e principles of optics, that
they will be made to converge: those which fall very obliquely will
either be reflected, or falling upon the uvea, or pigmentum nigrum,
which covers the ciliary ligaments, will be suffocated, and prevented
from entering the internal parts of the eye: those which fall more
directly, as was before said, become converging, in which state they
fall upon the anterior surface of the crystalline humour, which,
having a greater refracting power than the aqueous humour, and its
surface being convex, will cause them to converge still more, in
which state they will fall upon the posterior surface of the
crystalline, or anterior surface of the vitreous humour; which having
a less refractive power than the crystalline, they will be refracted
from the perpendicular; but, as they fall upon a concave surface, it
is evident, from the principles of optics, that they will be made to
converge still more: in which state they will go on to the retina,
and if the eye is well formed, the refraction of these several
humours will be just sufficient to bring them to a point or focus on
the retina.

The same thing will happen to rays flowing from every other visible
point of the object: the rays which flow from every point will be
collected into a corresponding point on the retina, and,
consequently, will paint the image of that object inverted; the rays
coming from the superior part of any object, being collected on the
inferior part of the retina, and vice versa, as is manifest from the
principles of optics.

If the rays are accurately, or very nearly, collected into a focus on
the retina, distinct vision will be produced; but if they be made to
converge to a point before or beyond the retina, the object will be
seen indistinctly; this is proved by holding a convex or concave
glass before the eye of a good sighted person: in the former case,
the rays will be made to converge to a point before they arrive at
the retina, and in the latter, to a point beyond it. In these cases,
it is plain that the rays which flow from a point in the object, will
not form a point, but a circular spot, upon the retina, and these
various circles intermixing with other, will render the image very
indistinct. This is well illustrated by the camera obscura, where if
you hold the paper nearer or more remote than the focal distance of
the lens, the picture will be indistinct.

So far then, in the theory of vision, are we led by the princip