Part 87

45. (Pg. 190) What proof of the truth of this theory of colours, may be afforded by the prism?

46. (Pg. 191) Why will green leaves, when exposed to the red ray, appear of a dingy brown?

47. (Pg. 191) Bodies, in general, when placed in a ray differing in colour from their own, appear of a mixed hue, what causes this?

48. (Pg. 191) Why will bodies of a pale, or light hue, most perfectly, assume the different colours of the spectrum?

49. (Pg. 192) Upon what property in a body, does the darkness of its colour depend?

50. (Pg. 192) Why do some bodies appear white, others black, and others of different colours?

51. (Pg. 192) From what cause do blue articles appear green, by candle-light?

52. (Pg. 193) What is believed to be the cause, of the red appearance of the sun, through a fog, or misty atmosphere?

53. (Pg. 193) From what is the blue colour of the sky, thought to arise?

54. (Pg. 193) What would be the colour of the sky, did not the atmosphere reflect light?

55. (Pg. 193) From what cause do some bodies change their colour, as leaves formerly green, become brown, and ink, yellow?

56. (Pg. 194) Why is a black dress, warmer in the sunshine, than a white one of the same texture?

CONVERSATION XVII.

ON THE STRUCTURE OF THE EYE, AND OPTICAL INSTRUMENTS.

DESCRIPTION OF THE EYE. OF THE IMAGE ON THE RETINA. REFRACTION BY THE HUMOURS 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 21.) it has two membranous coats, or coverings; the external one, a a a, is called the sclerotica, this is commonly known under the name of the white of the eye; it has a projection in that part of the eye which is exposed to view, b b, which is called the transparent 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 envelops the eye, is called the choroid, c c c; this has an opening in front, just beneath the cornea, which forms the pupil, or sight of the eye, d d, through which the rays of light pass into the eye. The pupil is surrounded by a coloured 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: the 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, 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.

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.