ON THE ADAPTATION OF THE PICTURES TO THE STEREOSCOPE.
—THEIR SIZE, POSITION, AND ILLUMINATION.
Having described the various forms of the stereoscope, and the method
of taking the binocular portraits and pictures to which it is to be
applied, we have now to consider the relation that ought to exist
between the instrument and the pictures,—a subject which has not been
noticed by preceding writers.
If we unite two dissimilar pictures by the simple convergency of
the optical axes, we shall observe a certain degree of relief, at a
certain distance of the eyes from the pictures. If we diminish the
distance, the relief diminishes, and if we increase it, it increases.
In like manner, if we view the dissimilar pictures in the lenticular
stereoscope, they have a certain degree of relief; but if we use
lenses of a higher magnifying power, so as to bring the eyes nearer
the pictures, the relief will diminish, and if we use lenses of a less
magnifying power, the relief will increase. By bringing the eyes nearer
the pictures, which we do by magnifying them as well as by approaching
them, we increase the distance between similar points of the two
pictures, and therefore the distance of these points, when united,
from any plane in the picture, that is, its relief will be diminished.
For the same reason, the diminution of the distance between similar
points by the removal of the eyes from the picture, will produce an
increase of relief. This will be readily understood if we suppose the
eyes R, L, in Fig. 24, to be brought nearer the plane MN, to R′L′, the
points 1, 1 and 2, 2 will be united at points nearer MN than when the
eyes were at R, L, and consequently their relief diminished.
Now we have seen, that in taking portraits, as explained in Fig. 46, we
view the two pictures, _ab, a′b′_, with the eyes at E and E′, exactly,
and with the same relief in the air, as when we saw the original AB,
from L, L′, and therefore E_c_ is the distance at which the dissimilar
pictures should be viewed in the stereoscope, in order that we may see
the different parts of the solid figure under their proper relief. But
the distance E_c_ = L_c_ is the conjugate focal length of the lens L,
if one lens is used, or the conjugate equivalent focal length, if two
achromatic lenses are used; and consequently every picture taken for
the stereoscope should be taken by a camera, the conjugate focal length
of whose lens corresponding to the distance of the sitter, is equal to
_five_ inches, when it is to be used in the common stereoscope, which
has generally that depth.
Between the pictures and the purely optical part of the stereoscope,
there are other relations of very considerable importance. The
exclusion of all external objects or sources of light, excepting that
which illuminates the pictures, is a point of essential importance,
though its advantages have never been appreciated. The spectacle
stereoscope held in the hand, the reflecting stereoscope, and the
open lenticular stereoscope, are all, in this respect, defective.
The binocular pictures must be placed in a dark box, in order to
produce their full effect; and it would be a great improvement on
the lenticular stereoscope, if, on the left and right side of each
eye-tube, a piece of brass were to be placed, so as to prevent any
light from entering the left angle of the left eye, and the right angle
of the right eye.[57] The eyes, thus protected from the action of all
external light, and seeing nothing but the picture, will see it with a
distinctness and brilliancy which could not otherwise be obtained.
[57] When any external light falls upon the eye, its picture is
reflected back from the metallic surface of the Daguerreotype, and a
negative picture of the part of the Daguerreotype opposite each eye is
mixed with the positive picture of the same part.
The proper illumination of the picture, when seen by reflected light,
is also a point of essential importance. The method universally adopted
in the lenticular stereoscope is not good, and is not the one which I
found to be the best, and which I employed in the first-constructed
instruments. The light which falls upon the picture is prevented from
reaching the observer only by its being incident at an angle greater
or less than the angle of reflexion which would carry it to his eyes.
A portion of the scattered light, however, does reach the eye, and in
Daguerreotypes especially, when any part of the surface is injured,
the injury, or any other imperfection in the plate, is more distinctly
seen. The illumination should be lateral, either by a different form of
window in the front, or by openings on the two sides, or by both these
methods.
When the lenticular stereoscope is thus fitted up, and the pictures
in this manner illuminated, the difference of effect is equally great
as it is between a picture as commonly seen, and the same picture
exhibited as a panorama or a diorama, in which no light reaches the
eyes but that which radiates from the painting itself, the reflexion
from the varnish being removed by oblique or lateral illumination.
The great value of transparent binocular slides, when the picture
is to be upon glass, is obvious from the preceding considerations.
The illumination is uniform and excellent, but care must be taken to
have the ground-glass in front of the picture, or the paper, when it
is used, of a very fine grain, so that it may throw no black specks
upon the sky or the lights of the picture. Another advantage of the
transparent slides is, that the pictures are better protected from
injury than those upon paper.
