Perspective is nothing else than seeing place [or objects] behind a
plane of glass, quite transparent, on the surface of which the
objects behind that glass are to be drawn. These can be traced in
pyramids to the point in the eye, and these pyramids are intersected
on the glass plane.
84.
Pictorial perspective can never make an object at the same distance,
look of the same size as it appears to the eye. You see that the
apex of the pyramid _f c d_ is as far from the object _c_ _d_ as the
same point _f_ is from the object _a_ _b_; and yet _c_ _d_, which is
the base made by the painter's point, is smaller than _a_ _b_ which
is the base of the lines from the objects converging in the eye and
refracted at _s_ _t_, the surface of the eye. This may be proved by
experiment, by the lines of vision and then by the lines of the
painter's plumbline by cutting the real lines of vision on one and
the same plane and measuring on it one and the same object.
85.