AN IMPROVED STEREOSCOPE
Year: 1857

page_text: oy the Ad.)
PROVISIONAL SPECIFICATION left by Walter Hardie at the Office of
the Commissioners of Patents, with bis Petition, on the 9th November
1857.
I, WALTER HARDIE, of No. 6, Pitt Street, in the City of Edinburgh,
Printer, do hereby declare the nature of the said Invention for." AN IMPROVED
STEREOSCOPE," to be as follows:-
The optical part of the instrument consists of two plane mirrors of metal or
thin silvered glass placed side by side, with their reflecting surfaces at right
angles to a vertical plane midway between them, but inclined to each other at
an angle, bearing a certain relation (indicated below) to the positions and
distances of the pictures. The pictures are viewed by reflection from these
mirrors, the eyes being placed so that the line joining them is perpendicular to
and bisected by the above-mentioned vertical plane.
The pictures must be placed one above the other, head to foot, inverted with
respect to the observer, and with a lateral separation of the corresponding
vanishing points, amounting to about two inches and a half (the distance
between the eyes); those points in the picture for the right eye being to the
right of the corresponding points in the other picture. The amount of their
vertical separation must be regulated, as indicated below, by their distance from
the mirrors and the angle of the mirrors. They may be mounted in this posi-
tion upon stiff card, which should have a fold or hinge upon a line parallel to

page_text: the horizons, and midway between them, as the pictures require to be inclined
to each other upon this line when placed in the stereoscope.
When viewed, the surfaces of the pictures must be respectively perpen-
dicular to two equal lines lying in the vertical plane above mentioned, and
proceeding from the point in which that plane is cut by the line of intersection
of the reflecting surfaces to points in the horizons of the pictures. These lines
should be less than the effective distance of the pictures (that is, the distance
from the eyes at which they are intended to be viewed) by two or three inches,
that being a convenient distance of the eyes from the intersection of the
mirrors.
Figure 1 is an orthographic projection of the essential parts on the vertical
plane above mentioned; F, H, is the picture; and M, O, the mirror for one
eye (say the right); F, H', and M', O', the picture and mirror for the other
eye and E is the point midway between the eyes when the instrument is in
use; I, C, I¹, C¹, are the lines above-mentioned proceeding from the inter-
section of the mirrors and meeting the picture planes perpendicularly in their
horizons at C, C¹, and they also indicate the positions of the planes of the
visual rays from the horizons incident upon the line of intersection of the
reflecting surfaces. The mirrors are so placed that these two planes of rays
I, C, I, C¹, are reflected into one plane represented by I, E, so that the eyes
at E see the horizons coincident in that plane. And since the picture surfaces
are perpendicular respectively to the equal lines I, C, I, C, it is evident that
their reflected images must be perpendicular to I, C", and equidistant from I,
and therefore coincident as at H", F". It is therefore obvious, that to produce
the coincidence of the planes of the reflected rays, the angle M, I, M¹, between
the mirrors must (as in Hadley's sextant) be equal to half the angle C, I, C.
The length of the mirrors in the direction M, O, M', 0, must be sufficient to
reach respectively the lines F, R, H, R, and F¹, L, H', L, drawn from the ex-
tremeties of the pictures to the points R and L, taken in the lines C, I, C, I,
produced so that I, R, I, I, respectively equal I, E.
To give a rectangular field of view of the greatest possible extent (the
dimensions and positions above mentioned being given) the sides of the mirrors
must be shaped in the manner indicated by the following construction:-
In Figure 2, M, O, M', O', represent the mirrors as in Figure 1, and m", I,
is the reflected image of M, I, which would be seen by the left eye in M¹, O'
if the inner edges of the mirrors met the middle vertical plane. Figure 3 is
an orthographic projection of the whole upon the plane, which cuts that of
Figure I perpendicularly in the line E, C", and is constructed as follows:-
Draw E, C, parallel to E, I, (Figure 2), and E, L, M, d, m", q, perpen-

