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.