such a degree as to be illegible. When such a coin is
laid upon the red-hot iron, the letters and figures become oxidated,
and the film of oxide radiating more powerfully than the rest of the
coin, the illegible inscription may be now distinctly read, to the
great surprise of the observer, who had examined the blank surface of
the coin previous to its being placed upon the hot iron. The different
appearances of the same coin, according as the raised parts are
polished or roughened, are shown in Fig. 23 and 24.
In order to explain the cause of these remarkable effects, we must
notice a method which has been long known, though never explained, of
deciphering the inscriptions on worn-out coins. This is done by merely
placing the coin upon a hot iron; an oxidation takes place over the
whole surface of the coin, the film of oxide changing its tint with the
intensity or continuance of the heat. The parts, however, where the
letters of the inscription had existed, oxidate at a different rate
from the surrounding parts, so that these letters exhibit their shape,
and become legible in consequence of the film of oxide which covers
them having a different thickness, and therefore reflecting a different
tint from that of the adjacent parts. The tints thus developed
sometimes pass through many orders of brilliant colours, particularly
_pink_ and _green_, and settle in a bronze, and sometimes a black tint,
resting upon the inscription alone. In some cases the tint left on the
trace of the letters is so very faint that it can just be seen, and may
be entirely removed by a slight rub of the finger.
[Illustration: _Fig. 23._]
[Illustration: _Fig. 24._]
When the experiment is often repeated with the same coin, and the
oxidations successively removed after each experiment, the film of
oxide continues to diminish, and at last ceases to make its appearance.
It recovers the property, however, in the course of time. When the
coin is put upon the hot iron, and consequently when the oxidation
is the greatest, a considerable smoke arises from the coin, and this
diminishes like the film of oxide by frequent repetition. A coin
which had ceased to emit this smoke, smoked slightly after having been
exposed twelve hours to the air. I have found, from numerous trials,
that it is always the raised parts of the coin, and in modern coins the
elevated ledge round the inscription, that become first oxidated. In an
English shilling of 1816, this ledge exhibited a brilliant yellow tint
before it appeared on any other part of the coin.
If we use a uniform and homogeneous disc of silver that has never been
hammered or compressed, its surface will oxidate equally, provided all
its parts are equally heated. In the process of converting this disc
into a coin, the _sunk_ parts have obviously been _most compressed_
by the prominent parts of the die, and the _elevated_ parts _least
compressed_, the metal being in the latter left as it were in its
natural state. The raised letters and figures on a coin have therefore
less density than the other parts, and these parts oxidate sooner or
at a lower temperature. When the letters of the legend are worn off by
friction, the parts immediately below them have also less density than
the surrounding metal, and the site as it were of the letters therefore
receives from heat a degree of oxidation, and a colour different from
that of the surrounding surface. Hence we obtain an explanation of the
revival of the invisible letters by oxidation.
The same influence of difference of density may be observed in
the beautiful oxidations which are produced on the surface of
highly-polished steel, heated in contact with air, at temperatures
between 430° and 630° of Fahrenheit.[13] When the steel has hard
portions called pins by the workmen, the uniform tint of the film of
oxide stops near these hard portions, which always exhibit colours
different from those of the rest of the mass. These parts, on account
of their increased density, absorb the oxygen of atmospheric air less
copiously than the surrounding portions. Hence we see the cause why
steel expanded by heat absorbs oxygen, which when united with the
metal, forms the coloured superficial film. As the heat increases,
a greater quantity of oxygen is absorbed, and the film increases in
thickness.
[13] See _Edinburgh Encyclopædia_, Art. STEEL, vol. xviii., p. 387.
These observations enable us to explain the legibility of inscriptions
in the dark, whether the coin is in a perfect state, or the letters
of it worn off. All _black_ or _rough_ surfaces radiate light more
copiously than _polished_ or _smooth_ surfaces, and hence the
inscription is _luminous_ when it is _rough_, and _obscure_ when it is
polished, and the letters covered with black oxide are more luminous
than the adjacent parts, on account of the superior radiation of light
by the black oxide which covers them.
By the means now described, invisible writing might be conveyed by
impressing it upon a metallic surface, and afterwards erasing it by
grinding and polishing that surface perfectly smooth. When exposed to
a proper degree of heat, the secret would display itself written in
oxidated letters. Many amusing experiments might be made upon the same
principle.
