imitate the colours and forms of nature, we are never deceived into
supposing that we have the real object before us. There must, therefore,
be something different in the conditions under which we see real objects
from those under which we view their pictures. The most favourable
circumstances for receiving an illusive impression of solidity from a
flat picture, is when we view it from a fixed position and with one eye.
This is because one means by which we unconsciously estimate distances
depends upon the changes in the perspective appearances of objects
caused by changes in our point of view. In many cases these changes in
the perspective are the only means we have of judging of the relative
distances of objects. But there is another circumstance which is still
more intimately connected with our perception of solidity. Each eye
receives a slightly different image of the objects before us (unless
these be extremely remote), inasmuch as they are viewed from a different
point. When the objects are very near, the two retinal images may differ
considerably, as the reader may convince himself by viewing with each
eye, alternately, objects immediately before him, while the other eye is
closed, and the head all the while motionless. The nearer objects will
plainly appear to shift their positions as seen against the back-ground
of the more distant objects; and a somewhat more careful observation
will reveal changes of perspective, or apparent form, in every one of
these objects. An extreme case is presented in that of a playing card,
or thin book, held in the plane which divides the eyes. The back or the
face, the one side or the other, will be seen, according as the right or
the left eye is opened. If we close the left eye, the displacement and
change of apparent form produced by a slight movement of the head are
sufficiently obvious; a movement of the head 2½ in. to the left causes a
decided change in the relative positions of adjacent objects. It is
plain, however, that it is precisely from a point 2½ in. to the left
that the left eye views these objects, and hence the perspective
appearance seen by the left eye must have the difference due to this
shifting of the point of view.
On the other hand, if one looks at a picture, or flat surface, placed
immediately in front, no change in the relative positions of its parts
is discernible by viewing it with either eye alternately. Not but that
there is a difference in the retinal images in the two cases, but there
is an absence of any point of comparison by which the change may be
judged. If we take a photograph of a statue, it will, when viewed by one
or the other eye, present the difference of the retinal images which is
due to a flat surface; the parts of the photographic image will be of
slightly different proportions as seen by each eye. If, instead of the
photograph we have before our eyes a statuette, each eye will see a
quite different view: the right eye will see a portion which is
invisible to the left eye, and _vice versâ_, and, in fact, we shall see
more than half round the object. Here, then, we have certain differences
of the retinal pictures when solid objects are viewed, and these
differences by innumerable repetitions have, unconsciously to ourselves,
become associated with notions of solidity, of something having length,
breadth, and depth, or thickness. The marvellous delicacy of these
perceptions will be alluded to hereafter.
Let us suppose that the lenses of two cameras are fixed in the positions
occupied by the two eyes, and that a photograph is taken in each camera,
the subject being, for example, a statuette. It is obvious that the
differences of the two photographs would correspond with the differences
of the two retinal images, and that, if a person could view with the
right eye only the photograph taken in the right-hand camera, and with
the left eye the left-hand photograph only, there would be formed on the
retinæ of his eyes images very nearly corresponding with those which the
actual object would produce, and the result would be, if these retinal
pictures occupied the proper position on the eyes, that the impression
of solidity would be produced, which is called the _stereoscopic
effect_.
This may be done without the aid of any instrument, as almost any person
may discover after some trials with nothing but a _stereoscopic slide_,
if he can succeed in maintaining the optic axis of his eyes quite
parallel. In such a case he will observe the stereoscopic effect by the
fusing together, as it were, into one sensation, of the impression
received by the right eye from the right photograph, with that received
by the left eye from the left photograph. But as each eye will, at the
same time, have the photograph intended for the other in the field of
view, the observer will be conscious of a non-stereoscopic image on each
side of the central stereoscopic one. These outside images are, however,
very distracting, for the moment the attention is in the least directed
to them, the optic axes converge to the one side or the other, losing
their parallelism, and the stereoscopic effect vanishes, because the
images no longer fall in the usual positions on the retinæ. It is, in
consequence, only after some practice that one succeeds in readily
viewing stereoscopic slides in this manner, but the acquirement is a
convenient one when a person has rapidly to inspect a number of such
slides, for he can see them stereoscopically without putting them in the
instrument. Many persons, however, find great difficulty in acquiring
this power. In such cases it is well to begin by separating the two
photographs by means of a piece of cardboard, covered with black paper
on both sides. When this is held in the plane between the eyes, each eye
sees only its own photograph, and the observer is not troubled with the
two exterior images. After a little practice in this way, the cardboard
may usually be dispensed with, and the observer will insensibly have
acquired the habit of viewing the slides stereoscopically, without any
aid whatever.
