OPTICAL ILLUSIONS CONTINUED—EXPERIMENTS—THE TALKING HEAD—GHOST
ILLUSIONS.
The enumeration of optical illusions is so considerable that we have
no intention of describing them all, and will merely cite a few other
examples. The following facts have been communicated to us by M.
Nachet:—
[Illustration: Fig. 132.—Hexagonal appearance formed by circles joined
together.]
When examining algæ under the microscope, we notice the spaces which
separate the streaks ornamenting the silicious covering of these
various organisms, and it is explained that they are formed by hexagons
visible only when we examine the object with a powerful microscope.
“For a long time,” says M. Nachet, “I occupied myself with the
examination of the hexagonal appearance of the points constituting
the streaks. Why should these hexagons show themselves, and how could
they be other than the visible base of small pyramids piled very
closely one on the other; and if this were the case, why were not the
points of the little pyramids visible? Or, was the structure before me
analogous to that of the eyes of insects? Then the carapace would be
but a surface of perforated polygonal openings. This latter hypothesis
was attractive enough, and would have explained many things; but some
careful observations with very powerful object-glasses, quite free
from blemishes, had shown me that these hexagons had round points,
contrary to the descriptions of micrographs. These observations,
corroborated by the micrographic photographs of Lackerbauer, the
much-regretted designer, and by Colonels Woodward and Washington, left
not the slightest doubt that it was necessary to discover why the
eye persistently saw hexagons where there were circles. To elucidate
this point, it was necessary to find some means of reproducing
artificially what nature had accomplished with so much precision on
the surfaces of algæ. After many fruitless attempts, I decided on
making a trial of a stereotype plate covered with dots arranged in
quincunxes, very close together” (figs. 132 and 133). “The result was
more successful than I had hoped; the effect produced is exactly that
of the arrangement of the so-called hexagons of the most beautiful
of the algæ, the _Pleurosigma angulata_. If these stereotypes are
examined with one eye only, we shall be immediately convinced that we
have to do with hexagonal polygons.” It is useless to give any long
exposition of a figure so clearly explanatory; it is simply an effect
of the contrast and opposition of the black and white in the sensation
of the retina. This effect is particularly striking with fig. 134, a
negative photograph heliographically engraved according to fig. 133.
In this the white points seem to destroy the black spaces, and to
approach each other tangentially, and the irradiation is so intense
that the white circles appear much larger than the black of fig. 133,
although of the same diameter. There are in these facts many points
which may interest not only students of micrography, but also artists.
As to the algæ, the origin of this investigation, it remains to be
discovered if these circles which cover their silicious carapace are
the projection of small hemispheres, or the section of openings made in
the thick covering. Certain experiments, however, seem to prove that
they are hemispheres, and the theory is also confirmed by a microscopic
photograph from Lackerbauer’s collection, magnified 3,000 diameters,
in which a black central point is seen in the centre of each circle, a
certain reflection of the luminous source reproduced in the focus of
each of the small demi-spheres which constitute the ornament of the
algæ. The microscope, which has progressively shown first the streaks,
then the hexagons, and then the round points, will surely clear up the
point some day or other.
[Illustration: Fig. 133.—Another figure of the same kind.]
[Illustration: Fig. 134.—Third figure.]
Mr. Silvanus P. Thompson, Professor of Physics at University College,
Bristol, has recently presented the French Society of Physical Science
with a curious example of optical illusion, the true cause of which is
not clearly known, but which we may compare with other facts made known
some time ago, of which no precise explanation has been given. Let us
first consider in what the effect discovered by Mr. S. P. Thompson
consists, according to the description that has been given of it by
M. C. M. Gariel; the illustrations here given will also allow of our
verifying the truth of the statements.
[Illustration: Fig. 135. Fig. 136.
Mr. Thompson’s optical illusion. Give a circular movement to these
figures, and the circles will appear to turn round.]