A series of curious and sometimes alarming deceptions, arises from the
representation of objects in perspective upon a plane surface. One of
the most interesting of these depends on the principles which regulate
the apparent direction of the eyes in a portrait. Dr. Wollaston has
thought this subject of sufficient importance to be treated at some
length in the Philosophical Transactions. When we look at any person
we direct to them both our face and our eyes, and in this position
the circular iris will be in the middle of the white of the eye ball,
or, what is the same thing, there will be the same quantity of white
on each side of the iris. If the eyes are now moved to either side,
while the head remains fixed, we shall readily judge of the change of
their direction by the greater or less quantity of white on each side
of the iris. This test, however, accurate as it is, enables us only to
estimate the extent to which the eyes deviate in direction from the
direction of the face to which they belong. But their direction in
reference to the person who views them is entirely a different matter;
and Dr. Wollaston is of opinion, that we are not guided by the eyes
alone, but are unconsciously aided by the concurrent position of the
entire face.
[Illustration: _Fig. 26._]
If a skilful painter draws a pair of eyes with great correctness
directed to the spectator, and deviating from the general position of
the face as much as is usual in good portraits, it is very difficult
to determine their direction, and they will appear to have different
directions to different persons. But what is very curious, Dr.
Wollaston has shown that the same pair of eyes may be made to direct
themselves either to or from the spectator by the addition of other
features in which the position of the face is changed. Thus, in Fig.
25, the pair of eyes are looking intently at the spectator, and the
face has a corresponding direction; but when we cover up the face in
Fig. 25 with the face in Fig. 26, which looks to the right, the eyes
change their direction, and look to the right also. In like manner,
eyes drawn originally to look a little to the right or the left of
the spectator, may be made to look directly at him by adding suitable
features.
[Illustration: _Fig. 25._]
The nose is obviously the principal feature which produces this
change of direction, as it is more subject to change of perspective
than any of the other features; but Dr. Wollaston has shown by a very
accurate experiment, that even a small portion of the nose introduced
with the features will carry the eyes along with it. He obtained four
exact copies of the same pair of eyes looking at the spectator, by
transferring them upon copper from a steel plate, and having added to
each of two pair of them a nose, in one case directed to the right,
and in the other to the left, and to each of the other two pair a very
small portion of the upper part of the nose, all the four pair of eyes
lost their front direction, and looked to the right or to the left,
according to the direction of the nose, or of the portion of it which
was added.
But the effect thus produced is not limited, as Dr. Wollaston
remarks, to the mere change in the direction of the eyes, “for a
total difference of character may be given to the same eyes by a due
representation of the other features. A lost look of devout abstraction
in an uplifted countenance, may be exchanged for an appearance of
inquisitive archness in the leer of a younger face turned downwards and
obliquely towards the opposite side,” as in Fig. 27, 28. This, however,
is perhaps not an exact expression of the fact. The new character
which is said to be given to the eyes is given only to the eyes in
combination with the new features, or, what is probably more correct,
the inquisitive archness is in the other features, and the eye does not
belie it.
Dr. Wollaston has not noticed the converse of these illusions, in which
a change of direction is given to fixed features by a change in the
direction of the eyes. This effect is finely seen in some magic lantern
sliders, where a pair of eyes is made to move in the head of a figure,
which invariably follows the motion of the eyeballs.
[Illustration: _Fig. 28._]
[Illustration: _Fig. 27._]
Having thus determined the influence which the general perspective of
the face has upon the apparent direction of the eyes in a portrait, Dr.
Wollaston applies it to the explanation of the well-known fact, that
when the eyes of a portrait look at a spectator in front of it they
will follow him, and appear to look at him in every other direction.
This curious fact, which has received less consideration than it
merits, has been often skilfully employed by the novelist, in alarming
the fears or exciting the courage of his hero. On returning to the
hall of his ancestors, his attention is powerfully fixed on the grim
portraits which surround him. The parts which they have respectively
performed in the family history rise to his mind: his own actions,
whether good or evil, are called up in contrast, and as the preserver
or the destroyer of his line, he stands, as it were, in judgment before
them. His imagination, thus excited by conflicting feelings, transfers
a sort of vitality to the canvas, and if the personages do not “start
from their frames,” they will at least bend upon him their frowns or
their approbation. It is in vain that he tries to evade their scrutiny.
Wherever he goes their eyes eagerly pursue him; they will seem even to
look at him over their shoulders, and he will find it impossible to
shun their gaze but by quitting the apartment.
As the spectator in this case changes his position in a horizontal
plane, the effect which we have described is accompanied by an apparent
diminution in the breadth of the human face, from only seven or eight
inches till it disappears at a great obliquity. In moving, therefore,
from a front view to the most oblique view of the face, the change in
its apparent breadth is so slow that the apparent motion of the head of
the figure is scarcely recognized as it follows the spectator. But if
the perspective figure has a great breadth in a horizontal plane, such
as a soldier firing his musket, an artilleryman his piece of ordnance,
a bowman drawing his bow, or a lancer pushing his spear, the apparent
breadth of the figure will vary from five to six feet or upwards till
it disappears, and therefore the change of apparent magnitude is
sufficiently rapid to give the figure the dreaded appearance of turning
round, and following the spectator. One of the best examples of this
must have been often observed in the foreshortened figure of a dead
body lying horizontally, which has the appearance of following the
observer with great rapidity, and turning round upon the head as the
centre of motion.