[Illustration:
FIG. 243.—_Wheatstone’s Reflecting Stereoscope._
]
Instruments have, however, been contrived which enable one to obtain the
desired result without effort; and one form of these is now tolerably
well known to everybody. The first stereoscope was the invention of
Wheatstone. The reflecting stereoscope is represented in Fig. 243, and
consists essentially of two plane metallic mirrors inclined to the front
of the instrument at angles of 45°, so that in each of them the observer
sees only the design which belongs to it. The rays reach the eyes as if
they came from images placed in front of the observer; and the two
images having the proper differences, produce together the impression of
solid objects.
[Illustration:
FIG. 244.
]
Brewster’s stereoscope—which is far more widely known than
Wheatstone’s—has two acute prisms, or, more usually, two portions of a
convex lens are cut out, and placed with their margins or thin parts
inwards, and they thus produce the same effect as would be obtained by
combinations of a prism with a convex lens. Another very common form of
the stereoscope has merely two convex lenses. The effect of the convex
lenses is to increase the apparent size of the images by diminishing the
divergence of the rays emitted by each point, producing the appearance
of larger designs seen at a greater distance. The effect of the prism is
to give the rays the direction which they would have if they proceeded
from an object placed in a position immediately between the two designs,
and an additional element by which we estimate distance, namely, the
convergence of the optic axes, is made to aid in the illusion, when the
rays proceeding from the two different pictures have approximately the
inclination that they would have if they emanated from real objects at
the place where the image is apparently formed. The box or case in which
the lenses or lenticular prisms are placed takes various forms. One of
the most common is represented in Fig. 244, but the stand on which it is
mounted is not a necessary part of the instrument, although it is
sometimes convenient. A handsome form is met with as a square case,
enclosing a number of photographic stereoscopic views mounted on an
endless chain in such a manner that they are brought successively into
view by turning a knob on the outside. When an instrument of this kind
is fitted up with a series of the beautiful landscape transparencies,
which are produced by certain continental photographers, a more perfect
reproduction of the impressions derived from nature, exclusive of
colour, cannot be conceived. We seem to be present on the very spots
which are so truthfully depicted by the subtile pencil of the sunbeam;
we feel that we have but to advance a foot in order to mix with the
passengers in the streets of Paris or of Rome, and that a single step
will bring us on the mountain-side, or place us on the slippery glacier;
at our own fireside we can feel the forty centuries looking down upon us
from the heights of those grand Egyptian pyramids, and find ourselves
bodily confronted with the mysterious Sphinx, still asking the solution
of her enigma. The truth and force with which these stereoscopic
photographs reproduce the relief of buildings are such, that when one
sees for the first time the real edifice of which he has once examined
the stereoscopic images, it no longer strikes him as new or unknown; for
he derives from the actual scene no impression of form that he has not
already received from the image.
[Illustration:
FIG. 245.
]
But of all subjects of stereoscopic photography the glaciers are,
perhaps, those which best show the power of the instrument as far
surpassing all other resources of graphic presentation. The most careful
painting fails to convey a notion of the strange glimmer of light which
fills the clefts of the ice, seen through the transparent substance
itself. The simple photograph commonly presents nothing but a confused
mass of grey patches; but combine in the stereoscope two such
photographs, each formed of nothing but slightly different grey patches,
and a surprising effect is at once produced: the masses of ice assume a
palpable form, and the beautiful effects of light transmitted or
reflected by the translucent solid reveal themselves. Another very
beautiful class of subjects for stereoscopic slides is found in those
marvellous instantaneous photographs, which seize and fix the images of
the waves as they dash upon the shore. Here a scene which has tasked the
power of the greatest painter is brought home to us with such force and
vividness that we all but hear the wild uproar of the breakers.
But for the art of photography the stereoscope would not thus be ready
to minister to our enjoyment, for no pictures wrought by man’s handiwork
could approach the requisite accuracy which the two stereoscopic
pictures must possess. All attempts to produce such pictures by
engraving or lithography have failed, except only in the case of linear
geometrical designs, such as representations of crystals. A very useful
and suggestive application of the stereoscope has been made to the
illustration of a treatise on _solid geometry_, where the lines
representing the planes, being drawn in proper perspective, the reader
by placing a simple stereoscope over the plates sees the planes stand
out in relief before him, and the multitude of lines, angles, &c., which
in a simple drawing might be distracting even for a practised
geometrician, assume a clear and definite form. The difference between
the two retinal pictures of objects is so slight, that when the objects
are at a little distance, ordinary observation fails to discover it
without the aid of special instruments; and an inspection of the pair of
photographs in a stereoscopic slide will convince any one that, even in
these, close and careful observation is required to perceive the
difference.
