_Emily._ And when we go from a strong light, into obscurity, we at first imagine ourselves in total darkness; for a sufficient number of rays cannot gain admittance into the contracted pupil, to enable us to distinguish objects: but in a few minutes it dilates, and we clearly perceive objects which were before invisible. _Mrs. B._ It is just so. The choroid _c c_, is embued with a black liquor, which serves to absorb all the rays that are irregularly reflected, and to convert the body of the eye, into a more perfect camera obscura. When the pupil is expanded to its utmost extent, it is capable of admitting ten times the quantity of light, that it does when most contracted. In cats, and animals which are said to see in the dark, the power of dilatation and contraction of the pupil, is still greater; it is computed that the pupils of their eyes may admit one hundred times more light at one time than at another. Within these coverings of the eye-ball, are contained, three transparent substances, called humours. The first occupies the space immediately behind the cornea, and is called the aqueous humour, _f f_, from its liquidity and its resemblance to water. Beyond this, is situated the crystalline humour, _g g_, so called from its clearness and transparency: it has the form of a lens, and refracts the rays of light in a greater degree of perfection, than any that have been constructed by art: it is attached by two muscles, _m m_, to each side of the choroid. The back part of the eye, between the crystalline humour and the retina, is filled by the vitreous humour, _h h_, which derives its name from a resemblance it is supposed to bear, to glass, or vitrified substances. [Illustration: PLATE XXII.] The membranous coverings of the eye are intended chiefly for the preservation of the retina, _i i_, which is by far the most important part of the eye, as it is that which receives the impression of the objects of sight, and conveys it to the mind. The retina is formed by the expansion of the optic nerve, and is of a most perfect whiteness: this nerve proceeds from the brain, enters the eye, at _n_, on the side next the nose, and is finely spread over the interior surface of the choroid. The rays of light which enter the eye, by the pupil, are refracted by the several humours in their passage through them, and unite in a focus on the retina. _Caroline._ I do not understand the use of these refracting humours: the image of objects was represented in the camera obscura, without any such assistance. _Mrs. B._ That is true; but the representation became much more strong and distinct, when we enlarged the opening of the camera obscura, and received the rays into it, through a lens. I have told you, that rays proceed from bodies in all possible directions. We must, therefore, consider every part of an object which sends rays to our eyes, as points from which the rays diverge, as from a centre.