The cause of this phenomenon is easily explained. Let us suppose a
portrait with its face and its eyes directed straight in front, so as
to look at the spectator. Let a straight line be drawn through the
tip of the nose and half way between the eyes, which we shall call the
middle line. On each side of this middle line there will be the same
breadth of head, of cheek, of chin, and of neck, and each iris will be
in the middle of the white of the eye. If we now go to one side, the
apparent horizontal breadth of every part of the head and face will
be diminished, but the parts on each side of the middle line will be
diminished equally, and at any position, however oblique, there will be
the same breadth of face on each side of the middle line, and the iris
will be in the centre of the white of the eyeball, so that the portrait
preserves all the characters of a figure looking at the spectator, and
must necessarily do so wherever he stands.
This explanation might be illustrated by a picture which represents
three artillerymen, each firing a piece of ordnance in parallel
directions. Let the gun of the middle one be pointed accurately to the
eye of the spectator, so that he sees neither its right side nor its
left, nor its upper nor its under side, but directly down its muzzle,
so that if there was an opening in the breech he would see through
it. In like manner the spectator will see the left side of the gun on
his left hand, and the right side of the gun on his right hand. If
the spectator now changes his place, and takes ever such an oblique
position, either laterally or vertically, he must still see the same
thing; because nothing else is presented to his view. The gun of the
middle soldier must always point to his eye, and the other guns to the
right and left of him. They must therefore all three seem to move as
he moves, and follow his eye in all its changes of place. The same
observations are of course applicable to buildings and streets seen in
perspective.
In common portraits the apparent motion of the head is generally
rendered indistinct by the canvas being imperfectly stretched, as
the slightest concavity or convexity entirely deforms the face when
the obliquity is considerable. The deception is therefore best seen
when the painting is executed on a very flat board, and in colours
sufficiently vivid to represent every line in the face with tolerable
distinctness at great obliquities. This distinctness of outline is
indeed necessary to a satisfactory exhibition of this optical illusion.
The most perfect exhibition, indeed, that I ever saw of it was in the
case of a painting of a ship upon a sign-board executed in strongly
gilt lines. It contained a view of the stern and side of a ship in the
stocks, and, owing to the flatness of the board and the brightness of
the lines, the gradual development of the figure, from the most violent
foreshortening at great obliquities till it attained its perfect form,
was an effect which surprised every person that saw it.
[Illustration: _Fig. 29._]
The only other optical illusion which our limits will permit us to
explain, is the very remarkable experiment of what may be truly called
_breathing light or darkness_. Let S be a candle whose light falls at
an angle of 56° 45´ upon two glass plates A, B, placed close to each
other, and let the reflected rays AC, BD, fall at the same angle upon
two similar plates, C, D, but so placed that the plane of reflexion
from the latter is at right angles to the plane of reflexion from the
former. An eye placed at E, and looking at the same time into the two
plates C and D, will see very faint images of the candle S, which by a
slight adjustment of the plates, may be made to disappear almost wholly
allowing the plate C to remain as it is, change the position of D, till
its inclination to the ray BD is diminished about 3°, or made nearly
53° 11´. When this is done, the image that had disappeared on looking
into D will be restored, so that the spectator at E, upon looking into
the two mirrors C, D, will see no light in C, because the candle has
nearly disappeared, while the candle is distinctly seen in D. If, while
the spectator is looking into these two mirrors, either he or another
person breathes upon them gently and quickly, the breath will revive
the extinguished image in C, and will extinguish the visible image
in D. The following is the cause of this singular result. The light
AC, BD, is polarized by reflexion from the plates A, B, because it is
incident at the polarizing angle of 56° 45´ for glass. When we breathe
upon the plates C, D, we form upon their surface a thin film of water,
whose polarizing angle is 53° 11´, so that if the polarized rays AC,
BD, fell upon the plates C, D, at an angle of 53° 11´, the candle from
which they proceeded would not be visible, or they would not suffer
reflexion from the plates C, D. At all other angles the light would
be reflected and the candles visible. Now the plate D is placed at an
angle of 53° 11´ and C at an angle of 56° 45´, so that when a film
of water is breathed upon them the light will be reflected from the
latter, and none from the former; that is, the act of breathing upon
the plates will restore the invisible and extinguish the visible image.