LXXXVII
caused upon the optic nerve by the image thrown upon the retina.
But this sensation will not tell us, by what object it was caused;
which object corresponds with this visual sensation, we know
only by a previous information, received through the sense of pal
pation (inclination of children to touch everything). Hence vision
is not a mere sensation, it is also the passing of a judgment!
(The same takes place with respect to all perceptions through
our senses.) A blind man, very well acquainted with the objects
around him, when restored to sight, would not know them by
means of his eyes. But more conclusions are necessary for
gaining the proper perception through a visual sensation: also
the magnitude and distance of an object we determine by in
ference. As to the magnitude of a body, we turn our eyes from
one extreme point to the other. The angle, which our eye thus
describes is called visual (optic) angle. 385. After much
experience we make the conclusion, that the greater the angle,
which our eye describes, the greater the object. Farther we
know, that the visual angle for one and the same object is not
always the same, but increases or decreases according to the
visual distance; also this and the magnitude of neighbouring
known objects is taken into consideration for determining the
magnitude of the object in question (the two lines of an alley
and of a railway seem to converge; a horizontal plane rises;