page_text: dicular to E, I, meeting E', C", in E¹, d, and q. Let E¹ be the point midway
between the eyes, and R, L, the places of the eyes. Through a, the point
midway between 9 and d, draw L, a, b, and R, x, a; take E, C, equal to
E, C, in Figure 1, and through C draw a, C", b, perpendicular to E¹, C¹¹;.
meeting L, a, b, and R, a, a, in a and b; then a, b, represents the width of
the binocular field of view at the distance E¹, C. Join L, a, and R, b, and
then draw to E', C", the perpendiculars O¹, i, k, O, e, f, M³, g, h.
The
Figures g, h, k, i, and c, d, f, e, are the orthographic projections of the mirrors.
M', 0, M, O, respectively. For it is obvious that if the corner d' of the
mirror were extended towards d, the reflected image y would approach q, and
become visible to L within g, i, while if g, i, were removed further to the right,
it would include y in the field of view, and in either case y would restrict the
field of view in that direction.
To reduce the projection of the mirror i, h, in Figure 3, to a plan in its own
plane, draw k, r, perpendicular to g, h. In Figure 4, take k, r, equal to the
length of the mirror M', O', (Figure 1), and through its extremities draw
perpendiculars g, h, i, k, making r, g, r, h, and i, k, equal to the corresponding
lines in Figure 3; then g, h, i, k, will be the outline of the mirror. In the
same way the outline of the other mirror may be found."
The inclination of the mirrors may be adjustable, but it is more convenient
to have it fixed, in which case it is obvious that that the angle C, I, C', (Figure
1) subtended by the horizons of the pictures, and the angle of inclination of
the pictures F, F, H, are also fixed; the former angle being twice the angle
M, I; M³, and the latter being the supplement of C, I, C', and consequently
that the horizons are equidistant from the line of intersection of the picture-
surfaces. It is also evident that the tallest pictures that can be thus placed,
so as to have the adjacent head and foot in contact at that line, must have the
horizon midway between the head and foot, as in Figure 1. If pictures of the
same size, having the horizon above or below the centre, were placed in
the same manner, (that is, with their horizons equidistant from the line of
intersection of their planes), their adjacent head and foot would be disjoined,
as in Figure 5, and the mirrors would require an upward or downward exten-
sion, corresponding to the head or foot of the picture. But the obvious
inconvenience of this arrangenient makes that of Figure 1 much preferable
for such pictures; as the slight error of position thereby given to the pictures
produces no appreciable defect in the linear perspective effect, and does not
interfere with the binocular union.
The height of the pictures intended to be viewed at any given distance is, of
course, limited by the fixed angle of the mirrors. This angle likewise deter-

page_text: mines the amount of vertical separation between the horizons; which must be
the same in all pictures viewed at the same distance, whatever their height may
be. There is no necessary limit to the breadth of the pictures; panoramic:
views and pictures broader than the field of view may be exhibited by being
drawn across the field of view. Narrow pictures, which show a blank margin
within the field of view, are most effective when the sides are made laterallý:
coincident, but this must not interfere with the proper horizontal separation of
the corresponding vanishing points.
Perfect accuracy in all the adjustments above described is not essential to
the stereoscopic effect. The most material adjustments are the parallelism of
the horizons-of the pictures, with a lateral separation of the points of sight not
exceeding two inches and a half, (an error in excess being of more consequence
than one in defect), and the distance of the pictures from the mirrors, upon
which depends the vertical coincidence of the reflected images.
For pictures intended to be viewed at a less effective distance than six or
seven inches (the ordinary limit of distinct vision), convex lenses or lenticular.
prisms, with their thin edges turned inwards, must be interposed between the
eyes and the pictures. The prismis give increased breadth to the field of view,
but with them the pictures should have a correspondingly increased lateral
separation, and the breadth of the mirrors should be extended outwards. The
same advantages may be gained in the case of large pictures viewed at greater
distances by the use of a pair of plane prisms; and with these also the pictures
and mirrors require the peculiar adjustments above mentioned.
If plane
prisms are used for small distances, simple lenses (not lenticular prisms) should
be used along with them.
use.
Figures 6 to 9 are Drawings of a portable folding stand or frame in which
the mirrors are mounted. It is adjustable to pictures varying in height from
the smallest up to ten inches, and intended to be viewed at distances ranging
from five inches to twenty inches; Figure 6 shews the stereoscope set up for
It is firmly held in this position by the india-rubber cords c, c, which
press the end of the frame a, b, e, d, against the inner side of the bar f of the
lower frame; the angle at h is the complement of the angle of the mirrors.
There are hinges at.g, h, i, k, which enable the instrument to be compactly
folded up, as shewn in Figure 9. The frame l, m, which carries the eye holes
and mirrors, together with the lenses and prisms, or lenticular prisms, slides
between the supports n, o, to adjust the distance between the mirrors and
pictures. The box containing the mirrors is hinged upon the frame at g, and
is kept open by the india-rubber spring P (Figure 9), and retained at the
proper angle by triangular leather sides q, which likewise limit the range of

page_text: view to the surface of the mirrors.
These sides double inwards like those of a
portemonnaie when the stereoscope is folded up; (the nearer one is omitted in
Figure 6, to show the interior of the box). Figure 7 shews the lenses or lenti-
cular prisms upon the inner side of the frame 1, m; they are joined together
like a pair of spectacles, and turn on a pivot placed midway between the eye
holes, so that they may be. shifted into the position indicated by the dotted
outline when not required. When plane prisms are employed to widen the
field of view, they may be permanently fitted into the eye holes. The frames
a, b, e, d, and r, s, support the pictures; r, s, is hinged at k to the piece t, u,
(Figure 8), which slides within a, b, e, d, to adjust it to the height of the
pictures. These are placed with the upper one in contact with the supports
n, o, and the hinge k is then brought to coincide with the fold between the
pictures; r, s, turns back only so far as to give the proper angle to the
pictures. The pictures being thus placed, their reflected images are made to
coalesce by raising or lowering the frame 1, m; the proper elevation might be
given at once to the frame l, m, by means of a graduated scale upon one of the
supports n, o; an index upon the frame beside the eye holes being brought
opposite the proper degree upon the scale to be marked in figures or the back
of each pair of pictures, or, instead of having the degree of elevation marked
on the back of the pictures, one side of a, b, e, d, might have a scale corre-
sponding to that on n, o, upon which the degree of elevation would be indicated
by the hinge k, when r, s, is adjusted to the height of the picture. When the
lenses or lenticular prisms are in use, a different degree of elevation is neces-
sary to give distinct vision to people with long and short sight; and, therefore,
in some cases the binocular union may not be effected, and the picture may be
seen doubled vertically; to correct this, it is only necessary to turn the lenses
on their pivot into an oblique position, so that the eyes look through the upper
and lower edges respectively, when they will act as prisms, displacing the
images vertically, and so may be made to unite them. Transparencies must
be placed with the foot at ; the stereoscope is held so that i, f, is vertical, the
pictures being towards the light, and the observer looks downward in the
direction x, y. The transparencies are held in their place by means of spring
catches on the frame, which grasp their margins or otherwise. This stand
might be simplified by omitting the frame r, s, the angle of the pictures being
in that case fixed by a couple of threads connecting their outer head and foot
corners; a still simpler form of the instrument has neither sliding parts nor
lenses; the eye hole board being fixed at the upper ends of n, o, and a pro-
longation of i, f, being substituted for r, s,; such an instrument, however, is
properly adapted for only one effective distance. Pictures having a less

page_text: effective distance may, indeed, be united by it if they have a sufficient vertical
separation given them, but their effect is not so natural as when viewed at
their proper distance.
When not in use the pictures may be folded together face to face. A
collection of pictures arranged and bound together as the leaves of a book, may 5
be exhibited in succession by turning over the leaves without removing them
from the stereoscope.