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ig. VI),
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y sees the
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; the blue
photograph with the eye, before which the red filter is held and the other eye
(blue filter) sees only the red one (fig. VIT).
So, since the combination has already been made by the printer, we have
only to look with adapted eye glasses or filters to get the right stereoscopic image.
It will always be necessary that the right eye only receives the image that
normally is presented to the right eye when looking at the object as shown in
fig. VIII.
If we should change the photographs in a way that the left cye receives the
image destined for the right and the right eye receives the image, destined for the
left one, then all things are reversed. All intersections of rays that belong together,
will give reconstructions which are opposite to normal; so we see points farther
on that should be nearer and other points nearer, that should be at a greater
distance. We call this pseudoscopic effect (fig. IX).
With normal binocular vision we can never make any mistake, for it is the
object itself that creates the right images in the right eye, and so at the right place.
When looking without a stereoscope, it is not difficult to understand that
we have the capacity of joining two stereo-photo's and getting a three-dimensional
reconstruction, as if -we looked normally. But we have to remember that in this
case the convergence of our eye-axes has to be incorrect, i.e. nearly parallel.
In case that the printer has brought both images together in such a way that
we can separate them for the construction of single retinal images, we know that
be has created the same situation as when normally looking with two eyes and so
the convergence is more or less normal.
Still we see considerable difference between a normal binocular image and
the three-dimensional image when seeing vectographs or anaglyphs. The differ-
ence is that when normally seeing, the object itself is in the place of the mental
reconstruction that the brain makes of the object; it is as if the object hides in the
image and does its work again and again by sending rays to our retina at the same
time that our brain is giving the reconstruction. When seeing a vectograph or an
anaglyph, we see the mental three-dimensional reconstruction, but now the object
itself is not there; our image only consists of a veil of intersecting points of rays.
In binocular vision that results in our mental reconstruction lying on the
object itself and being unable to pass that object, while the mental three-dimen-
sional reconstruction of the object by vectographs or anaglyphs is not brought to
a standstill by the object itself and so we see it over and through tables, paper and
other things. That is the difference.
It is as if in the first case our reconstruction is materialized, whereas in the
second case it is quite unmaterialized.
But what about the stereoscopic image we get when looking through a
stereoscope?
The stereoscope is nothing more than an instrument that helps to bring our
eye-axes more or less parallel, but always in such a position that we have our
object as in fig. IV c. We already know what must be the result.
All constructions of stereoscopes follow the same principle, whether they have
magnifying glasses, mirrors to enlarge our eye base, or not. The only thing that
is interesting is that the eye-axes have an intersection that does not lie on the
combined objects themselves, but at a much larger distance (fig. IVc).
So again double images have to appear and it is only the place where the
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