The first illustration consists of a series of concentric circles of
about the width of a millimetre, separated by white intervals of the
same size (fig. 135). These dimensions are not absolute; they vary with
the distance, and may even be a few inches in width if it is desired
to show the phenomenon to a rather numerous auditory. If we hold the
design in the hand, and give it a twirl by a little movement of the
wrist, the circle appears to turn round its centre, and the rotation
is in the same direction, and is equally swift; that is to say, the
circle appears to accomplish a complete turn, whilst the cardboard
really accomplishes one in the same direction. For the second effect
we draw a dark circle, in the interior of which are placed a number
of indentations at regular intervals (fig. 136). Operating in the
same manner as described above, this notched wheel appears to turn
round its centre, but this time in a different direction from the real
movement. In this, however, as in the other design, the effect is more
satisfactory if we do not look directly at it; the movements also are
particularly striking in combinations such as that represented in
fig. 137, in which the multiplicity of circles does not allow us to
fix one specially. We may add that the same effects may be obtained
with eccentric wheels, or even with other curves than circles. By
means of a photograph on glass, Mr. Thompson has been able to reflect
these designs on a screen where they were obtained on a large scale; a
circular movement was communicated to the photographic plate, so that
the design moved in a circular manner on the screen, and in this case
also there existed the illusion that every circle seemed turning round
its centre. And what is the explanation of these curious effects? Mr.
Thompson does not believe (and we share his opinion) that the faculty
possessed by the retina of preserving images during a certain time
(_persistence of impressions on the retina_) can entirely explain these
phenomena. Without desiring to formulate a decided theory, Mr. Thompson
is of opinion that we may class these facts with others which have been
known for some time, and that perhaps it is necessary to attribute to
the eye some new faculty which may explain the whole at once.
[Illustration: Fig. 137.—Another figure of Mr. Thompson’s. The
different circles appear to turn round if we give the design a rotating
movement.]
Brewster and Adams have described phenomena which are equally curious,
the principal of which we will describe, adding also some analogous
investigations due to Mr. Thompson. The result seems to be that there
exists in the eye a badly-defined purpose of nature, which in a certain
way _compensates_ (Brewster) for the real phenomenon, because it has a
contrary effect, which will continue for some time after the cessation
of the phenomena, and which gives by itself a sensation contrary to
that which the real movement would have produced. Thus, after having
fixed our eyes for two or three minutes on a rushing waterfall, if we
suddenly turn our glance on the adjacent rocks, the latter appear to
move from top to bottom. It is not a question here of the effect of
the relative movement to be observed on regarding _simultaneously_ the
falling water and the rocks; if one can succeed in abstracting oneself
to such an extent that the water appears motionless, the rocks appear
to take a contrary movement. In the phenomenon we describe there is no
simultaneous comparison; the eyes are turned _successively_ first on
the water, and then on the rocks. In a rapid river, such as the Rhine
above the fall at Schaffhausen, the stream is not equally swift in
every part, and the current is noticeably more rapid in the middle of
the river than near the banks. If we look fixedly at the centre of the
stream, and then suddenly turn our eyes towards the banks, it will
appear as though the river were flowing back towards its source. This
kind of _compensation_ does not only produce an apparent displacement,
but also changes in size. When travelling at great speed in a railway
train, the objects of the surrounding country as one flies by them
gradually appear smaller and smaller. If, when this occurs, we suddenly
remove our eyes to the interior of the railway carriage, and fix them
on immovable objects, such as the sides of the compartment, or the
faces of our travelling companions, the images on the retina will
really preserve the same size, and yet the objects will appear larger.
Such are some of the interesting facts among those discovered by Mr.
Thompson; and though we do not intend to push the inquiry further,
we think it may not be without interest to describe here another
illusion of that organ whose properties are in every way so curious and
remarkable.
[Illustration: Fig. 138.—Experiment on complementary colours.]
[Illustration: Fig. 139.—Design for experiment or the _punctum cæcum_.]
Another experiment to show the existence of impressions received by the
retina can be made with the figure above (138). If the gaze be fixed
upon the dark spot in the centre of the white figure for about half a
minute, and the eyes then directed to the ceiling, or a sheet of white
paper, the white figure will be reproduced _in black_. This result is
based upon the principle of complementary colours. A red design, for
instance, will be reproduced in green.
There is a dark spot in every human eye—that is, a spot which is
insensible to light. The eye is generally regarded as a perfect
instrument, but it is not yet so by any means. One of our great
philosophers remarked that if an instrument were sent home to him so
full of errors he would feel justified in returning it to the optician.
