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occurrence of clouds far to the west, and were towards the east, the same shadows became in fact passing to the place whence they seemed to visible, when considerably fore-shortened, and originate; and the circumstances of the case seem could be observed moving on and changing with to have been as follow the atmosphere contained the clouds themselves. All these phenomena, a slight haze, which allowed the sun's beams to with their variations, were easily referrible to pass forward with but little interruption, but their causes, and may be observed at almost any was yet in sufficient quantity to refleci a consi- sun-set in five weather; but the effect of the derable portion of light to the eye. The sun first evening, so similar in kind, though so difwas just setting ; the clouds very far to the west, ferent in appearance, was not again remarked. and out of sight from the place where the ob- It is with a view of guarding persons who may server stood, stopped the light wherever they observe the same effect, against any mistake as interfered, and cast immense horizontal, or nearly to its origin, that the appearance, with its nature, horizontal, shadows along the sky, parallel to has been thus particularly described. each other, and over the head of the observer. 320. The elevation of coasts, ships, and mounThe difference between these shadows and the tains, above their usual level, when seen in the intervening illuminated parts could not be ob- distant horizon, has been long known and deserved over head, or on the right or left hand, scribed under the name of Looming. The name i. e. perpendicular to their direction, because of of mirage has been applied by the French to the want of sufficient depth, as it were, in the the same class of phenomena; and the appellaparts thus circumstanced, to make them visible; tion of Fata Morgana has been given by the but, as they receded from the observer in the di- Italians to the singular appearances of the same rection from the sun, they became fore-shortened, kind which have been repeatedly seen in the and then from the greater depth of mass, and straits of Messina. See Fata MORGANA. consequently greater number of particles looked 321. The phenomena of the mirage are most at, became visible. This is at least one reason frequently seen in the case of ships when they why they were so visible towards the east; but are just beginning to appear above the visible another is the probable existence of more haze horizon. Mr. Huddart, Dr. Vince, and captain in that direction than towards the west, or to the Scoresby, have described various appearances of right or left of the observer's situation : the rays this kind, of which the following are the most could not be seen between the sun and the ob- interesting :server, though the sun was out of sight, and 322. On the 1st of August, 1798, Dr. Vince consequently the general light, it may be sup- observed at Ramsgate a ship which appeared posed, not too great; which seems to imply as at A, fig. 2, the top mast being the only part that less haze existed in that direction; and its of it that was seen above the horizon. An inpresence was fully proved towards the east by verted image of it was seen at B immediately ihe dull red color which the moon assumed above the real ship A, and an erect image at C, upon rising a short time after the appearance both of them being complete and well defined. had ceased. The convergence of the rays to The sea was distinctly seen between them as at one spot, and that opposite the sun, was merely v w. As the ship rose to the horizon the image an effect of perspective, and requires no expla- C gradually disappeared, and while this was nation here.
going on the image B descended, but the main319. Although the appearance on this evening mast of B did not meet the mainmast of A. The was exceedingly beautiful and rare, and the two images B C were perfectly visible when the more striking from the absence, to the observer, whole ship was actually below the horizon. of the sun or clouds, and the complete insulation 323. While navigating the Greenland Sea on of the phenomenon, yet by close observation, the 28th of June, 1820, captain Scoresby obupon other evenings, it was found that partial served about eighteen or nineteen sail of ships effects of the same kind were very common;
at the distance of from ten to fifteen miles. He and, from the manner in which these could be saw them from the mast head, beginning to observed, the explanation above given fully con- change their form. One was drawn out, or firmed. On several evenings after, when ob- elongated, in a vertical plane; another was conserving the sun set from the neighbouring hill of tracted in the same direction; one bad an inSt. