Transcriber’s Notes:
Underscores “_” before and after a word or phrase indicate _italics_
in the original text.
Equal signs “=” before and after a word or phrase indicate =bold=
in the original text.
Small capitals have been converted to SOLID capitals.
Illustrations and footnotes have been moved so they do not break up
paragraphs.
Antiquated spellings have been preserved.
Typographical and punctuation errors have been silently corrected.
In the Table of Contents, Chapters XV and XVI were corrected from page
200 to 211 and 216 respectively.
Figure 46 was incorrectly labelled Figure 45, this has been corrected.
THE STEREOSCOPE
ITS HISTORY, THEORY, AND CONSTRUCTION
WITH ITS APPLICATION TO THE FINE AND USEFUL ARTS
AND TO EDUCATION.
BY
SIR DAVID BREWSTER, K.H., D.C.L., F.R.S., M.R.I.A.,
VICE-PRESIDENT OF THE ROYAL SOCIETY OF EDINBURGH, ONE OF THE EIGHT
ASSOCIATES OF THE IMPERIAL INSTITUTE OF FRANCE, OFFICER OF THE
LEGION OF HONOUR, CHEVALIER OF THE PRUSSIAN ORDER OF MERIT,
HONORARY OR CORRESPONDING MEMBER OF THE ACADEMIES
OF PETERSBURGH, VIENNA, BERLIN, COPENHAGEN,
STOCKHOLM, BRUSSELS, GÖTTINGEN, MODENA,
AND OF THE NATIONAL INSTITUTE OF
WASHINGTON, ETC.
WITH FIFTY WOOD ENGRAVINGS.
LONDON:
JOHN MURRAY, ALBEMARLE STREET.
1856.
[_The Right of Translation is reserved._]
EDINBURGH:
T. CONSTABLE, PRINTER TO HER MAJESTY.
CONTENTS
PAGE
INTRODUCTION, 1
CHAP. I.—HISTORY OF THE STEREOSCOPE, 5
II.—ON MONOCULAR VISION, OR VISION WITH ONE EYE, 38
III.—ON BINOCULAR VISION, OR VISION WITH TWO EYES, 47
IV.—DESCRIPTION OF THE OCULAR, REFLECTING,
AND LENTICULAR STEREOSCOPES, 53
V.—ON THE THEORY OF THE STEREOSCOPIC VISION, 76
VI.—ON THE UNION OF SIMILAR PICTURES IN
BINOCULAR VISION, 90
VII.—DESCRIPTION OF DIFFERENT STEREOSCOPES, 107
VIII.—METHOD OF TAKING PICTURES FOR THE STEREOSCOPE, 131
IX.—ON THE ADAPTATION OF THE PICTURES TO
THE STEREOSCOPE.—THEIR SIZE, POSITION,
AND ILLUMINATION, 159
X.—APPLICATION OF THE STEREOSCOPE TO PAINTING, 166
XI.—APPLICATION OF THE STEREOSCOPE TO SCULPTURE,
ARCHITECTURE, AND ENGINEERING, 183
XII.—APPLICATION OF THE STEREOSCOPE TO NATURAL HISTORY, 189
XIII.—APPLICATION OF THE STEREOSCOPE TO
EDUCATIONAL PURPOSES, 193
XIV.—APPLICATION OF THE STEREOSCOPE TO PURPOSES
OF AMUSEMENT, 204
XV.—ON THE PRODUCTION OF STEREOSCOPIC
PICTURES FROM A SINGLE PICTURE, 211
XVI.—ON CERTAIN FALLACIES OF SIGHT IN THE
VISION OF SOLID BODIES, 216
XVII.—ON CERTAIN DIFFICULTIES EXPERIENCED IN
THE USE OF THE STEREOSCOPE, 231
ON THE STEREOSCOPE.
INTRODUCTION.
The _Stereoscope_, a word derived from =στέρεος=, _solid_, and
σκόπειν, to _see_, is an optical instrument, of modern invention, for
representing, in apparent relief and solidity, all natural objects and
all groups or combinations of objects, by uniting into one image two
plane representations of these objects or groups as seen by each eye
separately. In its most general form the Stereoscope is a _binocular_
instrument, that is, is applied to _both_ eyes; but in two of its
forms it is _monocular_, or applied only to _one_ eye, though the use
of the other eye, without any instrumental aid, is necessary in the
combination of the two plane pictures, or of one plane picture and its
reflected image. The Stereoscope, therefore, cannot, like the telescope
and microscope, be used by persons who have lost the use of one eye,
and its remarkable effects cannot be properly appreciated by those
whose eyes are not equally good.
When the artist represents living objects, or groups of them, and
delineates buildings or landscapes, or when he copies from statues or
models, he produces apparent solidity, and difference of distance from
the eye, by light and shade, by the diminished size of known objects as
regulated by the principles of geometrical perspective, and by those
variations in distinctness and colour which constitute what has been
called aerial perspective. But when all these appliances have been
used in the most skilful manner, and art has exhausted its powers,
we seldom, if ever, mistake the plane picture for the solid which it
represents. The two eyes scan its surface, and by their distance-giving
power indicate to the observer that every point of the picture is
nearly at the same distance from his eye. But if the observer closes
one eye, and thus deprives himself of the power of determining
differences of distance by the convergency of the optical axes, the
relief of the picture is increased. When the pictures are truthful
photographs, in which the variations of light and shade are perfectly
represented, a very considerable degree of relief and solidity is
thus obtained; and when we have practised for a while this species of
monocular vision, the drawing, whether it be of a statue, a living
figure, or a building, will appear to rise in its different parts from
the canvas, though only to a limited extent.
In these observations we refer chiefly to ordinary drawings held in the
hand, or to portraits and landscapes hung in rooms and galleries, where
the proximity of the observer, and lights from various directions,
reveal the surface of the paper or the canvas; for in panoramic and
dioramic representations, where the light, concealed from the observer,
is introduced in an oblique direction, and where the distance of the
picture is such that the convergency of the optic axes loses much of
its distance-giving power, the illusion is very perfect, especially
when aided by correct geometrical and aerial perspective. But when
the panorama is illuminated by light from various directions, and the
slightest motion imparted to the canvas, its surface becomes distinctly
visible, and the illusion instantly disappears.
The effects of stereoscopic representation are of a very different
kind, and are produced by a very different cause. The singular relief
which it imparts is independent of light and shade, and of geometrical
as well as of aerial perspective. These important accessories, so
necessary in the visual perception of the drawings _in plano_, avail
nothing in the evolution of their _relievo_, or third dimension. They
add, doubtless, to the beauty of the binocular pictures; but the
stereoscopic creation is due solely to the superposition of the two
plane pictures by the optical apparatus employed, and to the distinct
and instantaneous perception of distance by the convergency of the
optic axes upon the similar points of the two pictures which the
stereoscope has united.
If we close one eye while looking at photographic pictures in the
stereoscope, the perception of relief is still considerable, and
approximates to the binocular representation; but when the pictures are
mere diagrams consisting of white lines upon a black ground, or black
lines upon a white ground, the relief is instantly lost by the shutting
of the eye, and it is only with such binocular pictures that we see the
true power of the stereoscope.
As an amusing and useful instrument the stereoscope derives much of
its value from photography. The most skilful artist would have been
incapable of delineating two equal representations of a figure or a
landscape as seen by two eyes, or as viewed from two different points
of sight; but the binocular camera, when rightly constructed, enables
us to produce and to multiply photographically the pictures which we
require, with all the perfection of that interesting art. With this
instrument, indeed, even before the invention of the Daguerreotype and
the Talbotype, we might have exhibited temporarily upon ground-glass,
or suspended in the air, the most perfect stereoscopic creations, by
placing a Stereoscope behind the two dissimilar pictures formed by the
camera.
CHAPTER I.
HISTORY OF THE STEREOSCOPE.
When we look with both eyes open at a sphere, or any other solid
object, we see it by uniting into one two pictures, one as seen by the
right, and the other as seen by the left eye. If we hold up a thin book
perpendicularly, and midway between both eyes, we see distinctly the
back of it and both sides with the eyes open. When we shut the right
eye we see with the left eye the back of the book and the left side of
it, and when we shut the left eye we see with the right eye the back of
it and the right side. The picture of the book, therefore, which we see
with both eyes, consists of _two_ dissimilar pictures united, namely, a
picture of the back and the left side of the book as seen by the left
eye, and a picture of the back and right side of the book as seen by
the right eye.
In this experiment with the book, and in all cases where the object is
near the eye, we not only see _different pictures_ of the same object,
but we see _different things_ with each eye. Those who wear spectacles
see only the left-hand spectacle-glass with the left eye, on the left
side of the face, while with the right eye they see only the right-hand
spectacle-glass on the right side of the face, both glasses of the
spectacles being seen united midway between the eyes, or above the
nose, when both eyes are open. It is, therefore, a fact well known to
every person of common sagacity that _the pictures of bodies seen by
both eyes are formed by the union of two dissimilar pictures formed by
each_.
This palpable truth was known and published by ancient mathematicians.
Euclid knew it more than two thousand years ago, as may be seen in the
26th, 27th, and 28th theorems of his Treatise on Optics.[1] In these
theorems he shews that the part of a sphere seen by both eyes, and
having its diameter equal to, or greater or less than the distance
between the eyes, is equal to, and greater or less than a hemisphere;
and having previously shewn in the 23d and 24th theorems how to find
the part of any sphere that is seen by one eye at different distances,
it follows, from constructing his figure, that each eye sees different
portions of the sphere, and that it is seen by both eyes by the union
of these two dissimilar pictures.
[1] Edit. of Pena, pp. 17, 18, Paris, 1577; or _Opera_, by Gregory, pp.
619, 620. Oxon. 1703.
More than _fifteen hundred_ years ago, the celebrated physician Galen
treated the subject of binocular vision more fully than Euclid. In
the _twelfth_ chapter of the tenth book of his work, _On the use of
the different parts of the Human Body_, he has described with great
minuteness the various phenomena which are seen when we look at
bodies with both eyes, and alternately with the right and the left.
He shews, by diagrams, that dissimilar pictures of a body are seen
in each of these three modes of viewing it; and, after finishing his
demonstration, he adds,—
“But if any person does not understand these
demonstrations by means of lines, he will finally
give his assent to them when he has made the
following experiment:—Standing near a column, and
shutting each of the eyes in succession;—when the
_right_ eye is shut, some of those parts of the
column which were previously seen by the _right_ eye
on the _right_ side of the column, will not now be
seen by the _left_ eye; and when the _left_ eye is
shut, some of those parts which were formerly seen
by the _left_ eye on the _left_ side of the column,
will not now be seen by the _right_ eye. But when we,
at the same time, open both eyes, both these will be
seen, for a greater part is concealed when we look
with either of the two eyes, than when we look with
both at the same time.”[2]
[2] _De Usu Partium Corporis Humani_, edit. Lugduni, 1550, p. 593.
In such distinct and unambiguous terms, intelligible to the meanest
capacity, does this illustrious writer announce the fundamental law
of binocular vision—the grand principle of the Stereoscope, namely,
that _the picture of the solid column which we see with both eyes is
composed of two dissimilar pictures, as seen by each eye separately_.
As the vision of the solid column, therefore, was obtained by the union
of these dissimilar pictures, an instrument only was wanted to take
such pictures, and another to combine them. The Binocular Photographic
Camera was the one instrument, and the Stereoscope the other.
The subject of binocular vision was studied by various optical writers
who have flourished since the time of Galen. Baptista Porta, one of
the most eminent of them, repeats, in his work _On Refraction_, the
propositions of Euclid on the vision of a sphere with one and both
eyes, and he cites from Galen the very passage which we have given
above on the dissimilarity of the three pictures seen by each eye and
by both. Believing that we see only with one eye at a time, he denies
the accuracy of Euclid’s theorems, and while he admits the correctness
of the observations of Galen, he endeavours to explain them upon other
principles.
[Illustration: FIG. 1.]
In illustrating the views of Galen on the dissimilarity of the three
pictures which are requisite in binocular vision, he employs a much
more distinct diagram than that which is given by the Greek physician.
“Let A,” he says, “be the pupil of the right eye, B that of the left,
and DC the body to be seen. When we look at the object with both eyes
we see DC, while with the left eye we see EF, and with the right eye
GH. But if it is seen with one eye, it will be seen otherwise, for when
the left eye B is shut, the body CD, on the left side, will be seen in
HG; but when the right eye is shut, the body CD will be seen in FE,
whereas, when both eyes are opened at the same time, it will be seen
in CD.” These results are then explained by copying the passage from
Galen, in which he supposes the observer to repeat these experiments
when he is looking at a solid column.
In looking at this diagram, we recognise at once not only the
principle, but the construction of the stereoscope. The double
stereoscopic picture or slide is represented by HE; the right-hand
picture, or the one seen by the right eye, by HF; the left-hand
picture, or the one seen by the left eye, by GE; and the picture of the
solid column in full relief by DC, as produced midway between the other
two dissimilar pictures, HF and GE, by their union, precisely as in the
stereoscope.[3]
[3] Joan. Baptistæ Portæ Neap., _De Refractione Optices parte_, lib. v.
p. 132, and lib. vi. pp. 143-5. Neap. 1593.
Galen, therefore, and the Neapolitan philosopher, who has employed
a more distinct diagram, certainly knew and adopted the fundamental
principle of the stereoscope; and nothing more was required, for
producing pictures in full relief, than a simple instrument for uniting
HF and GE, the right and left hand dissimilar pictures of the column.
[Illustration: FIG. 2.]
In the treatise on painting which he left behind him in MS.,[4]
Leonardo da Vinci has made a distinct reference to the dissimilarity
of the pictures seen by each eye as the reason why “a painting, though
conducted with the greatest art, and finished to the last perfection,
both with regard to its contours, its lights, its shadows, and its
colours, can never shew a _relievo_ equal to that of the natural
objects, unless these be viewed at a distance and with a single
eye,”[5] which he thus demonstrates. “If an object C be viewed by a
single eye at A, all objects in the space behind it—included, as it
were, in a shadow ECF, cast by a candle at A—are invisible to an eye at
A; but when the other eye at B is opened, part of these objects become
visible to it; those only being hid from both eyes that are included,
as it were, in the double shadow CD, cast by two lights at A and B and
terminated in D; the angular space EDG, beyond D, being always visible
to both eyes. And the hidden space CD is so much the shorter as the
object C is smaller and nearer to the eyes. Thus he observes that
the object C, seen with both eyes, becomes, as it were, transparent,
according to the usual definition of a transparent thing, namely, that
which hides nothing beyond it. But this cannot happen when an object,
whose breadth is bigger than that of the pupil, is viewed by a single
eye. The truth of this observation is, therefore, evident, because a
painted figure intercepts all the space behind its apparent place, so
as to preclude the eyes from the sight of every part of the imaginary
ground behind it. Hence,” continues Dr. Smith, “we have one help to
distinguish the place of a near object more accurately _with both eyes
than with one_, inasmuch as we see it more detached from other objects
beyond it, _and more of its own surface, especially if it be roundish_.”
[4] _Trattata della Pictura, Scultura, ed Architettura._ Milan, 1584.
[5] Dr. Smith’s _Compleat System of Opticks_, vol. ii., Remarks, pp. 41
and 244.
We have quoted this passage, not from its _proving_ that Leonardo da
Vinci was acquainted with the fact that each eye, A, B, sees dissimilar
pictures of the sphere C, but because it has been referred to by Mr.
Wheatstone as the only remark on the subject of binocular vision which
he could find “after looking over the works of many authors who might
be expected to have made them.” We think it quite clear, however, that
the Italian artist knew as well as his commentator Dr. Smith, that each
eye, A and B, sees dissimilar parts of the sphere C. It was not his
purpose to treat of the binocular pictures of C, but his figure proves
their dissimilarity.
The subject of binocular vision was successfully studied by Francis
Aguillon or Aguilonius,[6] a learned Jesuit, who published his Optics
in 1613. In the first book of his work, where he is treating of the
vision of solids of all forms, (_de genere illorum quæ τὰ στέρεα [ta
sterea] nuncupantur_,) he has some difficulty in explaining, and fails
to do it, why the two dissimilar pictures of a solid, seen by each
eye, do not, when united, give a confused and imperfect view of it.
This discussion is appended to the demonstration of the theorem, “that
when an object is seen with two eyes, two optical pyramids are formed
whose common base is the object itself, and whose vertices are in the
eyes,”[7] and is as follows:—
[6] _Opticorum Libri Sex Philosophis juxta ac Mathematicis utiles._
Folio. Antverpiæ, 1613.
[7] In FIG. 1, AHF is the optical pyramid seen by the eye A, and BGE
the optical pyramid seen by the eye B.
“When one object is seen with two eyes, the angles at the vertices of
the optical pyramids (namely, HAF, GBE, Fig. 1) are not always equal,
for beside the direct view in which the pyramids ought to be equal,
into whatever direction both eyes are turned, they receive pictures of
the object under inequal angles, the greatest of which is that which
is terminated at the nearer eye, and the lesser that which regards the
remoter eye. This, I think, is perfectly evident; but I consider it as
worthy of admiration, how it happens that bodies seen by both eyes are
not all confused and shapeless, though we view them by the optical axes
fixed on the bodies themselves. For greater bodies, seen under greater
angles, appear lesser bodies under lesser angles. If, therefore, one
and the same body which is in reality greater with one eye, is seen
less on account of the inequality of the angles in which the pyramids
are terminated, (namely, HAF, GBE,[8]) the body itself must assuredly
be seen greater or less at the same time, and to the same person that
views it; and, therefore, since the images in each eye are dissimilar
(_minime sibi congruunt_) the representation of the object must appear
confused and disturbed (_confusa ac perturbata_) to the primary sense.”
[8] These angles are equal in this diagram and in the vision of a
sphere, but they are inequal in other bodies.
“This view of the subject,” he continues, “is certainly consistent with
reason, but, what is truly wonderful is, that it is not correct, for
bodies are seen clearly and distinctly with both eyes when the optic
axes are converged upon them. The reason of this, I think, is, that
the bodies do not appear to be single, because the apparent images,
which are formed from each of them in separate eyes, exactly coalesce,
(_sibi mutuo exacte congruunt_,) but because the common sense imparts
its aid equally to each eye, exerting its own power equally in the
same manner as the eyes are converged by means of their optical axes.
Whatever body, therefore, each eye sees with the eyes conjoined, the
common sense makes a single notion, not composed of the two which
belong to each eye, but belonging and accommodated to the imaginative
faculty to which it (the common sense) assigns it. Though, therefore,
the angles of the optical pyramids which proceed from the same object
to the two eyes, viewing it obliquely, are inequal, and though the
object appears greater to one eye and less to the other, yet the same
difference does not pass into the primary sense if the vision is made
only by the axes, as we have said, but if the axes are converged on
this side or on the other side of the body, the image of the same body
will be seen double, as we shall shew in Book iv., on the fallacies of
vision, and the one image will appear greater and the other less on
account of the inequality of the angles under which they are seen.”[9]
[9] Aguilonius, _Opticorum_, lib. ii. book xxxviii. pp. 140, 141.
Such is Aguilonius’s theory of binocular vision, and of the union
of the two dissimilar pictures in each eye by which a solid body is
seen. It is obviously more correct than that of Dr. Whewell and Mr.
Wheatstone. Aguilonius affirms it to be contrary to reason that two
dissimilar pictures can be united into a clear and distinct picture,
as they are actually found to be, and he is therefore driven to call
in the aid of what does not exist, a _common sense_, which rectifies
the picture. Dr. Whewell and Mr. Wheatstone have cut the Gordian knot
by maintaining what is impossible, that in binocular and stereoscopic
vision a long line is made to coincide with a short one, and a large
surface with a small one; and in place of conceiving this to be done by
a common sense overruling optical laws, as Aguilonius supposes, they
give to the tender and pulpy retina, the recipient of ocular pictures,
the strange power of contracting or expanding itself in order to
equalize inequal lines and inequal surfaces!
[Illustration: FIG. 3.]
In his fourth and very interesting book, on the fallacies of distance,
magnitude, position, and figure, Aguilonius resumes the subject of the
vision of solid bodies. He repeats the theorems of Euclid and Gassendi
on the vision of the sphere, shewing how much of it is seen by each
eye, and by both, whatever be the size of the sphere, and the distance
of the observer. At the end of the theorems, in which he demonstrates
that when the diameter of the sphere is equal to the distance between
the eyes we see exactly a hemisphere, he gives the annexed drawing
of the mode in which the sphere is seen by each eye, and by both. In
this diagram E is the right eye and D the left, CHFI the section of
that part of the sphere BC which is seen by the right eye E, BHGA the
section of the part which is seen by the left eye D, and BLC the half
of the great circle which is the section of the sphere as seen by both
eyes.[10] These three pictures of the solids are all dissimilar. The
right eye E does not see the part BLCIF of the sphere; the left eye
does not see the part BLCGA, while the part seen with both eyes is the
hemisphere BLCGF, the dissimilar segments BFG, CGF being united in its
vision.[11]
After demonstrating his theorems on the vision of spheres with one
and both eyes,[12] Aguilonius informs us, before he proceeds to the
vision of cylinders, that it is agreed upon that it is not merely true
with the sphere, but also with the cylinder, the cone, and all bodies
whatever, that the part which is seen is comprehended by tangent rays,
such as EB, EC for the right eye, in Fig. 3. “For,” says he, “since
these tangent lines are the outermost of all those which can be drawn
to the proposed body from the same point, namely, that in which the
eye is understood to be placed, it clearly follows that the part of
the body which is seen must be contained by the rays touching it on
all sides. For in this part no point can be found from which a right
line cannot be drawn to the eye, by which the correct visible form is
brought out.”[13]
[10] It is obvious that a complete hemisphere is not seen with both
eyes.
[11] Aguilonius, _Opticorum_, lib. iv. pp. 306, 307.
[12] In the last of these theorems Aguilonius describes and explains,
we believe for the first time, the _conversion of relief_ in the vision
of convex and concave surfaces. See Prop. xciv. p. 312.
[13] Id., p. 313.
Optical writers who lived after the time of Aguilonius seem to have
considered the subject of binocular vision as exhausted in his
admirable work. Gassendi,[14] though he treats the subject very
slightly, and without any figures, tells us that we see the left side
of the nose with the left eye, and the right side of it with the
right eye,—two pictures sufficiently dissimilar. Andrew Tacquet,[15]
though he quotes Aguilonius and Gassendi on the subject of seeing
distances with both eyes, says nothing on the binocular vision of
solids; and Smith, Harris, and Porterfield, only touch upon the subject
incidentally. In commenting on the passage which we have already
quoted from Leonardo da Vinci, Dr. Smith says, “Hence we have one help
to distinguish the place of a near object more accurately with both
eyes than with one, inasmuch as we see it more detached from other
objects beyond it, _and more of its own surface, especially if it be
roundish_.”[16] If any farther evidence were required that Dr. Smith
was acquainted with the dissimilarity of the images of a solid seen by
each eye, it will be found in his experiment with a “long ruler placed
between the eyebrows, and extended directly forward with its flat
sides, respecting the right hand and the left.” “By directing the eyes
to a remote object,” he adds, “the _right side of the ruler seen by
the right eye_ will appear on the left hand, and _the left side on the
right hand_, as represented in the figure.”[17]
[14] _Opera_, tom. ii. p. 394. Lugduni, 1658.
[15] _Opera Mathematica Optica_, tribus libris exposita, p. 136.
[16] _Opticks_, vol. ii., Remarks, pp. 41 and 245.
[17] Id., vol. i. p. 48, Fig. 196.
In his Treatise on Optics, published in 1775, Mr. Harris, when speaking
of the visible or apparent figures of objects, observes, that “we have
other helps for distinguishing _prominences of small parts_ besides
those by which we distinguish distances in general, as their degrees
of light and shade, and _the prospect we have round them_.” _And by
the parallax, on account of the distance betwixt our eyes, we can
distinguish besides the front part of the two sides of a near object
not thicker than the said distance, and this gives a visible relievo
to such objects, which helps greatly to raise or detach them from the
plane in which they lie. Thus the nose on a face is the more remarkably
raised by our seeing both sides of it at once._“[18] _That is, the
relievo is produced by the combination of the two dissimilar pictures
given by each eye._
Without referring to a figure given by Dr. Porterfield, in which he
actually gives drawings of an object as seen by each eye in binocular
vision,[19] the one exhibiting the object as seen endwise by the right
eye, and the other the same object as seen laterally by the left eye,
we may appeal to the experience of every optical, or even of every
ordinary observer, in support of the fact, that the dissimilarity of
the pictures in each eye, by which we see solid objects, is known to
those who have never read it in Galen, Porta, or Aguilonius. Who has
not observed the fact mentioned by Gassendi and Harris, that their left
eye sees only the left side of their nose, and their right eye the
right side, two pictures sufficiently dissimilar? Who has not noticed,
as well as Dr. Smith, that when they look at any thin, flat body, such
as a thin book, they see both sides of it—_the left eye only the left
side of it, and the right eye only the right side_, while the back, or
the part nearest the face, is seen by each eye, and both the sides and
the back by both the eyes? What student of perspective is there—master
or pupil, male or female—who does not know, as certainly as he knows
his alphabet, that the picture of a chair or table, or anything else,
drawn from _one point of sight_, or as seen by one eye placed in that
point, is _necessarily dissimilar_ to another drawing of the same
object taken from another point of sight, or as seen by the other eye
placed in a point 2½ inches distant from the first? If such a person is
to be found, we might then admit that the dissimilarity of the pictures
in each eye was not known to every student of perspective.[20]
[18] _Treatise on Optics_, p. 171; see also sect. 64, p. 113.
[19] _Treatise on the Eye_, vol i. p. 412, Plate 5, Fig. 37.
[20] As Mr. Wheatstone himself describes the dissimilar pictures or
drawings as “two different projections of the same object seen from
_two points of sight_, the distance between which is equal to the
interval between the eyes of the observer,” it is inconceivable on what
ground he could imagine himself to be the discoverer of so palpable and
notorious a fact as that the pictures of a body seen by two eyes—_two
points of sight_, must be dissimilar.
Such was the state of our knowledge of binocular vision when two
individuals, Mr. Wheatstone, and Mr. Elliot, now Teacher of Mathematics
in Edinburgh, were directing their attention to the subject. Mr.
Wheatstone communicated an important paper on the Physiology of Vision
to the British Association at Newcastle in August 1838, and exhibited
an instrument called a Stereoscope, by which he united the two
dissimilar pictures of solid bodies, the =τὰ στέρεα=, (_ta sterea_ of
Aguilonius,) and thus reproduced, as it were, the bodies themselves.
Mr. Wheatstone’s paper on the subject, which had been previously
read at the Royal Society on the 21st of June, was printed in their
Transactions for 1838.[21]
[21] _Phil. Trans._, 1838, pp. 371-394.
Mr. Elliot was led to the study of binocular vision in consequence
of having written an Essay, so early as 1823, for the Class of Logic
in the University of Edinburgh, “On the means by which we obtain
our knowledge of distances by the Eye.” Ever since that date he was
familiar with the idea, that the relief of solid bodies seen by the
eye was produced by the union of the dissimilar pictures of them in
each eye, but he never imagined that this idea was his own, believing
that it was known to every student of vision. Previous to or during the
year 1834, he had resolved to construct an instrument for uniting two
dissimilar pictures, or of constructing a stereoscope; but he delayed
doing this till the year 1839, when he was requested to prepare an
original communication for the Polytechnic Society, which had been
recently established in Liverpool. He was thus induced to construct
the instrument which he had projected, and he exhibited it to his
friends, Mr. Richard Adie, optician, and Mr. George Hamilton, lecturer
on chemistry in Liverpool, who bear testimony to its existence at that
date. This simple stereoscope, without lenses or mirrors, consisted of
a wooden box 18 inches long, 7 broad, and 4½ deep, and at the bottom
of it, or rather its farther end, was placed a slide containing two
dissimilar pictures of a landscape as seen by each eye. Photography did
not then exist, to enable Mr. Elliot to procure two views of the same
scene, as seen by each eye, but he drew the transparency of a landscape
with three distances. The _first_ and most remote was the moon and
the sky, and a stream of water from which the moon was reflected, the
two moons being placed nearly at the distance of the two eyes, or 2½
inches, and the two reflected moons at the same distance. The _second_
distance was marked by an old cross about a hundred feet off; and the
_third_ distance by the withered branch of a tree, thirty feet from the
observer. In the right-hand picture, one arm of the cross just touched
the disc of the moon, while, in the left-hand one, it projected over
one-third of the disc. The branch of the tree touched the outline of a
distant hill in the one picture, but was “a full moon’s-breadth” from
it on the other. When these dissimilar pictures were united by the
eyes, a landscape, certainly a very imperfect one, was seen in relief,
composed of three distances.
Owing, no doubt, to the difficulty of procuring good binocular
pictures, Mr. Elliot did not see that his contrivance would be very
popular, and therefore carried it no farther. He had never heard of Mr.
Wheatstone’s stereoscope till he saw his paper on Vision reprinted in
the _Philosophical Magazine_ for March 1852, and having perused it, he
was convinced not only that Mr. Wheatstone’s theory of the instrument
was incorrect, but that his claim to the discovery of the dissimilarity
of the images in each eye had no foundation. He was, therefore, led
to communicate to the same journal the fact of his having himself,
thirteen years before, constructed and used a stereoscope, which was
still in his possession. In making this claim, Mr. Elliot had no
intention of depriving Mr. Wheatstone of the credit which was justly
due to him; and as the claim has been publicly made, we have described
the nature of it as a part of scientific history.
In Mr. Wheatstone’s ingenious paper of 1838, the subject of binocular
vision is treated at considerable length. He gives an account of
the opinions of previous writers, referring repeatedly to the works
of Aguilonius, Gassendi, and Baptista Porta, in the last of which
the views of Galen are given and explained. In citing the passage
which we have already quoted from Leonardo da Vinci, and inserting
the figure which illustrates it, he maintains that Leonardo da Vinci
was not aware “that the object (C in Fig. 2) presented a different
appearance to each eye.” “_He failed_,” he adds, “_to observe this,
and no subsequent writer, to my knowledge, has supplied the omission.
The projection of two obviously dissimilar pictures on the two retinæ,
when a single object is viewed, while the optic axes converge, must
therefore be regarded as a new fact in the theory of vision._” Now,
although Leonardo da Vinci does not state in so many words that he was
aware of the dissimilarity of the two pictures, the fact is obvious in
his own figure, and he was not led by his subject to state the fact
at all. But even if the fact had not stared him in the face he must
have known it from the Optics of Euclid and the writings of Galen,
with which he could not fail to have been well acquainted. That the
dissimilarity of the two pictures is _not a new fact_ we have already
placed beyond a doubt. The fact is expressed in words, and delineated
in drawings, by Aguilonius and Baptista Porta. It was obviously known
to Dr. Smith, Mr. Harris, Dr. Porterfield, and Mr. Elliot, before it
was known to Mr. Wheatstone, and we cannot understand how he failed
to observe it in works which he has so often quoted, and in which he
professes to have searched for it.
This remarkable property of binocular vision being thus clearly
established by preceding writers, and admitted by himself, as the cause
of the vision of solidity or distance, Mr. Wheatstone, as Mr. Elliot
had done before him, thought of an instrument for uniting the two
dissimilar pictures optically, so as to produce the same result that is
obtained by the convergence of the optical axes. Mr. Elliot thought of
doing this by the eyes alone; but Mr. Wheatstone adopted a much better
method of doing it by reflexion. He was thus led to construct an
apparatus, to be afterwards described, consisting of two plane mirrors,
placed at an angle of 90°, to which he gave the name of _stereoscope_,
anticipating Mr. Elliot both in the construction and publication of his
invention, but not in the general conception of a stereoscope.
After describing his apparatus, Mr. Wheatstone proceeds to consider
(in a section entitled, “Binocular vision of objects of different
magnitudes”) “what effects will result from presenting similar images,
differing only in magnitude, to analogous parts of the retina.” “For
this purpose,” he says, “two squares or circles, _differing obviously_
but not extravagantly in size, may be drawn on two separate pieces of
paper, and placed in the stereoscope, so that the reflected image of
each shall be equally distant from the eye by which it is regarded. _It
will then be seen that notwithstanding this difference they coalesce
and occasion a single resultant perception._” The fact of coalescence
being supposed to be perfect, the author next seeks to determine the
difference between the length of two lines which the eye can force
into coalescence, or “the limits within which the single appearance
subsists.” He, therefore, unites two images of equal magnitude, by
making one of them visually less from distance, and he states that,
“by this experiment, _the single appearance of two images of different
size is demonstrated_.” Not satisfied with these erroneous assertions,
he proceeds to give a sort of rule or law for ascertaining the
relative size of the two unequal pictures which the eyes can force
into coincidence. The inequality, he concludes, must not exceed the
difference “between the projections of the same object when seen in the
most oblique position of the eyes (_i.e._, both turned to the extreme
right or the extreme left) ordinarily employed.” Now, this rule, taken
in the sense in which it is meant, is simply _a truism_. It merely
states that the difference of the pictures which the eyes _can_ make to
coalesce is equal to the difference of the pictures which the eyes do
make to coalesce in their most oblique position; but though _a truism_
it is not _a truth_, first, because no real coincidence ever can take
place, and, secondly, because no apparent coincidence is effected when
the difference of the picture is greater than what is above stated.
From these principles, which will afterwards be shewn to be erroneous,
Mr. Wheatstone proceeds “to examine _why_ two dissimilar pictures
projected on the two retinæ _give_ rise to the perception _of an object
in relief_.” “I will not attempt,” he says, “at present to give the
complete solution of this question, which is far from being so easy
as at first glance it may appear to be, and is, indeed, _one of great
complexity_. I shall, in this case, merely consider the most obvious
explanations which might be offered, and shew their insufficiency to
explain the whole of the phenomena.
