OPTICAL PHENOMENA.

277. Various and interesting effects result from the decomposition, reflection, and refraction of light, in its passage through the atmosphere when in different conditions of medium or density.

278. A ray of light is composed of seven different colors; viz. red, orange, yellow, green, blue, indigo and violet.

279 All that endless variety and combination of tints displayed by the sky, the flowers of the field, the rich hues of the autumnal woods, and the gorgeous plumage of birds-in short, the colors of all objects, whether opaque bodies or transparent media arise from their varying capacity of absorbing or reflecting certain rays. The reflection of all the rays causes white, and the absorption of all black.

280. The cerulean tint of the sky is caused by the blue rays being more abundantly reflected or scattered by the aerial particles; the greater paleness of the blue arises from the existing vesicular (a) vapor in the lower strata of the atmosphere.

281. In the eastern horizon at sunrise, and in the western at sunset, the color of the sky is usually red or yellow. This is owing to the solar beams having a greater portion of the atmosphere to traverse in order to reach us, most of the blue rays are lost by absorption and reflection in their passage, while the red and yellow, which have the greater momentum, (variously modified by reflected and peculiar atınospheric conditions) are reflected.

282. The golden, rosy, purple and fiery hues of the clouds and sky, are caused by the presence of aqueous and vesicular vapor in the air in different stages of condensation.

283. The scintillation of the stars is also due to the same cause.

284. Rainbows.-A pencil of light on entering a rain-drop undergoes refraction; this pencil or ray consisting of different colors, which pos sesses different degrees of refrangibility, emerges, after being reflected at the back part of the drop decomposed into its primitive colors. Thus, from the action of the aqueous globules upon the luminous rays, arises the colored bow we see projected on the cloud. The more brightly the sun shines, and copious the rain, the more vivid and distinct are the colors of the bow.

285. Both solar and lunar rainbows are due to the same cause.

286. Halos.-Colored circles of varying diameter are occasionally seen surrounding the sun and moon when the sky is invested with light clouds. They are more frequently observed around the moon, but this is simply owing to the sun's light being generally too dazzling to admit of colors being distinguished near his disc. They are caused by the inflection of light by the globules of visible vapor, minute crystals of ice and snow; or that property of rays to bend and divide as they pass near the borders, ridges and angles of surfaces.

287. When the halos are well defined, and closely encircle the luminous body, it is because the atmosphere is surcharged with humidity, large globules being formed. And hence there is truth in the popular remark, that a dense halo portends rain.

FIG. 11.

288. Parhelia. Luminous circles, arcs, and horizontal bands, with mock-suns (parhelia,) and mockmoons, (paraselena,) in the neighborhood of the sun and moon are rare meteorological spectacles in our latitude, but very common in the arctic regions, where snowy spiculæ or minute crystals of ice float abundantly in the air, the angular of light in different

289. In the severe winter of 1615, in Iceland, the sun when seen, was always accompanied by two, four, five, and even nine of these illusions. The above cut represents a remarkable example seen and described by Capt. Parry, which continued from noon until six o'clock in the evening, during his winter sojourn at Melville Island.

290. Mirage.-Illusions with reference to terrestrial objects are due

to extraordinary reflection and refraction, caused by peculiar conditions of

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the air as to density. The mirage,(a) the delusive appearance of water, common in the plains of Asia and Africa on the verge of the horizon, is a well known instance.

291. In Egypt the general aspect of the country is that of a plain, with gentle eminences on which the villages are seated. When the weather is calm and the ground is highly heated, the landscape at a distance assumes the appearance of a pure transparent lake. The villages seem built on islands, their respective objects, houses and trees, are vividly reflected as if in water, and multiplied and enlarged images are produced. So perfect is the representation, that experienced travelers, oppressed with heat and thirst, have accepted it as a reality. This was the case with the French army during the celebrated campaign in Egypt.

"Still the same burning sun! no cloud in Heaven!
The hot air quivers, and the sultry mist
Floats o'er the desert, with a show

Of distant waters mocking their distress."

292. The phenomenon as above described depends upon the earth being highly heated, and the resulting evaporation. But the temperature of the air rapidly diminish 's from the surface of the ground; hence there are striking diversities in the density of the lower strata, and the rays of light are unequally refracto and reflected at the surfaces of strata of different densities.

293. Instances of the sun and moon being seen above the horizon when actually below it, of mountains and coasts unseen before, brought within sight of the spectator-of persons having their shadows depicted upon the clouds-of vessels being seen sailing in the air, high above the surface of the water, are not uncommon in certain localities and latitudes.

(a) Serab is the Arabic term for what the French call mirage. It occurs as a commo: nblem of deceit in oriental writings. The Koran says, "The actions of unbelievers are like the serab

of the plain; he who is thirsty takes it for water, and finds it to be nothing."

294. A singular spectacle is occasionally exhibited in the narrow strait between Messina and Reggio. A variety of images, men, houses, cattle, rocks, and trees are seen, pictured on the surface of the water, and in the air immediately over it. Multiplied images of the same object occur, or two images, one in a natural and the other in an inverted position; and they have been obseryed to be fringed with red, green, blue, and other prismatic colors.

This exhibtion is called by the Sicilians fata morgana, a title of uncertain derivation, but supposed to refer to a vulgar presumption of the spectacle being called into existence by fairy art or an enchanter's wand.

FIG. 14.

295. Dr. Buchan relates, that while on the cliff near Brighton, England with a companion, watching the sun, when rising, Nov. 18, 1804, he saw, just as the solar disc emerged above the surface of the water, the face of the cliff represented precisely opposite to him, with a neighboring wind-mill, his own figure, and that of his friend, all faithfully depicted at some height above the sea. The appearance lasted about ten minutes. There was a dense fog upon the water at the time.

296. As two travelers were standing on the summit of Ben Lomond, August 19, 1820, watching the sunset, the attention of one of the party was arrested by the appearance of two gigantic figures pictured upon the cloud in the east, apparently standing on an enormous pedestal. He pointed out the phenomenon to his companion, and immediately one of the figures was observed to strike the other on the shoulder, and point towards them; they waved their hats and umbrellas, and the shadowy figures made a similar movement, faithfully imitating every gesticulation. The spectacle continued about fifteen minutes.

297. The spectre of Brocken(a)-the colossal figure of a man observed to walk the ridge at sunrise, is an analogous example.

(a) One of the Hartz Mountains.