dimension. We can hardly prove this. The dimensions which an object apparently 
has for us is again a matter of experience, for this cannot only be derived from 
the dimensions of the retinal image. 
This is soon to be perceived when various people are asked how large they 
see the moon. The dimensions which they give differ fantastically. 
From experience we compare what we see as regards colour and size with 
what we know and then from this results our personal *observation". 
Besides these totally personal observations as to the form and the classifi- 
cation of the image, certain rules are to be recognised. 
We mean this: everyone who fixes his eye upon a ball does not see a square 
object. When a figure has been drawn on the ball we all see this in the same 
place on the whole if we turn the eye towards the same place of observing. 
This fact is very important. Of course little differences in observing 
the shape are possible as a consequence of variation of our eye-lens; it is also 
possible to have variations in the shape of the figure in the image, when we 
make recognition difficult by vague colours and contours or if we observe 
when it is more or less dark, but when the portrait of a cat is pictured on the 
ball we do not see a dog, as long as the portrait has been duly pictured. 
The relation between the object itself and the image which we get of that 
object is so closely tied down with mathematical rules, that we often make the 
mistake of thinking that the image we form of that object really is the 
object, that is to say that we really optically observe the object itself. The 
analogy is deceitful as well. 
We can aim at every point of the object if we want to shoot at it and hit 
the mark, supposing that we are good marksmen and have good rifles. We 
observe the movements of the object as well. 
This teaches us immediately that other persons, fully independent of us. 
see the movements in the same way. 
Yet, reality can already play us a trick now, because we switch on our 
speed of reaction, which is entirely subjective. 
It may happen that the one observes a movement in a somewhat different 
sense than another, even we can see a fact occur differently, if circumstances 
are too difficult for normal observation. 
If however we take people of normal psychical condition, who are normally 
sighted, and we have the observation taking place calmly and without involving 
difficulties, then the image observed will be the same for several observers, 
taking into consideration the restrictions mentioned before: variations in lens 
and colour sensibility. 
The fact that the relation between the object and the image which we make 
of it is so accurately tied down by rules, is also the cause of the error made by 
the uninitiated, as if we project outside ourselves our image according to the 
rules of optics. Optics as such does not play a part at all in the creation of our 
mental image. 
Now it should be interesting to know whether that close connection between 
our image and the object (as we stated when viewing with one eye only) also 
holds good in the case of seeing binocularly. 
This we shall discuss later when treating binocular vision. Besides being, 
able to fix our eye (that is our eye-axis) upon an object, we can also (once more 
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IH