But the eye has its dark place, the _punctum cæcum_, and it can be
discovered by covering the left eye with the hand, and holding the
present page at arm’s length with the other. Then fix the gaze on the
small cross in the picture, and bring the book close up. At a little
distance the white ball will disappear from the page (fig. 139).
[Illustration: Fig 140.—An optical illusion.]
The illustration (fig. 140) shows us a very curious optical illusion,
and one very easy to practise. Roll up a sheet of paper, and look
through it, as through a telescope, with the right eye, _keeping both
eyes open_. Then place the left hand open palm towards you against the
roll of paper, you will then appear to be looking through a hole in
your left hand. Sometimes the effect is produced without holding up the
other hand to the roll, as shown in fig. 140.
Among optical illusions there are a great number that may be produced
by means of mirrors. The divided telescope is an example. The
apparatus, raised on a firm stand, allows of one apparently seeing an
object through a stone or other opaque object, as shown in fig. 141.
The illustration shows the arrangement of the apparatus. The observer,
looking through it, plainly perceives the object through the glass;
the image is reflected four times before reaching his eye, by means of
small mirrors concealed in the instrument.
[Illustration: Fig. 141.—A divided telescope.]
Convex or concave mirrors distort images in a singular manner, and
produce very interesting effects. _Anamorphoses_ constitute particular
objects belonging specially to the class of experiments relating to
cylindrical mirrors. They are images made according to determined
rules, but so distorted that when regarding them fixedly we can only
distinguish confused strokes. When they are seen reflected in the
curved mirrors, they present, on the contrary, a perfectly regular
appearance. Fig. 142 exhibits an Anamorphosis made by a cylindrical
mirror. It will be seen that the confused image of the horizontal paper
is reflected in the cylindrical mirror, producing the figure of a
juggler. It is easy to contrive similar designs one’s-self; and comical
mirrors may also be employed which produce particular effects of a no
less interesting kind. The next illustration is of a set of figures
which in a cylindrical mirror look like the ten of hearts (fig. 143).
[Illustration: Fig. 142.—Cylindrical Mirror and Anamorphosis.]
[Illustration: Fig. 143.—Anamorphus design for the ten of hearts.]
One of the most remarkable applications of mirrors in amusing
experiments is undoubtedly that of the severed and _talking head_. A
few years back this trick obtained considerable success in Paris and a
number of other towns. The spectators beheld a small space set apart,
in which was placed a table on three legs; on this table was a human
head, placed in cloth on a dish. The head moved its eyes and spoke;
it evidently belonged to a man whose body was completely hidden. The
spectators thought they saw an empty space beneath the table, but the
body of the individual who was really seated there was concealed by
two glasses placed at an inclination of 45° to the walls on the right
and left. The whole was arranged in such a manner that the reflection
of the walls coincided with the visible part of the wall at the back
of the room. The three walls were painted in one colour, and a subdued
light increased the illusion, the effect of which was remarkable (fig.
144).
[Illustration: Fig. 144.—The talking head.]
The spectres designed by Robin also attracted considerable public
attention within recent times. They were images formed by the medium
of transparent glass. Glass panes often produce the phenomenon of
spectres. In the evening, when it is dark out of doors, it is easy
to prove that the reflection of objects in a lighted room can be
reproduced behind the window panes by reason of the darkness outside.
If we approach the window pane, we see also the real objects outside,
a balcony, tree, etc. These real objects mingle with the reflected
image, and combine to produce very curious effects. In this way M.
Robin has contrived the effects of the theatre. He throws on the stage
the reflection of a person dressed as a Zouave, and he himself, armed
with a sabre, stabs the spectre through the body. A great number of
other singular effects have been obtained in the same manner. Pepper’s
Ghost was managed in this way.
[Illustration: Fig. 145.—The ghost effect.]
Within recent times, images produced in a similar way have been
utilized to facilitate the study of drawing. A piece of glass is fixed
vertically on a black board (fig. 146). A model to copy from is placed
on one side of the piece of glass, and is arranged so that the visual
ray passes obliquely through the glass, and we perceive the reflection
of the design very clearly on the other side. It is then very easily
reproduced with a pencil on a sheet of white paper by tracing the
outlines.
[Illustration: Fig. 146.—Drawing by reflection.]