Some of the principles of stereoscopic drawings may be seen exemplified
by the pair we give in Fig. 245. With this figure the reader may attempt
the experiment of seeing the stereoscopic effect without the
stereoscope. When he has succeeded in doing this, or when he fuses the
images together by placing a simple stereoscope over the page, he will
find the result very singular; for he will receive the impression of a
solid crystal of some dark polished substance—black lead, for
instance—placed on a surface of the same material. The edges of the
solid will appear to have a certain lustre, such as one sees on the
edges of a real crystal. The reason of this impression being produced by
two drawings, one of which is formed by black lines on a white ground,
while the other has white lines on a black ground, is probably due to
the circumstance that we very often see in nature the _lustrous_ edges
of an object with one eye only. That is, one eye is in the path of the
rays which are regularly reflected from the object, while the other is
not,—a fact which may be verified in an instant by looking first with
one eye and then with the other, at a polished pencil, or similar
object, when placed in a certain position.
There is a kind of modification of the reflecting stereoscope, known
under the name of the _pseudoscope_, which is highly instructive, as
showing how much our notions of the solidity of objects are due to the
differences of the retinal images. In the pseudoscope the rays reach the
eyes after passing through rectangular prisms in such a manner that
objects on the right appear on the left, and objects on the left appear
on the right; but the images agree by reason of the symmetry of the
reflection, although the image of the objects that without the
instrument would be formed in the right eye is, by the action of the
prisms, formed in the left eye, and _vice versâ_. The impressions
produced are very curious: convex bodies appear concave—a coin, for
example, seems to have the image hollowed out, a pencil appears a
cylindrical cavity, a globe seems a concave hemisphere, and objects near
at hand appear distant, and so on. These illusions are, however, easily
dispelled by any circumstance which brings before the mind our knowledge
of the actual forms, and by a mental effort it is possible to perceive
the actual forms even with the pseudoscope, and indeed to revert
alternately, with the same object, from convexity to concavity. This
last effect is very curious, for the object appears to abruptly change
its form, becoming alternately hollow and projecting, according as the
mind dwells upon the one notion or the other; but the experiment is
attended with a feeling of effort, which is very fatiguing to the eyes.
Professor Helmholtz has contrived another very curious instrument,
depending on the same principles as the stereoscope. He terms it the
_telestereoscope_, and while the effect of the pseudoscope is to reverse
the relief of objects, the telestereoscope merely exaggerates this
relief; hence this instrument is well adapted for making those objects
which from their distance present no stereoscopic effect, stand out in
relief. The distance between our eyes is not sufficiently great to give
us sensibly different views of very distant objects, and what the
telestereoscope does is virtually to separate our eyes to a greater
distance. Fig. 246 is a horizontal section of the instrument. L and R
represent the position of the eyes of the spectator; _a, b_, are two
plane mirrors at 45° to his line of sight; A, B, are two larger plane
mirrors, respectively nearly parallel to the former. _c d a_ L and _f g
b_ R show the paths of rays from distant objects, and it is obvious that
the right eye will obtain a view of the objects identical with that
which would be presented to an eye at R´, while the left eye has
similarly the picture of the objects as seen from the point L´. The four
mirrors are mounted in a box, and means are provided for adjusting the
positions of the larger mirrors, as may be required. With this
instrument the distant objects in a landscape—a range of mountains, for
example—which present to the naked eye little or no appearance of
relief, have their projections and hollows revealed in the most curious
manner.
[Illustration:
FIG. 246.—_The Telestereoscope._
]
It is upon a similar principle that stereoscopic views of some of the
celestial bodies have been obtained. Admirable stereoscopic slides of
the moon have been produced by photographing her at different times,
when the illumination of the surface is the same, but when, in
consequence of her _libration_, somewhat different views of our
satellite are presented to us. Two such photographs, properly combined
in the stereoscope, give not only the spherical form in full relief, but
all the details of the surface: the mountains, craters, valleys, and
plains are seen in their true relative projection.
The telestereoscope may be inverted, so to speak, and its effect
reversed; for an arrangement of mirrors similarly disposed, but on such
a scale as will permit the eyes to be respectively in the lines _c d_
and _f g_, would reflect from objects in the direction L R rays which
would have but little of the difference of direction to which the
stereoscopic effect is due. Hence solid objects viewed with such an
instrument appear exactly like flat pictures, the effect being far more
marked than in simply viewing them with one eye.
An ingenious method of exhibiting a stereoscopic effect to an audience
has been contrived by Rollmann. He draws on a black ground two linear
stereoscopic designs—that for the left eye with red lines, that for the
right eye with blue. Each individual in the audience is provided with a
piece of blue glass and a piece of red: he places the red glass before
the left eye, the blue glass before the right: each eye thus receives
only the picture intended for it, for the blue lines cannot be seen
through the red glass, or the red lines through the blue glass. The
diagrams may, of course, be projected on a screen by a magic lantern, in
which case the circumstances are even more favourable. Duboscq has
arranged a kind of opera-glass, so that a person may view appropriate
designs on the large scale, and arrangements have been also contrived by
which the stereoscopic effect may be seen in moving figures.
