therefore, yellow, or has a slight tendency to reflect several rays which produce a dingy brown color.

An ink-spot on linen at first absorbs all the rays; but, exposed to the air, it undergoes a chemical change, and the spot partially regains its tendency to reflect colors, but with a preference to reflect the yellow rays, and such is the color of the iron-mould.

Emily. Bodies, then, far from being of the color which they appear to possess, are of that color which they have the greatest aversion to, which they will not incorporate with, but reject and drive from them.

Mrs. B. It certainly is so; though I scarcely dare venture to advance such an opinion whilst Caroline is contemplating her beautiful rose.

Caroline. My poor rose! you are not satisfied with depriving it of color, but even make it have an aversion to it; and I am unable to contradict you.

Emily. Since dark bodies absorb more solar rays than light ones, the former should sooner be heated if exposed to the sun?

Mrs. B. And they are found by experience to be so. Have you never observed a black dress to be warmer than a white one?

Emily. Yes, and a white one more dazzling: the black is heated by absorbing the rays, the white dazzling by reflecting them.

Caroline. And this was the reason that the brown paper was burnt in the focus of the lens, whilst the white paper exhibited the most luminous spot, but did not take fire.

Mrs. B. It was so. It is now full time to conclude our lesson. At our next meeting, I shall give you a description of the eye.

CONVERSATION XVII.

OPTICS.

ON THE STRUCTURE OF THE EYE, AND OPTICAL INSTRUMENTS.

Description of the Eye.-Of the Image on the Retina.—Refraction of the Humors of the Eye. Of the Use of Spectacles. Of the Single Microscope.-Of the Double Microscope.-Of the Solar Microscope.Magic Lanthorn.-Refracting Telescope.-Reflecting Telescope.

MRS. B.

THE body of the eye is of a spherical form: (fig. 1. plate XXI.) it has two membranous coverings; the external one, a a a, is called the sclerotica; this has a projection in that part of the eye which is exposed to view, bb, which is called the cornea, because, when dried, it has nearly the consistence of very fine horn, and is sufficiently transparent for the light to obtain free passage through it.

The second membrane which lines the cornea, and envelopes the eye, is called the choroid, c c c; this has

an opening in front, just beneath the cornea, which forms the pupil, dd, through which the rays of light pass into the eye. The pupil is surrounded by a colored border, called the iris, e e, which by its muscular motion, always preserves the pupil of a circular form, whether it is expanded in the dark, or contracted by a strong light. This you will understand better by examining fig. 2.

Emily. I did not know that the pupil was susceptible of varying its dimensions.

Mrs. B. The construction of the eye is so admirable, that it is capable of adapting itself, more or less, to the circumstances in which it is placed. In a faint light the pupil dilates so as to receive an additional quantity of rays, and in a strong light it contracts, in order to prevent the intensity of the light from injuring the optic nerve. Observe Emily's eyes, as she sits looking towards the windows: her pupils appear very small, and the iris large. Now, Emily, turn from the light, and cover your eyes with your hand, so as entirely to exclude it for a few moments.

Caroline. How very much the pupils of her eyes are now enlarged, and the iris diminished. This is, no doubt, the reason why the eyes suffer pain, when from darkness they suddenly come into a strong light; for the pupil being dilated, a quantity of rays must rush in before it has time to contract.

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,