“_It may be supposed_ that we see only one point of a field of view
distinctly at the same instant, the one, namely, to which the optic
axes are directed, while all other points are seen so indistinctly that
the mind does not recognise them to be either single or double, and
that the figure is appreciated by successively directing the point of
convergence of the optic axes successively to a sufficient number of
its points to enable us to judge accurately of its form.
“That there is _a degree of indistinctness_ in those parts of the field
of view to which the eyes are not immediately directed, and which
increases with the distance from that point, cannot be doubted; and
it is also true that the objects there obscurely seen are _frequently
doubled_. In _ordinary_ vision, it may be said, this indistinctness and
duplicity are not attended to, because the eyes shifting continually
from point to point, every part of the object is successively rendered
distinct, and the perception of the object is not the consequence
of a single glance, during which _a small part of it only is seen
distinctly_, but is formed from a comparison of all the pictures
successively seen, while the eyes were changing from one point of an
object to another.
“_All this is_ IN SOME DEGREE _true, but were it entirely so_ no
appearance of relief should present itself when the eyes remain
intently fixed on one point of a binocular image in the stereoscope.
But in performing the experiment carefully, it will be found, provided
the picture do not extend far beyond the centres of distinct vision,
that the image is still seen single, and in relief, when in this
condition.”[22]
[22] _Phil. Trans._, 1838, pp. 391, 392.
In this passage the author makes a distinction between _ordinary
binocular vision_, and binocular vision through the stereoscope,
whereas in reality there is none. The theory of both is exactly the
same. The muscles of the two eyes unite the two dissimilar pictures,
and exhibit the solid, in ordinary vision; whereas in stereoscopic
vision the images are united by reflexion or refraction, the eyes in
both cases obtaining the vision of different distances by rapid and
successive convergences of the optical axes. Mr. Wheatstone notices
_the degree of indistinctness_ in the parts of the picture to which
the eyes are not immediately directed; but he does not notice the
“_confusion and incongruity_” which Aguilonius says ought to exist,
in consequence of some parts of the resulting relievo being seen of
one size by the left eye alone,—other parts of a different size by the
right eye alone, and other parts by both eyes. This confusion, however,
Aguilonius, as we have seen, found not to exist, and he ascribes it to
the influence of a _common sense_ overruling the operation of physical
laws. Erroneous as this explanation is, it is still better than that of
Mr. Wheatstone, which we shall now proceed to explain.
In order to disprove the theory referred to in the preceding extract,
Mr. Wheatstone describes two experiments, which he says _are equally
decisive against it_, the first of them only being subject to rigorous
examination. With this view he draws “two lines about two inches long,
and inclined towards each other, on a sheet of paper, and having
caused them to coincide by converging the optic axes to a point nearer
than the paper, he looks intently on the upper end of the resultant
line without allowing the eyes to wander from it for a moment. The
_entire line will appear single_, and in its proper relief, &c....
The eyes,” he continues, “sometimes become fatigued, which causes the
line to become double at those parts to which the optic axes are not
fixed, _but in such case all appearance of relief vanishes_. The same
experiment may be tried with small complex figures, but the pictures
should not extend too far beyond the centre of the retinæ.”
Now these experiments, if rightly made and interpreted, are not
_decisive against_ the theory. It is not true that the entire line
appears single when the axes are converged upon the upper end of the
resultant line, and it is not true that the disappearance of the
relief when it does disappear arises from the eye being fatigued.
In the combination of more complex figures, such as two similar
rectilineal figures contained by lines of unequal length, neither the
inequalities nor the entire figure will appear single when the axes are
converged upon any one point of it.
In the different passages which we have quoted from Mr. Wheatstone’s
paper, and in the other parts of it which relate to binocular vision,
he is obviously halting between truth and error, between theories
which he partly believes, and ill-observed facts which he cannot
reconcile with them. According to him, certain truths “may be supposed”
to be true, and other truths may be “in some degree true,” but “not
entirely so;” and thus, as he confesses, the problem of binocular and
stereoscopic vision “is indeed one of great complexity,” of which “he
will not attempt at present to give the complete solution.” If he had
placed a proper reliance on the law of visible direction which he
acknowledges I have established, and “with which,” he says, “the laws
of visible direction for binocular vision ought to contain nothing
inconsistent,” he would have seen the impossibility of the two eyes
uniting two lines of inequal length; and had he believed in the law of
distinct vision he would have seen the impossibility of the two eyes
obtaining single vision of any more than one point of an object at a
time. These laws of vision are as rigorously true as any other physical
laws,—as completely demonstrated as the law of gravity in Astronomy,
or the law of the Sines in Optics; and the moment we allow them to be
tampered with to obtain an explanation of physical puzzles, we convert
science into legerdemain, and philosophers into conjurors.
Such was the state of our stereoscopic knowledge in 1838, after the
publication of Mr. Wheatstone’s interesting and important paper.
Previous to this I communicated to the British Association at
Newcastle, in August 1838, a paper, in which I established the law
of visible direction already mentioned, which, though it had been
maintained by preceding writers, had been proved by the illustrious
D’Alembert to be incompatible with observation, and the admitted
anatomy of the human eye. At the same meeting Mr. Wheatstone exhibited
his stereoscopic apparatus, which gave rise to an animated discussion
on the theory of the instrument. Dr. Whewell maintained that the
retina, in uniting, or causing to coalesce into a single resultant
impression two lines of different lengths, had the power either of
contracting the longest, or lengthening the shortest, or what might
have been suggested in order to give the retina only half the trouble,
that it contracted the long line as much as it expanded the short one,
and thus caused them to combine with a less exertion of muscular power!
In opposition to these views, I maintained that the retina, a soft
pulpy membrane which the smallest force tears in pieces, had no such
power,—that a hypothesis so gratuitous was not required, and that the
law of visible direction afforded the most perfect explanation of all
the stereoscopic phenomena.
In consequence of this discussion, I was led to repeat my experiments,
and to inquire whether or not the eyes in stereoscopic vision _did
actually_ unite the two lines of different lengths, or of different
apparent magnitudes. I found that they did not, and that no such union
was required to convert by the stereoscope two plane pictures into
the apparent whole from which they were taken as seen by each eye.
These views were made public in the lectures on the _Philosophy of the
Senses_, which I occasionally delivered in the College of St. Salvator
and St. Leonard, St. Andrews, and the different stereoscopes which I
had invented were also exhibited and explained.
In examining Dr. Berkeley’s celebrated Theory of Vision, I saw the vast
importance of establishing the law of visible direction, and of proving
by the aid of binocular phenomena, and in opposition to the opinion of
the most distinguished metaphysicians, that we actually see a third
dimension in space, I therefore submitted to the Royal Society of
Edinburgh, in January 1843, a paper _On the law of visible position in
single and binocular vision, and on the representation of solid figures
by the union of dissimilar plane pictures on the retina_. More than
twelve years have now elapsed since this paper was read, and neither
Mr. Wheatstone nor Dr. Whewell have made any attempt to defend the
views which it refutes.
