it is a common remark, that people of a sallow complexion appear fairer or whiter by candle-light. Mrs. B. The yellow cast of their complexion is not so striking, when every object has a yellow tinge. Emily. Pray, why does the sun appear red through a fog? Mrs. B. It is supposed to be owing to the red rays having a greater momentum, which gives them power to traverse so dense an atmosphere. For the same reason, the sun generally appears red at rising and sitting as the increased quantity of atmosphere, which the oblique rays must traverse, loaded with the mists and vapours which are usually formed at those times prevents the other rays from reaching us. Caroline. And, pray, why are the skies of a blue colour. Mrs. B. You should rather say, the atmosphere; for the sky is a very vague term, the meaning of which it would be difficult to define philosophically. Caroline. But the colour of the atmosphere should be white, since all the rays traverse it in their passage to the earth. Mrs. B. Do not forget that we see none of the rays which pass from the sun to the earth, excepting those which meet our eyes; and this happens only if we look at the sun, and thus intercept the rays, in which case, you know, the sun appears white. The atmosphere is a transparent medium, through which the sun's rays pass freely to the earth; but when reflected back into the atmosphere, their momentum is considerably diminished; and they have not all of them power to traverse it a second time. The momentum of the blue rays is least; these, therefore, are the most impeded in their return, and are chiefly reflected by the atmosphere: this reflection is performed in every possible direction; so that whenever we look at the atmosphere, some of these rays fall upon our eyes; hence we see the air of a blue colour. If the atmosphere did not reflect any rays, though the objects on the surface of the earth would be illumined, the skies would appear perfectly black. Caroline. Oh, how melancholy that would be; and how pernicious to the sight, to be constantly viewing 952. Why does the sun appear red in the morning and when seen through fog or clouds? -953. Why does the sky or atmosphere appear blue? 954. How would the sky appear if the atmosphere reflected none of the rays of light? bright objects against a black sky! But what is the reason that bodies often change their colour; as leaves which wither in autumn, or a spot of ink which produces an ironmould on linen? Mrs. B. It arises from some chemical change, which takes place in the internal arrangement of the parts, by which they lose their tendency to reflect certain colours, and acquire the power of reflecting others. A withered leaf thus no longer reflects the blue rays; it appears, therefore, yellow, or has a slight tendency to reflect several rays which produce a dingy brown colour. 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 colours, but with a preference to reflect the yellow rays, and such is the colour of the iron-mould. Emily. Bodies, then, far from being of the colour which they appear to possess, are of that colour 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 are not satisfied with depriving it of colour, 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 At our next meeting, I shall give you a deeye. our lesson. scription of the 955. What is the reason that they often change their colour? -956. What dress is warmest, a black or a white one ?-957. Why is a black one warmest ? -958. Why is a white more dazzling than a black dress? CONVERSATION XVII. OPTICKS. ON THE STRUCTURE OF THE EYE, AND OPTICAL INSTRU- Description of the Eye; Of the Image on the Retina; MRS. B. 959 THE body of the eye is of a spherical form) (fig. 1, 9 plate XXI.) It has two membraneous coverings; the exter nal one, a a a, is called the sclerotica; this has a projection in that part of the eye which is exposed to view, b b, which is called the cornea, because, when dried, it has 12 nearly the consistence of very fine horn, and is sufficient ly 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, 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 -960. Which 959. What is the form of the body of the eye? figure represents an eye?-961. What is the external covering of the eye called?-962. Which part of the eye is called the cornea? -963. From what does the cornea take its name? 964. What part of the eye is called the choroid?-965. What part of the figure represents the choroid?-966. What is that part of the eye called through which the light passes?- -967. By what part of the figure is the pupil represented? -968. By what is the pupil of the eye surrounded?- -969. What represents the iris in the figure? -970. Is the pupil of the eye always of the same size? 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 optick 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, and we clearly perceive objects which were before invisible. Mrs. B. It is just so. The choroid cc, is imbued 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 their pupils may receive 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 971. When is it dilated, and when contracted?-972. Why does it give the eyes pain on first going into a bright light from a dark room?- -973. Why does it seem much darker on first going out in the night, than after we have been out a short time? -974. How much more light is admitted when the pupil is extended to the utmost, than when most contracted?- -975. Why can cats, horses, and some other animals, see better in the night than we can?-976. How much is it thought the pupil of their eyes extend and contract?977. What is contained within the coverings of the eye-ball? occupies the space immediately behind the cornea, and is called the aqueous humour ff from its liquidity and its resemblance to water., Beyond this is situated the crystalline humour, g g, sb 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, hh, which derives its name from a resemblance it is supposed to bear to glass or vitrified substances. [ The membraneous 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 consists of an expansion of the optick nerve, of a most perfect whiteness: it proceeds from the brain, enters the eye, at n, on the side next the nose, and is finely spread over the interiour 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 is represented in the camera obscura, without any such assistance. Mrs. B. That is true; but the representation would be much more strong and distinct, if we enlarge 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. 978. What are the three humours called?- -979. From what does the aqueous humour derive its name?- -980. From what does the crystalline humour derive its name?-981. From what does the vitreous humour derive its name?- -982. For what are the membraneous coverings of the eye chiefly intended?983. Which part of the figure exhibits the retina?- -984. Of what does the retina consist ?- -985. How is the light which enters the pupil affected by the several humours ?- -986. What would be the consequence if the light admitted by the pupil were not refracted by the humours? |