stereoscopic image is lying that we have to discuss now. 
Place where we observe the stereoscopic, three-dimensional reconstruction. 
One of the figures which Wheatstone left us to explain clearly the problem 
of stereoscopic vision, is the figure with which the eye-axes of a person have been 
converged at a fixed point F. 
He maintains that two points a and b, lying along the respective eye-axes 
(at practically equal distances from our eye), should raise a fused image in F 
(fig. X). 
This assertion is entirely wrong and though this principle was already put 
forward more than a century ago, the exactness of this seems to be so obvious that 
nearly everybody has simply accepted this and to-day one is still convinced that 
the thesis is right. 
Nevertheless it is completely wrong and it is an indication of a lack of 
critical sense that no one has ever checked and rectified the facts. 
What is the case in reality? 
Let us take any given point in our field of observation and suppose that 
point is the point in question F. Now we take two matches and we put them 
upright before our eyes in such a way that the matches (thus the points a and 4) 
cover the point F before each of our eyes. 
Far from seeing the Wheatstone phenomenon now and thus seeing in F a 
fused image of a and b (thus both matches), three images are formed of the two 
matches, that is to say two single images of one of the matches and one combined 
image which is in fact not lying in F but is hiding F. 
We can demonstrate that this fused image (essentially a stereoscopic image) 
cannot lie in F. 
For that purpose we have only to make the following experiment. 
We move the matches towards us in the same position. Undoubtedly we now 
see the following occur: 
The fused image comes nearer when moving the matches towards us and the 
single images do so too. 
Making the reversed motion with the two matches, that is to say moving 
them farther away from us, thus nearer to F, then the reverse happens. 
F being a fixed point, the fused image which is moving nearer or farther 
cannot lie in F. What is more, it only reaches F at the moment that we have 
almost reached F with the matches. That last moment we can hardly observe. 
Too often do we experience how long wrong assertions can remain in cur- 
rency if only a semblance of truth attaches to them. Let us hope, however, that 
this time the wrong figure of Wheatstone will be hushed up, once and for all. 
But we have not yet finished with it, for, of course, we want to know where 
the fused image is indeed situated. 
For that purpose let us make another experiment. 
We take two smooth, round pencils, a red and a blue one, instead of the 
matches. We put them again in a and b, in other words along the lines of the 
converging eye-axes and we observe that the single images (the outer two) are 
respectively red and blue, red on the side we hold the red one and blue on the 
side we hold the blue pencil. The fused image has a combined colour and is evi- 
dently composed of red + blue, as we should expect. 
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