_Seeing_ is that great faculty by which the form, colour, and motion of objects is represented to our ideas. The principal conveyer of this wonderful sensation, is the _optic nerve_; the eye is a telescope finished to the greatest perfection; or like a _camera obscura_, where objects are pictured to the utmost nicety, and a due proportion observed in their nature. It is a globe enrolled with a strong tendinous coat, and filled with three kinds of humours perfectly clear and transparent, which differ in nothing but their various degrees of liquidity and form. Each of these humours is enclosed in a delicate thin transparent membrane. The external coat of the eye is called the _cornea_, from its resemblance to horn, being a continuation of the _dura mater_ and _pia mater_; and is every where opaque, except at the entrance of the light, which is called _pupilla_, and projects somewhat beyond the spherical form of the eye, similar to the crystal of a watch. Inside the _cornea_ it is lined with a spreading of the optic nerves, called the _retina_, where objects are reflected; by which the ideas are immediately conveyed to the brain. The humours are the following: The _aqueous humour_, the most external, which is designed for collecting the rays of light; the second is the _crystaline humour_, which is in the form of a magnifying lens, and invested with a subtile transparent tegument, and so curiously fixed, as to be contracted and dilated, according as the objects are near, or removed from the eye; and accordingly the crystaline humour requires to be flattened or thickened; the last is the _vitrous humour_, which fills the remaining cavity of the eye; this humour serves to spread the objects again on the _retina_, and arrange them in their natural order, so that the picture of the objects viewed, may be preserved in its due proportion and regularity. Now the act of vision is performed thus: The convex transparent part of the _cornea_ faces at once every object which comes within the direct lines thereof, and this is termed a _full view_.[8] This view enters through the aqueous humour, where the whole is contracted according to its convex and concave form, so as to make it fit to enter the _pupilla_, which is a muscular continuation of the _sclerotis_, and called the _uvea_; and from thence the whole view passes through the crystaline lens, by which the view is again magnified, and passed through the vitrous, and the whole depicted on the _retina_, in the same concave curve, and at equal angles in which it was first received, and apparently the same magnitude; thence representing the objects in their natural perfection, figure, colour, magnitude, and motion. Footnote 8: It is the general received opinion, that objects are reversed in the representation on the _retina_ of the eye. This is however a mistake; I hope on another subject to prove this in a full and satisfactory manner.