It is obvious from these considerations that the size of the pictures
is determined, as well as the distance at which they are to be viewed.
Much ignorance prevails upon this subject, both among practical
photographers and optical writers. Large binocular pictures have been
spoken of as desirable productions, and it has been asserted, and
claimed too, as a valuable property of the reflecting stereoscope, that
it allows us to use larger pictures than other instruments. There never
was a greater mistake. If we take a large picture for the stereoscope
we must place it at a great distance from the eye, and consequently use
a large stereoscope. A small picture, seen distinctly near the eye, is
the very same thing as a large picture seen at a greater distance. The
size of a picture, speaking optically and correctly, is measured by the
angle which it subtends at the eye, that is its apparent magnitude. A
portrait three inches high, for example, and placed in the lenticular
stereoscope five inches from the eye, has the same apparent size as a
Kit Cat portrait in oil the size of life, three feet high, seen at the
distance of five feet, the distance at which it is commonly examined;
and if we increase the magnifying power so as to see the three-inch
picture at the distance of two inches, it will have the same apparent
size as the three feet oil portrait seen at the distance of two feet.
If the pictures used in the stereoscope were imperfect pictures that
would not bear being magnified, it would be improper to use them; but
the Daguerreotypes, and the transparent pictures, which are taken
by the first artists, for the lenticular stereoscope, will bear a
magnifying power ten times greater than that which is applied to them.
If we take a large picture for the stereoscope, we are compelled by
pictorial truth to place it at a distance from the eye equal to the
equivalent focal distance of the camera. Every picture in every camera
has the same apparent magnitude as the object which it represents;
whether it be a human figure, or the most distant landscape; and if
we desire to see it in its true relief in the stereoscope, we must
place it at a distance from the eye equal to the focal length of the
lens, whether it be an inch or a foot high. There is, therefore,
nothing gained by using large pictures. There is, on the contrary, much
inconvenience in their use. They are in themselves less portable, and
require a larger stereoscope; and we believe, no person whatever, who
is acquainted with the perfection and beauty of the binocular slides in
universal use, would either incur the expense, or take the trouble of
using pictures of a larger size.
In the beautiful combination of lenticular stereoscopes, which was
exhibited by Mr. Claudet, Mr. Williams, and others, in the Paris
Exhibition, and into which six or eight persons were looking at the
same time, binocular pictures of a larger size could not have been
conveniently used.
But, independently of these reasons, the question of large pictures
has been practically settled. No such pictures are taken by the
Daguerreotypists or Talbotypists, who are now enriching art with
the choicest views of the antiquities, and modern buildings, and
picturesque scenery of every part of the world; and even if they could
be obtained, there are no instruments fitted for their exhibition. In
the magnificent collection of stereoscopic pictures, amounting to above
a thousand, advertised by the London Stereoscopic Company, there are
no fewer than _sixty_ taken in Rome, and representing, better than a
traveller could see them there, the ancient and modern buildings of
that renowned city. Were these _sixty_ views placed on the sides of a
revolving polygon, with a stereoscope before each of its faces, a score
of persons might, in the course of an hour, see more of Rome, and see
it better, than if they had visited it in person. At all events, those
who are neither able nor willing to bear the expense, and undergo the
toil of personal travel, would, in such a panorama,—an analytical view
of Rome,—acquire as perfect a knowledge of its localities, ancient
and modern, as the ordinary traveller. In the same manner, we might
study the other metropolitan cities of the world, and travel from them
to its river and mountain scenery,—admiring its noble castles in our
descent of the Rhine,—its grand and wild scenery on the banks of the
Mississippi, or the Orinoco,—the mountain gorges, the glaciers, and the
peaks of the Alps and the Ural,—and the more sublime grandeur which
reigns among the solitudes of the Himalaya and the Andes.
The following general rule for taking and combining binocular pictures
is the demonstrable result of the principles explained in this chapter:—
_Supposing that the camera obscura employed to take binocular
portraits, landscapes, &c., gives perfect representations of them, the
relief picture in the stereoscope, produced by their superposition
and binocular union, will not be correct and truthful, unless the
dissimilar pictures are placed in the stereoscope at a distance from
the eyes, equal to the focal distance, real or equivalent, of the
object-glass or object-glasses of the camera, and, whatever be the size
of the pictures, they will appear, when they are so placed, of the same
apparent magnitude, and in the same relief, as when they were seen from
the object-glass of the camera by the photographer himself._