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the coin we observe under the bad left eye is of the same quality as that which is 
observed by our good eye. 
When we do the reverse, then we observe before our right eye a more or less 
vague image, equal to that normally seen by the left eye. 
Evidently with binocular vision the one eye receives and the other eye 
observes this image; in more scientific terms, the perception of the one eye 
raises the apperception of the other one with binocular vision. 
As we saw before the point on which we concentrate is beyond our will the 
point of intersection of the eye-axes, the two mental images meet at this point of 
concentration and by that we get the “impression” that binocular vision is the 
combined function of the right and the left eye with monocular vision. That is 
not the case at all, e.g. exp. 3 shows that the image seen with the left eye, has 
been formed by the right one and reversed, while with monocular vision the 
image of the left eye is also formed by the left eye. 
When viewing an object with both eyes, the combination takes place near the 
object, but this combination must also be the image seen by the left eye but 
formed by the right one and the image seen by the right eye should be formed by 
the left one. 
Ibe influence of binocular vision on the forming of a stereoscopic image. 
Human beings have the capability, when looking with both eyes, not only of 
evoking an impression of that part of the world that makes an image on the 
retinas of both our eyes, but also they have the capability of making that image 
move in a direction parallel to our eye base. 
So we can, when looking in the direction of a single object, bring this object 
in focus. Then we only see one sharp image of that object (fig. IVa). 
But we are also able to bring our eyes in the direction of that object and 
focus too far. The result is that in view of that object we stare with eye-axes that 
have a direction to a point that is too far away, so that the object lies within the 
angle formed by our eye-axes (fig. IVc). 
In this case we become aware that it is quite impossible to see only one 
object. An object near F may be seen as a single image, but O can only be seen as 
a double image, according to fig. II. | 
When shutting the right eye suddenly, the left part of this double image will 
disappear; when shutting the left eye the right part disappears. 
If we devote our attention to what is happening, it is also observed that the 
left part of the image that disappeared when shutting the right eye, is seen in the 
direction of the left eye-axis; whereas the right part that disappeared when 
shutting the left eye, is seen in the direction of the right eye-axis. 
The more F is chosen at a greater distance, the more our eye-axes will have 
to diverge and so the more both parts of the double image will be separated. 
When looking to infinity, that is to say with F lying in infinity, both eye- 
axes have to be parallel. 
Persons who practise a good deal can also make their eye-axes diverge. The 
writer can make his eyes diverge in such a way that the double objects are sepa- 
rated about 15 cm, that is about double the eye base. 
We discussed, however, that we can also look too short. Now again we see a 
double image. With relation to point F we are squinting. But now the object :s 
5