The third humour of the eye, is the vitreous; it is the largest of all the humours, filling up the whole of that part of the eye which lies behind the crystalline humour. It is thicker than the aqueous, but thinner than the crystalline humour; on its back part is spread the retina, and in the middle of its fore part is a small cavity, in which the whole posterior surface of the crystalline lens lies; this humour is also enclosed in a very fine capsule, called tunica vitrea; this capsule at the edge of the crystalline humour is divided into two membranes, of which the one is continued over the whole anterior surface of the vitreous humour, and lines that cavity into which the back part of the crystalline is received; the other passes over the crystalline humour, and covers all its fore part, by which means these two humours are closely connected together. The weights of the aqueous, crystalline, and vitreous humours in a human eye, are, according to the accurate Petit, at a medium, to each other, as 1, 1, and 25. It was thought necessary to premise this general description of the structure of the eye, in order that what we are going to add in the remaining part of this Lecture may be the more easily comprehended. A more distinct idea will perhaps be had from a contemplation of the following figure, which represents the section of an eye by a vertical plane passing through its centre. [FIGURE] 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.