We may divide the illusions to which the sense of sight is liable into four kinds. First, mental, or those arising in the brain itself, and only referred to the eye. Second, those produced by the structure of the eye. Third, those arising from the impressions of outward objects on the retina. Fourth, those produced by various combinations of the foregoing. It is only the second and third we shall have occasion to touch upon. But before we can well understand their nature, it will be necessary to get a slight knowledge of the structure of the eye, and some idea respecting the nature of light.
With perhaps the exception of the ear, the eye is the most wonderful example of the infinite skill of the Creator. A more exquisite piece of mechanism it is impossible for the human mind to conceive.
The annexed diagram (Fig. 1) of a horizontal section of this organ will give a better idea of its general structure than whole pages of letter-press. It will be seen to consist of a globe of three envelopes or coats, which are kept distended by three transparent humours or lenses: the aqueous (c), the crystaline (f), and the vitreous (g). The outer coat (a) is dense, white, and fibrous. In front of the eye it gives place to a perfectly transparent one, called the cornea (d). The next coat, the choroid (b), is vascular, very black on its internal surface, in order that light falling on it through the pupil (h) may not be reflected. The pupil is an opening through a diaphragm which is called the iris (i), from its colour varying in different individuals. It has the power of expanding and contracting the pupil, for the purpose of regulating the supply of light to the retina (c), or third and last coat which lies immediately on the choroid. It is transparent, very complex, and the only part of the eye we shall carefully consider.
The following diagram (Fig. 2) represents a section of it magnified 250 diameters, a is called the limitary membrane, and forms its innermost surface, or that which is next the vitreous humour; b consists of the layer of optic nerve fibres; c is a layer of grey nerve cells; d, two layers in which the principal retinal blood-vessels are spread out; c, two layers of granular matter; f Jacob's membrane, or layer of rods and cones. Fig. 3 will give some idea of the supposed connexion between these various parts, the same letters referring to the same parts as in Fig. 2.
When a ray of light enters the eye, it passes through the humors or lenses, and is formed by them into an image, on the choroid, of the object looked at. The extremities of the rods and cones