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FIRST CLASS OF OBJECTS FOR THE SUBJECT. 75
The third process by which the Understanding converts
sensation into perception, consists in constructing bodies
out of the simple surfaces hitherto obtained that is, in
adding the third dimension. This it does by estimating
the expansion of bodies in this third dimension in Space
which is known to the Understanding a priori through
Causality, according to the degree in which the eye is
affected by the objects, and to the gradations of light and
shade. In fact, although objects fill Space
in all three
dimensions, they can only produce an impression upon the
eye with two ;
for the nature of that organ is such, that
our sensation, in seeing, is merely plauimetrical, not stereo-
metrical. All that is stereometrical in our perception is
a/lded by the Understanding, which has for its sole data
the direction whence the eye receives its impression, the
limits of that impression, and the various gradations of light
and dark: these data directly indicate their causes, and
enable us to distinguish whether what we have before us
is a disk or a ball. This mental process, like the preceding
ones, takes place so immediately and with such rapidity,
that we are conscious of nothing but the result. It is this
which makes perspective drawing so difficult a problem,
that it can only be solved by mathematics and has to be
learnt ; although all it has to do, is to represent the sen
sation of seeing as it presents itself to our Understanding
as a datum for the third process : that is, visual sen
sation in its merely planimetrical extension, to the two
dimensions of which extension, together with the said data
in them, the Understanding forthwith adds the third, in
contemplating a drawing as well as in contemplating reality.
Perspective drawing is, in fact, a sort of writing which can
be read as easily as printed type, but which few are able to
write ; precisely because our intellect, in perceiving, only
apprehends effects with a view to constructing their causes,
immediately losing sight of the former as soon as it has