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Now however we let the red pencil make a small quick tumbling motion in 
the direction of the eye-axis, along which it is situated (if need be several times 
at a stretch) and after that we stop the red pencil just like the other one. The 
fused image is obviously red now. 
After this we do the same with the blue pencil while keeping the red one still. 
The fused image now is a blue pencil ?). 
From this experiment it is obvious that the fused (stereoscopic) image 1s 
twofold. 
What did we do? By turns we compelled the eyes to devote our attention 
to one of the pencils (that which was moving) Le. at first we especially 
observed the red pencil with the left eye. 
After this we especially observed the blue one with the other eye by attracting 
attention for that eye to the moving pencil. From this experiment it appears that 
we are able to compel ourselves to give priority to a certain eye. The other one 
then only plays a secondary role. 
When we move neither of the two pencils, then the fused image looks more 
red or blue, in accordance with the colour of the pencil which is brought along 
the eye-axis of our best eye and to which more attention is paid. 
But with our experiments the distance of a and b from our eye and from F 
was arbitrary. 
All the observed phenomena remain however the same, whether we keep the 
pencils nearer or farther, as long as we do not approach F too much. 
Thus the object can be put into an infinite number of places between our 
eye-axis and the point F we pay attention to. 
It follows that with normal binocular vision we are confronted with the 
special case, that we pay attention to the point F itself. We only observe a normal 
image as we observe this daily, if we have our eyes (at least approximately) con- 
verged at the point to which we are devoting attention. 
From the fact that we see the stereoscopic image lying near the moving 
pencil, thus near the red as well as near the blue one, it should follow that in fact 
we have to do with two stereoscopic images, but we get the impression to see only 
one, for the situation of both is the same. 
Each of the stereoscopic images “appears” to consist of the object to which 
we pay our attention and a moved or a broadcasted image of the other one. 
So the explanation would be that we essentially observe four images of two 
objects but that the inner two, which both are fused images, can be observed by 
us as one image only (fig. XII A). 
To explain the things mentioned we revert to our experiment with the stere- 
oscope, by which we put under one of the glasses, only the right one, an object 
e.g. a photo. We then see this photo lying under the left glass too. If we 
had brought under the left glass the accessory stereoscopic photo, then under each 
of the glasses a stereoscopic couple would have been seen. 
However, we see the whole only as one stereoscopic image and we now know 
that this phenomenon has to do with the normal binocular vision, assuming that 
we observe objects, which are not placed in the point of intersection or point of 
convergence of the eye-axes. 
1) Persons with strongly differing eyes will observe that one of the images is continually inclined 
to return.