Every student of this interesting subject should examine a few
stereoscopic images produced by simple lines representing geometrical
figures, or the photographs of the model of a crystal, as these exhibit
in the most striking manner the conditions requisite for the production
of stereoscopic effects. A person having a little skill in perspective
and geometry might construct the two stereoscopic images of a body
defined by straight lines, but the drawings must be executed with
extreme exactitude, for the least deviation would produce the most
marked effect in the stereoscopic appearance. The production of
stereoscopic photographs now forms a considerable branch of industrial
art. At first, these photographs were made by taking the two different
views with the same camera at two operations. But there were
difficulties in obtaining uniformity of depth in the impressions, and
the change in the shadows produced by the earth’s rotation showed
itself—although the interval between the two exposures might not exceed
three or four minutes. The increased shadows in such cases show
themselves in the stereoscope, like dark screens suspended in the air.
It was Sir David Brewster who, in 1849, first proposed the plan now
universally adopted, of producing the views simultaneously by twin
cameras forming their images on different parts of the same sensitive
plate, the centres of the lenses being placed at the same distance apart
as a man’s eyes, that is, from 2½ to 3 in. This is, of course, the only
manner in which instantaneous views can be secured. Helmholtz, however,
advocates the photographs of remote objects being taken at a much
greater distance apart, for they otherwise present little appearance of
relief. By selecting from an assortment of slides, two views of the
Wetterhorn, taken from different points in the Grindelwald valley, and
combining these in the stereoscope, he found that a far more distinct
idea of the modelling of the mountain could be thus obtained than even a
spectator of the actual scene would receive by viewing the mountain from
any one point. Such a mode of combining the photographs would produce in
the stereoscope the same effect as the telestereoscope would in the
landscape, but the effect would be caused to a proportionately far
higher degree.
The date of Wheatstone’s first publication regarding the stereoscope was
1833; but a complete description and theory of the instrument was not
published until five years afterwards. Brewster first made public, in
1843, his invention of the stereoscope with lenses, which is now so
familiar to us, and few scientific instruments have become so quickly
and extensively popular; certainly no other simple and inexpensive
instrument has contributed so largely to the amusement and instruction
of our domestic circles. And, to the philosopher who studies the nature
of our perceptions, the stereoscope has been even more instructive, for,
instead of vague surmises, it provided him with the solid ground of
experiment on which to found his theories. The literature of this one
subject—stereoscopic effect—is extensive enough to occupy a tolerably
long book-shelf. It dates from 300 B.C., when Euclid touched upon the
subject in his Optics; and after a lapse of more than eighteen centuries
it was taken up by Baptista Porta, in 1583; but the whole development of
this subject belongs almost entirely to the last half-century.
[Illustration:
FIG. 247.
]
The part which the muscles of the eyes take in our perceptions of form
has been already alluded to, and it may be interesting to illustrate
this point by a curious example or two of illusions arising from their
movements. If our reader will glance at Fig. 247, he will see that the
lines, _a b_ and _c d_, appear to be farther apart towards the centre
than at the ends, while _f g_ and _h i_, on the other hand, appear
nearest together in the middle. He will hardly be convinced that in each
case the lines are quite parallel until he has actually measured the
distances. A still more striking example of the same kind of illusion is
shown by Fig. 248, due to Zöllner. This appears a sort of pattern, in
which the broad bands are not upright, but sloping alternately to the
right and left, and with the spaces between the lines wider at one end
than the other. The lines in the figure are, however, strictly parallel.
The illusion by which they appear divergent and convergent is still more
strongly felt when the book is held so that the wider bands are inclined
at an angle of 45° to the horizon. There is another illusion here with
reference to the short lines, which will appear to be opposite to the
white spaces on the other side of the long lines to which they are
attached. That these illusions are really due to movements of the eyes
may be proved by viewing the designs in any manner which entirely
prevents the movement, as by fixing the gaze on one spot in the case of
Fig. 247, when the illusion will vanish; but this plan is not so easily
applied to Fig. 248. A convincing proof, however, will be found in the
appearance of these figures when they are viewed by the instantaneous
light of the electric spark, as when a Leyden jar is discharged in a
dark room. The reader viewing the figures, held near the place where the
spark appears, will see them distinctly without the illusions as to the
non-parallelism of the lines. In the absence of an electrical machine,
or coil and jar, the reader may have an opportunity of seeing the
figures by flashes of lightning at night, when the result will be the
same.
[Illustration:
FIG. 248.
]