he prominences
and depressions of objects, and that those shades uniformly vary, when the
sense of touch observes any variation. Hence when the retina becomes
stimulated by colours or shades of light in a certain form, as in a
circular spot; we know by experience, that this is a sign, that a tangible
body is before us; and that its figure is resembled by the miniature figure
of the part of the organ of vision, that is thus stimulated.

Here whilst the stimulated part of the retina resembles exactly the visible
figure of the whole in miniature, the various kinds of stimuli from
different colours mark the visible figures of the minuter parts; and by
habit we instantly recall the tangible figures.

Thus when a tree is the object of sight, a part of the retina resembling a
flat branching figure is stimulated by various shades of colours; but it is
by suggestion, that the gibbosity of the tree, and the moss, that fringes
its trunk, appear before us. These are ideas of suggestion, which we feel
or attend to, associated with the motions of the retina, or irritative
ideas, which we do not attend to.

So that though our visible ideas resemble in miniature the outline of the
figure of coloured bodies, in other respects they serve only as a language,
which by acquired associations introduce the tangible ideas of bodies.
Hence it is, that this sense is so readily deceived by the art of the
painter to our amusement and instruction. The reader will find much very
curious knowledge on this subject in Bishop Berkley's Essay on Vision, a
work of great ingenuity.

The immediate object however of the sense of vision is light; this fluid,
though its velocity is so great, appears to have no perceptible mechanical
impulse, as was mentioned in the third Section, but seems to stimulate the
retina into animal motion by its transmission through this part of the
sensorium: for though the eyes of cats or other animals appear luminous in
obscure places; yet it is probable, that none of the light, which falls on
the retina, is reflected from it, but adheres to or enters into combination
with the choroide coat behind it.

The combination of the particles of light with opake bodies, and therefore
with the choroide coat of the eye, is evinced from the heat, which is given
out, as in other chemical combinations. For the sunbeams communicate no
heat in their passage through transparent bodies, with which they do not
combine, as the air continues cool