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Show 8 NOTE IlL--COLOURED CLOUDS. Eve's ji!ken couch with gorgeous tints adorn, And fire the arrow)' throne of rijing morn. C ANTO I. 1. I 19. THE rays from the rifing and fetting fun are refraCted by our fpherical at~ofphere, hence the moll: refrangible rays, as the violet, indigo, and blue are refleCted 111 greater quantiti es from the morning and evening lkies; and the leafl: refra~gible ones, as red and orange, are !aft feen about the fetting fun. Hence ~r. Beguelm .obferved that. the fhadow of his finger on his pocket-book was much bluer rn the mornmg and evenmg, when the fhaJow was about eight times as long as the body from which it was projected. Mr. Melville obferves, that the blue rays being more refrangible are bent down in the evenings by our atmofphere, while the red and orange being lefs refrangible continue to pafs on and tinge the morning and evening clouds with their colours. See Priefl:ley's Hillary of Li ght and Colours, p. 440. But as the particles of air, like thofe of water, are themfelves blue, a blue fhadow may be feen at all times of the day, though much more beautifully in the mornings and evenings, or by means of a candle in the middle of the day. For if a lbadow on a piece of white paper is produced by placing your finger between the paper and a candle in the day light, the fhadow will appear very blue; the yellow light of the candle upon the other parts of the paper apparently deepens the blue by its cantrall:, t!1cfe colours being oppofite to each other, as explained in note II. Colours are produced from clouds or milts by refraCtion, as well as by refleCtion. In riding in the night over an unequal country I obferved a very beautiful coloured halo round the moon, whenever I was covered with a few feet of mift, as I afcended from the vallie~; which ceafed to appear when I rofe above the mill:. This I fuppofe was owing to the thinnefs of the ftraturn of mill:, in which I was immerfed ; had it been thicker, the colours refracted by the fmall drops, of which a fog confifts, would not have palfed through it down to my eye. There is a bright fpot feen on the cornea of the eye, when we face a window, which is much attended to by portrait painters; this is the light refleCled from the fpherical furface of the polifhed cornea, and brought to a focus; if the obferver is placed in this focus, he fees the image of the window ; if he is placed before or behind the focus, he only fees a luminous fpot, which is more luminous and of lefs extent, the nearer he approaches to the focus. The luminous appearance of the eyes of animals in the dufky corners of a room, or in holes in the earth, may arife in fome in!l:ances from the fame principle; viz. the reflection of the light from the fpherical cornea; which will be coloured red or blue in fome degree by the morning, evening, or meridian light; or by the objects from which that light is previou!ly reflected. In the cavern at Colebrook Dale, where the mineral tar exfudes, the eyes of the horfe, which was drawing a cart from within Non: IV. C 0 MET S. towards the mouth of it, appeared like two balls of phofphorus, when he was above roo yards ofF, and for a long time before any other part of the animal was villble. I n thi" cafe I fufpetl: the l11minous appc:uance to have been owing to the light, which had en tered ~he eye~ b:ing reflected from the back furface of the vi treous humour, and thence emergIng agam 1n parallel rays from the animals eye, as it docs from the back fur face of the drops of the rainbow, and from the water-drops which li e, perhaps without contact, 0 11 cabbage-leaves, and have the brilliancy of quickfilver. This ac aunts for this luminous appearance being be~ feen in thofe animals which have large :~perturcs in th eir iris, as in cats and horfes, <tnd JS the only part viGble in obfcure pl <Jces, becaufe this is a better refl ect in.g fu~face th an any other part of the animal. If any of th efe emergent rays from 1he anu~al s eye can be. fupp~fed to have been re/1etl:ed from the choroid coat thro 11 gh the femi-tranfparent retma, tillS wmdd account for the coloured ~!are of the eyes of dew:> or cats and rabbits in dark corners. :) 9 NOTE lV.-COMETS. Alarm with comel-b!aze the .fapphire p!ainJ '!'be wan jlars glimmering tb1rough its .filver trai1t. C ANTO I. l. IJ3· ·THERE have been many theories invented to account for the tails of comets. Sir 1faac Newton thinks that they con fill of rare vapours raifed from the nucleus of the comet, and fo ~arefied by the fun's heat as to have their general gravitation diminifhed, and. that they 111 confequence afcend oppo!ite to the fun, and from thence refleet the rays of hght. Dr. Halley compares the light of the tails of comets to the ftreams of the aurora borea~is, and other elelhic effluvia. Philof. Tranf. No. 347· Dr. H~mdton obfe~ves, that the light of fmall fl:ars is feen undiminifhed through bot~ the l1ght of the tails of comets, and of the aurora borealis, and has farther j Jlullrated thetr electric analogy, and adds that the tails of cornets confl!l: of a lucid felf-n 1 inin(7 fubfl:ance. which has not the power of refracting or refleCting the rays of light. Effays~ _ . The tat! of the comet of I 744 at one time appeared to extend above r6 degrees from Its body, and mull: have .thence been above twenty three millions of miles long. And the comet of I68o, according to the calculations of Dr. Halley on November the I It] 1 , was not above one femi-diameter of the earth, or lefs that 4000 miles to the northwa rd. of.the way of the earth; at which time had the earth been in that part of its orbit, what 011ght h4ve been the confequence! no one would probably have; (urvived. to hav:c .regi£lered the treme.ndous effetls. B |
