THE STEREOSCOPE.
HAVING devoted so much space in the preceding chapters to optical
amusements of a purely recreative character, it is only right that we
should now say a few words on certain instruments of a less frivolous
character than those we have lately been considering, and which deserve
at our hands the most serious attention. We shall, therefore, in the
present chapter, speak of an ingenious instrument which serves to show
in relief the images of objects depicted on a flat surface. We have
already seen, that although we have two eyes, provided with lenses and
screens by means of which the images of things around us are formed, we
only perceive a single object; and the student has no doubt long since
wondered why nature has bestowed two eyes upon us, when only one would
have apparently served the same purpose. This question was for a long
time a complete puzzle to philosophers, and it was not until Professor
Wheatstone made his experiments on binocular vision in 1838, that the
matter received a satisfactory explanation. He showed that each eye
receives a different impression of any object upon the retina, and that
it is in consequence of the union of these slightly dissimilar images
that the sensation of relief is experienced. A one-eyed man or a Cyclops
would only partially perceive relief in the objects presented to his
view, in consequence of a single image being sent to his brain. He
would, no doubt, after examining the things he saw with his hands, know
they were solid, and generally see them so; but if a new object were
presented to his view he would have some difficulty in knowing whether
it had a flat surface or not.
[Illustration:
FIG. 67.
]
The principle of binocular vision may be explained as follows: If a
playing die, such as is represented in fig. 67, be held out at arm’s
length in the position indicated in the figure, and looked at first with
the left eye and then with the right, we shall find that in the first
case we see a little of the three dots on the left-hand side, and in the
second we lose sight of the three dots and see a little of the single
one on the right-hand side. The images seen by each eye are, therefore,
slightly dissimilar, and it stands to reason that, if by any means we
can combine two slightly dissimilar flat pictures of a solid object, we
shall see it in relief. This was proved practically by Professor
Wheatstone, who constructed an instrument capable of effecting the
desired union, and which has since been called the stereoscope, from two
Greek words signifying ‘to see solid.’ The instrument remained for a
long time fallow, so to speak, from the difficulty of drawing two
pictures that should be identical in size and details, although
dissimilar in the arrangement of their perspective. It was, therefore,
not until photography enabled us to do this with the greatest ease and
exactitude that the stereoscope became common. The instrument first
devised by Professor Wheatstone, was what is termed a reflecting
stereoscope, and was expensive to make and cumbrous to use. It was
modified by Sir David Brewster, by the substitution of prisms for
reflectors, and was thus made cheaper and more portable. The refracting
form of stereoscope is so familiar to most people, that it really needs
no description. It will only be necessary to mention that the prisms
used in the eye-pieces are made by cutting a double convex lens in two,
and reversing the halves. They are so placed that the centre of each
prism is just in the centre of each eye; but as the eyes of different
people vary in distance, an arrangement is generally added so that the
eye-pieces may slide from side to side. Being cut from lenses, the
prisms have a magnifying power; consequently other means are provided
for sliding them up and down to suit the length of focus in different
eyes.
[Illustration:
FIG. 68.—Stereoscope.
]
In fig. 69 we can follow the path of the rays proceeding from each
picture, and reach the eyes apparently from a spot exactly between the
two.
In the reflecting stereoscope two mirrors are joined together at right
angles to each other, the two pictures being placed at each side, at a
distance corresponding to their size. The reflecting instrument,
although not so portable, is in some sort superior to the other,
inasmuch as pictures of any size can be seen by it, whilst in the
smaller instrument the size of the photograph is limited by the distance
at which the eyes are placed.
[Illustration:
FIG. 69.—The Principle of the Refracting Stereoscope.
]
It should be mentioned, that no optical instrument of any kind is
absolutely necessary to obtain a stereoscopic effect from two suitable
drawings or photographs, as it is quite possible by a little management
of the eyes to cause the two images to combine with each other.
Referring again to fig. 67, it will be perceived that the two figures of
the dice are about an inch and a half from each other. Holding the book
at about ten inches from the eye, they are viewed by squinting strongly
until the _right_ eye looks at the _left_ die, and the _left_ eye at the
right. This may be also done by converging the eyes on a point beyond
the centre of the figure, which may be easily done by looking at a point
midway between the two. In both cases the images at first appear
doubled, and we see four dice, but a little practice will soon enable
you to cause the two inside images to coalesce, and so give the effect
of relief. It is true that even then three images are seen, but the eye
soon grows accustomed to neglect them altogether. This habit is a very
pleasant acquirement for the London _flâneur_, who can thus see in
perfection the numberless stereoscopic views now shown in our
shop-windows without the intervention of an instrument of any kind.
The method of photographing subjects for the stereoscope is very simple,
and consists in taking two views of the object to be depicted, from two
different points. According to the distance of these points from each
other, so will the resulting pictures appear in greater or less relief.
This is readily seen in some stereoscopic portraits which have been
taken at a large angle, and consequently show such increased relief as
to produce distortion. Theoretically, the interval of the two points of
view ought to be two inches and a half, that being the average distance
between the two eyes; but in practice it is better to increase it in the
case of portraits or other near objects to about twelve inches, and in
that of views to even several feet. Brewster’s original rule for taking
stereoscopic photographs, was to place the cameras one foot apart for
every twenty-five feet of distance. The beautiful stereoscopic pictures
of the moon photographed by Mr. Warren de la Rue were taken at more than
1,000 miles’ distance, in order to obtain the necessary relief. The
principle of the stereoscope has received many useful applications in
the way of book illustrations, art teaching, and anatomical
demonstration, and has thus gained a position among philosophical
instruments that it did not at first possess.
A combination of the principles of the phenakistiscope (fig. 4) and
stereoscope, has resulted in the invention of an instrument called the
stereotrope. A number of binocular photographs of some object in
motion—a steam-engine, for instance—are taken when the moving parts are
in different positions, and mounted on two revolving discs, the images
being combined by means of a pair of semi-lenses, as in the ordinary
refracting stereoscope.
We cannot leave this subject without describing the pseudoscope, also
the invention of Professor Wheatstone. If a stereoscopic pair of
photographs of some solid body—a ball, for instance—are mounted the
reverse way, that is to say, if the picture intended to be looked at by
the right eye is placed on the left, the relief of the object will be
reversed, and the ball will appear as a hollow hemisphere. If,
therefore, we can by means of lenses or prisms cause the image of any
natural object, as seen by the right eye, to be conveyed to the left,
and _vice versâ_, we shall see the relief reversed. A conical cap will
appear in relief as a cone, a globe will look like a hollow sphere, and
the human face will take the semblance of the inside of a mask. The same
deception may be effected by looking at a seal through a short-focused
lens, so that the image shall seem reversed. In this case, the light
coming apparently from the wrong side, and shining on the parts in
relief, gives them the appearance of being hollow. An intaglio will, of
course, appear in relief when so looked at. Photographs of gems and
bas-reliefs will also present a pseudoscopic appearance, if looked at in
a light coming from the opposite side to that in which they were taken.
The same appearance may be seen sometimes in wall papers having patterns
painted in strong relief.
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