les of
optics, and we can with certainty, by their assistance, affirm, that
if the eye is sound, and the image of an object distinctly painted
upon the retina, it will be seen distinctly, erect, and of its proper
colours: so far we can proceed on safe and sure grounds, but if we
venture further, we shall find ourselves bewildered in the regions of
hypothesis and fancy. The machinery by which nature connects the
material and immaterial world is hidden from our view; in most cases
we must be satisfied with knowing that there are such connexions, and
that these connexions invariably follow each other, without our being
able to discover the chain that goes between them. It is to such
connexions that we give the name of laws of nature; and when we say
that one thing produces another by a law of nature, this signifies no
more, than that one thing, which is called the cause, is constantly
and invariably followed by another, which we call the effect, and
that we know not how they are connected. But there seems a natural
propensity in the mind of man, to endeavour to account for every
phenomenon that falls under his view, which has given rise to a
number of absurd and romantic conjectures in almost every branch of
science. From this source has risen the vibration of the fibres of
the optic nerve, or the undulation of a subtile ether, or animal
spirits, by which attempts have been made to explain the theory of
vision; but all of them are absurd and hypothetical.

Kepler was the first who had any distinct notion of the formation of
the pictures of objects on the bottom of the eye; this discovery he
published about the year 1600. Joannes Baptista Porta had indeed got
some rude notion of it prior to the time of Kepler, but as he knew
nothing of the refraction made by the humours of the eye, his
doctrine was lame and defective, for he imagines that the images are
painted on the surface of the crystalline humour.

The disputes concerning the theory of vision had very much divided
the ancient philosophers; some of them imagining that vision was
caused by the reception of rays into the eye; while a great many
others thought it more agreeable to nature, that certain emanations,
which they called visual rays, should flow from the eye to the
object.

We shall now inquire more particularly how each part of the eye is
peculiarly fitted to produce distinct vision. Though the eye is
composed of different humours, yet one might have been su