ON THE PRODUCTION OF STEREOSCOPIC PICTURES FROM A SINGLE PICTURE.
Those who are desirous of having stereoscopic _relievos_ of absent or
deceased friends, and who possess single photographic portraits of
them, or even oil paintings or miniatures, will be anxious to know
whether or not it is possible to obtain from one plane picture another
which could be combined with it in the stereoscope; that is, if we
consider the picture as one seen by either eye alone, can we by any
process obtain a second picture as seen by the other eye? We have no
hesitation in saying that it is impossible to do this by any direct
process.
Every picture, whether taken photographically or by the eye, is
necessarily a picture seen by one eye, or from one point of sight; and,
therefore, a skilful artist, who fully understands the principle of
the stereoscope, might make a copy of any picture as seen by the other
eye, so approximately correct as to appear in relief when united with
the original in the stereoscope; but the task would be a very difficult
one, and if well executed, so as to give a relievo without distortion,
the fortune of the painter would be made.
When the artist executes a portrait, he does it from one point of
sight, which we may suppose fixed, and corresponding with that which
is seen with his _left_ eye. If he takes another portrait of the same
person, occupying exactly the same position, from another point of
sight, two and a half inches to the right of himself, as seen with
his _right_ eye, the two pictures will differ only in this, that
each point in the head, and bust, and drapery, will, in the second
picture, _be carried farther to the left of the artist_ on the plane
of representation. The points which project most, or are most distant
from that plane, will be carried farther to the left than those which
project less, the extent to which they are carried being proportional
to the amount of their projection, or their distance from the plane.
But since the painter cannot discover from the original or left-eye
plane picture the degree of prominence of the leading points of the
head, the bust, and the drapery, he must work by guess, and submit his
empirical touches, step by step, to the judgment of the stereoscope. In
devoting himself to this branch of the art he will doubtless acquire
much knowledge and dexterity from experience, and may succeed to a very
considerable extent in obtaining pictures in relief, if he follows
certain rules, which we shall endeavour to explain.
If the given portrait, or picture of any kind, is not of the proper
size for the stereoscope, it must be reduced to that size, by taking
a photographic copy of it, from which the right-eye picture is to be
drawn.
[Illustration: FIG. 50.]
In order to diminish the size of the diagram, let us suppose that the
plane on which the portrait is taken touches the back of the head,
and is represented in section by AB, Fig. 50. We must now assume,
under the guidance of the original, a certain form of the head, whose
breadth from ear to ear is EE″, N being the point of the nose in the
horizontal section of the head, E″NEN′, passing through the nose N, and
the lobes EE″ of the two ears. Let L, R be the left and right eyes of
the person viewing them, and LN the distance at which they are viewed,
and let lines be drawn from L and R, through L, N, E and E″, meeting
the plane AB on which the portrait is taken in _e′_, E‴, _n_, N′, _e_,
and E′. The breadth, E‴_e′_, and the distances of the nose from the
ears N′E′, N′E‴, being given by measurement of the photograph suited
to the stereoscope, the distances NN′, EE′, E″E‴ may be approximately
obtained from the known form of the human head, either by projection
or calculation. With these data, procured as correctly as we can, we
shall, from the position of the nose _n_, as seen by the right eye R,
have the formula
LR × NN′
N′_n_ = ————————
NL.
The distance of the _right_ ear _e′_, from the right-eye picture, will
be,
_ne′_ = _e′_N - N′_n_;
LR × EE′
and as E′_e_ = —————————
EL.
The distance of the _left_ ear _e_, in the right-eye picture, from the
nose _n_, will be
_ne_ = N′_n_ + N′E′ - E′_e_.
In order to simplify the diagram we have made the original, or left-eye
picture, a front view, in which the nose is in the middle of the face,
and the line joining the ears parallel to the plane of the picture.
When the position of the nose and the ears has been thus approximately
obtained, the artist may, in like manner, determine the place of the
pupils of the two eyes, the point of the chin, the summit of the
eyebrows, the prominence of the lips, and the junction of the nose with
the teeth, by assuming, under the guidance of the original picture,
the distance of these different parts from the plane of projection.
In the same way other leading points in the figure and drapery may be
found, and if these points are determined with tolerable accuracy the
artist will be able to draw the features in their new place with such
correctness as to give a good result in the stereoscope.
In drawing the right-eye picture the artist will, of course, employ as
the groundwork of it a faint photographic impression of the original,
or left-eye picture, and he may, perhaps, derive some advantage from
placing the original, when before the camera, at such an inclination
to the axis of the lens as will produce the same diminution in the
horizontal distance between any two points in the head, at a mean
distance between N and N′, as projected upon the plane AB. The line
N′E‴, for example, which in the left-eye photograph is a representation
of the cheek NE″, is reduced, in the right-eye photograph, to _ne′_,
and, therefore, if the photograph on AB, as seen by the right eye, were
placed so obliquely to the axis of the lens that N′_e_ was reduced
to _ne′_, the copy obtained in the camera would have an approximate
resemblance to the right-eye picture required, and might be a better
groundwork for the right-eye picture than an accurate copy of the
photograph on AB, taken when it is perpendicular to the axis of the
lens.
In preparing the right-eye picture, the artist, in place of using
paint, might use very dilute solutions of aceto-nitrate of silver,
beginning with the faintest tint, and darkening these with light till
he obtained the desired effect, and, when necessary, diminishing the
shades with solutions of the hypo-sulphite of soda. When the picture
is finished, and found satisfactory, after examining its relief in the
stereoscope, a negative picture of it should be obtained in the camera,
and positive copies taken, to form, with the original photographs, the
pair of binocular portraits required.