EXPLANATION.

NOO represents the optic nerve.
The outmost line ALLB represents the sclerotic coat, and the
part ACB the transparent cornea.
The line ALLB, immediately within the former, represents the
choroides; the part APB is the iris or uvea, in which the hole at P
is the pupil.
The line FOOG is the retina.
The cavity ACBEMDA is the aqueous humour.
DE is the crystalline lens or humour.
The space DFOOGE, lying behind the crystalline, represents the
vitreous humour.
BE and AD is the ligamentum ciliare.

_Nature and Properties of Light._

After this short description of the human eye, I shall next proceed
to take notice of some of the properties of light; but shall confine
myself to such as are absolutely necessary for explaining the
phenomena of vision, as far as that can be done from optical
principles.

1. It is, I believe, generally at present agreed, that light consists
of exceedingly small particles of matter, projected with great
velocity in all directions from the luminous or radiant body. This
hypothesis, to which no solid objection has yet been made, appears to
be more simple than any other; and is so consistent with all the
phenomena yet observed, that we have great reason to think it true:
however, as it is not absolutely and directly demonstrated, it may
have been wrong in optical writers to have given this hypothesis (for
it can only be called a hypothesis) as a definition of light.

2. The space through which light passes is, by opticians, called a
medium, and it is observed, that, when light passes through a medium,
either absolutely void, or containing matter of an uniform density,
and of the same kind, it always proceeds in straight lines.

3. Those rays of light which come directly from a luminous body to
the eye, only give us a perception of light; but when they fall upon
other bodies, and are from them reflected to the eye, they give us an
idea or perception of those bodies.

4. When a ray of light passes out of one medium into another of
different density, it is bent out of its course, and is said to be
refracted. We must, however, except those rays which fall in a
direction perpendicular to the surface of the refracting medium; as
the refractive force acts in the same direction in which those rays
move, they will not be turned out of their course, but proceed in the
same direction they had before they entered the refracting medium.
When a ray passes out of a rarer into a