denser medium, it will be
refracted, or bent towards a line which is perpendicular to the
surface which separates the media at the point where it falls; but
when it passes out of a denser into a rarer medium, it will be bent
from the perpendicular.

5. Whenever the rays, which come from all the points of any object,
meet again in so many points, after they have been made to converge
by refraction, there they will form the picture of the object,
distinct, and of the same colours, but inverted. This is beautifully
demonstrated by a common optical instrument, the camera obscura. If a
double convex lens, be placed in the hole of a window shutter in a
dark room, and a sheet of white paper be placed at a certain distance
behind the lens; a beautiful, but inverted picture of the external
objects will be formed: but if the paper be held nearer, or more
remote than this distance, so that the rays from each point shall not
meet at the paper, but betwixt it and the lens, or beyond the paper,
the picture will be indistinct and confused.

_Of the Manner in which Vision is performed._

From the just mentioned properties of light, and the description we
have given of the eye, it will not be difficult to explain the theory
of vision, so far as it depends upon optical principles. For the eye
may, with great propriety, be compared to a camera obscura; the rays
which flow from external objects, and enter the eye, painting an
inverted picture of those objects on the retina: if you carefully
dissect from the bottom of an eye, newly taken out of the head of an
animal, a small portion of the tunica sclerotica and choroides, and
place this eye in a hole made in the window shutter of a dark
chamber, so that the bottom of the eye may be towards you; the
pictures or images of external objects will be painted on the retina
in lively colours, but inverted.

In order to see how the several parts of the eye contribute to
produce this effect, let us follow the rays proceeding from a
luminous point, and see what will happen to them from the
beforementioned properties of light.

Since the rays of light flow from every visible point of a body in
every direction, some of them, issuing from this point, will fall
upon the cornea, and, entering a medium of greater density, will be
refracted towards the perpendicular, and as they fall upon a convex
spherical surface, nearly in a parallel state, the pupil being so
extremely small, it is evident, from th