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The connection of the pencil of rays of our mental image with the optical 
course of rays is only this, that the pencil of rays of the x number of points of the 
object which we observe, is to our eye congruent to the pencils of rays from the 
instrument in our orbit to the image that we “observe” projected in the field of 
observation (except psychical aberrations and except for infirmities of our lenses). 
Both pencils of rays, that from the image, observed by our right "mental 
eye" and that from the image observed by our left *mental eye", produce a system 
of intersections quite equal to those with normal vision, for in the two cases both 
the mental images cover each other, as we saw before. 
The difference with our binocular vision in our normal field of observation 
is only that in the last case the object itself lies in the place where our mental 
spatial image forms from the masses if intersections of corresponding rays, while 
when observing separated objects, placed within or outside our angle of conver- 
gence, the spatial intersection image floats near one of the flat stereoscopic photos. 
Our inborn and developed experience taught us in both cases, without fits and 
starts, to go fumbling along this whole mental image with our perception or with 
our *concentration". 
We want to call special attention to the fact that in our opinion the formed 
three-dimensional image lies nearly in the plane of the objects presented to our 
eyes, thus in the Wheatstone case nearly in the plane brought through a and b 
perpendicularly on our eye-axes. “Nearly”, because one cannot speak of a fixed 
position, neither with normal vision nor with stereoscopic vision. 
This is not only due to the action of parallaxes, through which part of all the 
points of the image is nearer to our eyes and another part further away (therefore 
we cannot speak only of a plane), but also because the ego is brought into action. 
And this complex of psychical and neurological reactions has raised properties in 
the mental image which were not present in the original object nor in the perceived 
retinal image. 
Here we have discussed, however, the case of normally reacting persons with 
normally functioning eyes and nerves. 
Finally we want to point out that in our opinion nothing indicates that 
amalgamation into a fused image should take place in the brain, but we think 
that fusion takes place in our field of observation of two images not yet fused. 
What, among other things, points to this is the fact that we are able to observe 
the fusion of two images but that the stereoscopic effect, especially for the unskill- 
ed, is brought about almost simultaneously and then, as it were, with a shock. 
Had fusion happened before in the brain, this would not be the case. 
Now we want to solve from the constructional point of view, too, the posi- 
tion of the stereoscopic image (fig. XIT). 
In figs. a and b we tried to show how observation works. It is certain that 
fig. 4 must represent the matter well in so far as we indeed observe the stereos- 
copic image in two places and well along both the eye-axes so far they are focussed 
upon F. It is certain too that we observe the outer by-images with interspaces, 
which entirely harmonize with the space between the points a and Pb. Yet it is 
certain that we do not observe four images and so we have to believe that fig. b is 
more correct and consequently the stereoscopic image, psychical and physical 
disturbances excepted, must lie in the median plane, the plane through our nose 
perpendicular to the eye-base (this brings fig. II to mind). 
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