prepared, at a small opening in the window-shutter through which the sun-light comes, a row of prismatic images of this opening will be seen on each side of it; their direction being perpendicular to the lines. olgu Thus, if figure 34. repre-bul Fig. 34. 188 od sents such a plate of glass, the white lines being the ruled lines, and the aperture is viewed through the point, L, a row of images will be seen on each side of L at the corresponding points, a a, b b, c c, and in a line perpendicular to the ruled lines. The tints b PRISMATIC IMAGES.im are in the same order as in the un oloid-niq 8 en gniasqe first experiment. and are caused by the decomposition which the rays of light undergo in passing by the edges of the unruled intervals of glass, dans no bovisado If another series of parallel lines is drawn at right angles to the first, a second row of colored images will start up in a direction perpendicular to that of the first images; and, if the ruled lines in both series are equally distant from each other, the first set of images a a a a, will all be situated at the same distance from L, each one on the middle of the side of a square, whose centre is L. The same will be true of the second set bb bb, and so of the rest. bio ad his image out to solos. If there were three series of ruled lines, equally inclined to each other, they would form regular six-sided figures or hexagons, and the images would be found on the sides of hexagons as in figure 35., where the white dots represent the places of the images. 473. Now it is evident, the number of series may be so increased, that the figures formed by the lines shall have so many sides as not to differ essentially from circles; and then the colored images would touch each Show, by the aid of figures 34., 35. and 36., the peculiar manner in which diffraction operates so as to produce coronas. other, forming rings of pris-bbim ad Fig. 35. nom matic images around the luminous point, the blue being the innermost color, and the red the outermost. Thus, for instance, if in figure 35., the number of series was so multiplied, the largest hexagon would be changed into the limiting circle, and the images, a a, b b, c c, &c., would form prismatic circles around L as a common 200 centre. Telimies ni begasus 918 becomes.9ts abuol 474. These rings appear in great beauty when a luminous object is viewed through a plate of ruled glass, the lines forming concentric circles, many hundreds, and even thousands being contained in an inch; the distance, number, and brilliancy of the rings increasing with the fineness of the lines, and the narrowness of the trans parent intervals. muisoqa odt lo solos do od diw gai 10475. 475. Similar prismatic rings are beheld, whenever the transparent intervals through which the object is seen are grouped symmetrically around a point. Thus Fraunhofer, in looking at a luminous object through a number of small glass balls of equal size, placed between two parallel plates of glass, saw it surrounded by several colored circles or coronas. Nor is this surprising, for the apertures between the balls through which the light came, are arranged concentrically around a point, like the transparent intervals in the case of the circularly ruled lines. This will be seen by a reference to figure 36., where the dark circles represent the balls, and the light parts between them the intervals through which the rays come from the luminous object, which is supposed to be situated behind the figure. Now it is obvious at a glance, if the eye is fixed upon B, that the intervals lie in the circumferences of circles, whose com mon centre is the middleg to Fig. 36. molto point of B. If we look at A, the intervals are arranged around this ball in a similar way, and so of any other ball. In viewing, therefore, a bright object through the balls, it should exhibit the same appearances as if seen through the circularly ruled glass, and this is found to be the case. GLASS BALLS. sdl bas oitsmerig mot blowb 476. Now the globules of vapor, of which fogs and clouds are composed, are arranged in a similar manner throughout the atmosphere, and act upon the light of the sun and moon as if they were so many small glass balls. When, therefore, the rays of the moon, for instance, reach the eye of the observer, after passing between the particles of light, interposing vapors, he will often see her orb surrounded by beautiful coronas, glowing with the rich colors of the spectrum. 477. Coronas are only seen when the globules of vapor are comparatively few, and are of equal size. If they are too numerous a dense cloud is formed, and the intervals being closed by the globules, no rays can pass through them. If they are few in number but differ in size, then the intervals are not symmetrically arranged, and the sun or moon will appear surrounded by a glory, or bright circle of white light. The distance of coronas from the luminous body is not always the same. The smaller the particles, the greater is the diameter of the rings. 478. When white clouds, having the form of the cirro-cumulus, float near the sun, bright, prismatic colors are often seen, by the aid of a blackened mirror, When are coronas only seen? What is the result when the particles of vapor differ in size? Why will the rings vary in magnitude? What is said respecting the edges of cirro-cumulus and cumulus clouds, when passing near the sun and moon? fringing the edges that are parallel to the horizon. The fringes are generally green within, bordered by two red lines. If the air is pure, and the moon shines brightly, the light and broken edges of cumulus clouds, as they pass near her disk, are sometimes seen in like manner fringed with prismatic hues, the purple tint being the nearest color, and the red the most distant. 479. ANTHELIA. Anthelia are coronas seen by reflec tion, when the back of the observer is towards the sun; and are so called from the Greek words anti, opposite, and helios, the sun. If the plain surface of the circularly ruled glass (Art. 474) is blackened, and the luminous object seen by reflection upon the ruled side, its image will appear surrounded by colored rings precisely like those that encircle the object itself, when viewed, as in the first case, by transmitted light. In analogy to this, when the shadow of a person is cast upon a stratum of vapor, the head of the observer, under favorable circumstances, is seen surrounded with prismatic circles. In 480. A beautiful display of this kind was witnessed from the summit of Mount Lafayette, fifteen miles from Mount Washington, on the 7th of August, 1826. the afternoon of the day in question, two gentlemen were standing upon this lofty eminence, a thunderstorm was raging beneath them, and a sea of vapor shut out the vales from view. A light mist was at this time falling, when suddenly the sun burst through the clouds above, and the observers saw their shadows resting upon the vapor before them, their heads surrounded with brilliant, prismatic rings. The circles were apparently ten or twelve feet in diameter, perfectly defined, and their tints were exceedingly rich and vivid. This phenomenon lasted for the space of twelve or fifteen minutes, when it gradually vanished. 481. In the polar seas, when the stratum of fog that Define anthelia. Relate the instances given. rests upon the ocean rises to the height of about three hundred feet, a person, stationed upon the mast of a ship, eighty or a hundred feet above the water, perceives in the fog opposite the sun, one or more circles around the shadow of his head. They are all concentric; their common centre being in the imaginary line drawn from the sun through the eye of the spectator to the fog beyond him. The number of circles varies from one to five, and when the sun is bright, or the fog thick and low, they are usually numerous and highly colored. 482. On the 23d of July, 1821, Scoresby saw four concentric circles around his head, with the series of colors arranged in the following order: 1st circle, white, yellow, red, purple. 2d circle, blue, green, yellow, red, purple. 3d circle, green, whitish, yellowish, red, purple. 4th circle, greenish white. The colors of the first and second rings were very brilliant, those of the third faint, and only seen at intervals, while the fourth exhibited only a slight tinge of green. According to Scoresby, anthelia are always seen in the polar regions whenever fog and sunshine occur at the same time. 483. Several philosophers have supposed that_anthelia, or coronas opposite to the sun, are caused by the passage of light through frozen particles of vapor, but this phenomenon has frequently occurred, when the temperature of the air was so high as to preclude this idea. Thus Kaemtz often beheld anthelia upon the Alps, when the temperature of the air was 50° Fah., at a short distance from the fog. Their explanation upon the principle of diffraction is the most satisfactory, and the truth of this theory is strongly confirmed by an observation of Kaemtz, who, on one occasion, first saw a corona when the cloud was between himself and the sun, What is the opinion of some philosophers in regard to anthelia ? What fact is stated by Kaemtz? |