In continuing my researches, I communicated to the Royal Society of
Edinburgh, in April 1844, a paper _On the knowledge of distance as
given by binocular vision_, in which I described several interesting
phenomena produced by the union of _similar_ pictures, such as those
which form the patterns of carpets and paper-hangings. In carrying on
these inquiries I found the reflecting stereoscope of little service,
and ill fitted, not only for popular use, but for the application
of the instrument to various useful purposes. I was thus led to the
construction of several new stereoscopes, but particularly to the
_Lenticular Stereoscope_, now in universal use. They were constructed
in St. Andrews and Dundee, of various materials, such as wood,
tin-plate, brass, and of all sizes, from that now generally adopted,
to a microscopic variety which could be carried in the pocket. New
geometrical drawings were executed for them, and binocular pictures
taken by the sun were lithographed by Mr. Schenck of Edinburgh.
Stereoscopes of the lenticular form were made by Mr. Loudon, optician,
in Dundee, and sent to several of the nobility in London, and in other
places, and an account of these stereoscopes, and of a binocular camera
for taking portraits, and copying statues, was communicated to the
Royal Scottish Society of Arts, and published in their Transactions.
It had never been proposed to apply the reflecting stereoscope to
portraiture or sculpture, or, indeed, to any useful purpose; but it was
very obvious, after the discovery of the Daguerreotype and Talbotype,
that binocular drawings could be taken with such accuracy as to
exhibit in the stereoscope excellent representations in relief, both
of living persons, buildings, landscape scenery, and every variety of
sculpture. In order to shew its application to the most interesting of
these purposes, Dr. Adamson of St. Andrews, at my request, executed
two binocular portraits of himself, which were generally circulated
and greatly admired. This successful application of the principle to
portraiture was communicated to the public, and recommended as an art
of great domestic interest.
After endeavouring in vain to induce opticians, both in London and
Birmingham, (where the instrument was exhibited in 1849 to the
British Association,) to construct the lenticular stereoscope, and
photographers to execute binocular pictures for it, I took with me
to Paris, in 1850, a very fine instrument, made by Mr. Loudon in
Dundee, with the binocular drawings and portraits already mentioned.
I shewed the instrument to the Abbé Moigno, the distinguished author
of _L’Optique Moderne_, to M. Soleil and his son-in-law, M. Duboscq,
the eminent Parisian opticians, and to some members of the Institute
of France. These gentlemen saw at once the value of the instrument,
not merely as one of amusement, but as an important auxiliary in the
arts of portraiture and sculpture. M. Duboscq immediately began to make
the lenticular stereoscope for sale, and executed a series of the most
beautiful binocular Daguerreotypes of living individuals, statues,
bouquets of flowers, and objects of natural history, which thousands of
individuals flocked to examine and admire. In an interesting article
in _La Presse_,[23] the Abbé Moigno gave the following account of the
introduction of the instrument into Paris:—
[23] December 28, 1550.
“In his last visit to Paris, Sir David Brewster intrusted the models
of his stereoscope to M. Jules Duboscq, son-in-law and successor of M.
Soleil, and whose intelligence, activity, and affability will extend
the reputation of the distinguished artists of the Rue de l’Odeon, 35.
M. Jules Duboscq has set himself to work with indefatigable ardour.
Without requiring to have recourse to the binocular camera, he has,
with the ordinary Daguerreotype apparatus, procured a great number
of dissimilar pictures of statues, bas-reliefs, and portraits of
celebrated individuals, &c. His stereoscopes are constructed with more
elegance, and even with more perfection, than the original English
(Scotch) instruments, and while he is shewing their wonderful effects
to natural philosophers and amateurs who have flocked to him in crowds,
there is a spontaneous and unanimous cry of admiration.”
While the lenticular stereoscope was thus exciting much interest in
Paris, not a single instrument had been made in London, and it was not
till a year after its introduction into France that it was exhibited in
England. In the fine collection of philosophical instruments which M.
Duboscq contributed to the Great Exhibition of 1851, and for which he
was honoured with a Council medal, he placed a lenticular stereoscope,
with a beautiful set of binocular Daguerreotypes. This instrument
attracted the particular attention of the Queen, and before the closing
of the Crystal Palace, M. Duboscq executed a beautiful stereoscope,
which I presented to Her Majesty in his name. In consequence of this
public exhibition of the instrument, M. Duboscq received several orders
from England, and a large number of stereoscopes were thus introduced
into this country. The demand, however, became so great, that opticians
of all kinds devoted themselves to the manufacture of the instrument,
and photographers, both in Daguerreotype and Talbotype, found it a most
lucrative branch of their profession, to take binocular portraits of
views to be thrown into relief by the stereoscope. Its application to
sculpture, which I had pointed out, was first made in France, and an
artist in Paris actually copied a statue from the _relievo_ produced by
the stereoscope.
Three years after I had published a description of the lenticular
stereoscope, and after it had been in general use in France and
England, and the reflecting stereoscope forgotten,[24] Mr. Wheatstone
printed, in the _Philosophical Transactions_ for 1852, a paper on
Vision, in which he says that he had previously used “an apparatus in
which prisms were employed to deflect the rays of light proceeding from
the pictures, so as to make them appear to occupy the same place;” and
he adds, “I have called it the _refracting_ stereoscope.”[25] Now,
whatever Mr. Wheatstone may have done with prisms, and at whatever time
he may have done it, I was the first person who published a description
of stereoscopes both with _refracting_ and reflecting prisms; and
during the three years that elapsed after he had read my paper, he made
no claim to the suggestion of prisms till after the great success of
the lenticular stereoscope. The reason why he then made the claim, and
the only reason why we do not make him a present of the suggestion,
will appear from the following history:—
[24] “Le fait est,” says the Abbé Moigno, “_que le stéréoscope par
réflexion était presque complètement oublié_, lorsque Sir David
Brewster construisit son stéréoscope par refraction que nous allons
décrire.”—_Cosmos_, vol. i. p. 4, 1852.
[25] _Phil. Trans._, 1852, p. 6.
In the paper above referred to, Mr. Wheatstone says,—“I recommend, as
a convenient arrangement of the _refracting_ stereoscope for viewing
Daguerreotypes of small dimensions, the instrument represented, (Fig.
4,) shortened in its length from 8 inches to 5, and lenses 5 inches
focal distance, placed before and close to the prisms.”[26] Although
this refracting apparatus, which is simply a _deterioration_ of the
lenticular stereoscope, is recommended by Mr. Wheatstone, nobody
either makes it or uses it. The semi-lenses or quarter-lenses of the
lenticular stereoscope _include a virtual and absolutely perfect
prism_, and, what is of far more consequence, each lens is a variable
lenticular prism, so that, when the eye-tubes are placed at different
distances, the lenses have different powers of displacing the pictures.
They can thus unite pictures placed at different distances, which
cannot be done by any combination of whole lenses and prisms.
[26] _Ibid._, pp. 9, 10.
In the autumn of 1854, after all the facts about the stereoscope
were before the public, and Mr. Wheatstone in full possession of all
the merit of having anticipated Mr. Elliot in the publication of
his stereoscopic apparatus, and of his explanation of the theory of
stereoscopic relief, such as it was, he thought it proper to revive
the controversy by transmitting to the Abbé Moigno, for publication in
Cosmos, an extract of a letter of mine dated 27th September 1838. This
extract was published in the _Cosmos_ of the 15th August 1854,[27] with
the following illogical commentary by the editor.
“Nous avons eu tort mille fois d’accorder à notre
illustre ami, Sir David Brewster, l’invention du
stéréoscope par réfraction. M. Wheatstone, en effet,
a mis entre nos mains une lettre datée, le croirait
on, du 27 Septembre 1838, dans lequel nous avons
lû ces mots écrits par l’illustre savant Ecossais:
‘I have also stated that you promised to order
for me your stereoscope, both with reflectors and
PRISMS. J’ai aussi dit (à Lord Rosse[28])
que vous aviez promis de commander pour moi votre
stéréoscope, celui avec réflecteurs et celui avec
prismes.’ Le stéréoscope par réfraction est donc,
aussi bien que le stéréoscope par réflexion, le
stéréoscope de M. Wheatstone, qui l’avait inventé
en 1838, et le faisait construire à cette époque
pour Sir David Brewster lui-même. Ce que Sir
David Brewster a imaginée, et c’est une idée très
ingénieuse, dont M. Wheatstone ne lui disputât
jamais la gloire, c’est de former les deux prismes
du stéréoscope par réfraction avec les deux moitiés
d’une même lentille.”
[27] Vol. v. livre viii. p. 241.
[28] Mr. Andrew Ross, the celebrated optician!
That the reader may form a correct idea of the conduct of Mr.
Wheatstone in making this claim indirectly, and in a foreign journal,
whose editor he has willingly misled, I must remind him that I
first saw the reflecting stereoscope at the meeting of the British
Association at Newcastle, in the middle of _August 1838_. It is proved
by my letter that he and I then conversed on the subject of _prisms_,
which at that time he had never thought of. I suggested prisms for
displacing the pictures, and Mr. Wheatstone’s natural reply was, that
they must be _achromatic prisms_. This fact, if denied, may be proved
by various circumstances. His paper of 1838 contains no reference to
prisms. If he had suggested the use of prisms in August 1838, he would
have inserted his suggestion in that paper, which was then unpublished;
and if he had _only once_ tried a prism stereoscope, he never would
have used another. On my return to Scotland, I ordered from Mr. Andrew
Ross one of the reflecting stereoscopes, and one made with achromatic
prisms; but my words do not imply that Mr. Wheatstone was the first
person who suggested prisms, and still less that he ever made or used
a stereoscope with prisms. But however this may be, it is a most
extraordinary statement, which he allows the Abbé Moigno to make, and
which, though made a year and a half ago, he has not enabled the Abbé
to correct, _that a stereoscope with prisms was made for me_ (or for
any other person) _by Mr. Ross_. I never saw such an instrument, or
heard of its being constructed: I supposed that after our conversation
Mr. Wheatstone might have tried achromatic prisms, and in 1848, when I
described my single prism stereoscope, I stated what I now find is not
correct, that _I believed_ Mr. Wheatstone had used _two_ achromatic
prisms. The following letter from Mr. Andrew Ross will prove the main
fact that he never constructed for me, or for Mr. Wheatstone, any
refracting stereoscope:—
”2, FEATHERSTONE BUILDINGS,
_28th September 1854_.
“DEAR SIR,—In reply to yours of the 11th
instant, I beg to state that I never supplied you
with a stereoscope in which prisms were employed in
place of plane mirrors. I have a perfect recollection
of being called upon either by yourself or Professor
Wheatstone, some fourteen years since, to make
achromatized prisms for the above instrument. I also
recollect that I did not proceed to manufacture them
in consequence of the great bulk of an achromatized
prism, with reference to their power of deviating a
ray of light, and at that period glass sufficiently
free from striæ could not readily be obtained, and
was consequently very high-priced.—I remain, &c. &c.
“ANDREW ROSS.
“To Sir David Brewster.”
Upon the receipt of this letter I transmitted a copy of it to the Abbé
Moigno, to shew him how he had been misled into the statement, “that
Mr. Wheatstone had caused a stereoscope with prisms to be constructed
for me;” but neither he nor Mr. Wheatstone have felt it their duty to
withdraw that erroneous statement.
In reference to the comments of the Abbé Moigno, it is necessary to
state, that when he wrote them he had in his possession my printed
description of the single-prism, and other stereoscopes,[29] in which
I mention my belief, now proved to be erroneous, that Mr. Wheatstone
had used achromatic prisms, so that he had, on my express authority,
the information which surprised him in my letter. The Abbé also must
bear the responsibility of a glaring misinterpretation of my letter
of 1838. In that letter I say that Mr. Wheatstone promised _to order
certain things from Mr. Ross_, and the Abbé declares, contrary to the
express terms of the letter, as well as to fact, _that these things
were actually constructed for me_. The letter, on the contrary, does
not even state that Mr. Wheatstone complied with my request, and it
does not even appear from it that the reflecting stereoscope was made
for me by Mr. Ross.
[29] The Abbé gave an abstract of this paper in the French journal _La
Presse_, December 28, 1850.
Such is a brief history of the lenticular stereoscope, of its
introduction into Paris and London, and of its application to
portraiture and sculpture. It is now in general use over the whole
world, and it has been estimated that upwards of half a million
of these instruments have been sold. A Stereoscope Company has
been established in London[30] for the manufacture and sale of the
lenticular stereoscope, and for the production of binocular pictures
for educational and other purposes. Photographers are now employed
in every part of the globe in taking binocular pictures for the
instrument,—among the ruins of Pompeii and Herculaneum—on the glaciers
and in the valleys of Switzerland—among the public monuments in the
Old and the New World—amid the shipping of our commercial harbours—in
the museums of ancient and modern life—in the sacred precincts of
the domestic circle—and among those scenes of the picturesque and the
sublime which are so affectionately associated with the recollection of
our early days, and amid which, even at the close of life, we renew,
with loftier sentiments and nobler aspirations, the youth of our being,
which, in the worlds of the future, is to be the commencement of a
longer and a happier existence.
[30] No. 54, Cheapside, and 313, Oxford Street. The prize of twenty
guineas which they offered for the best short popular treatise on the
Stereoscope, has been adjudged to Mr. Lonie, Teacher of Mathematics in
the Madras Institution, St. Andrews. The second prize was given to the
Rev. R. Graham, Abernyte, Perthshire.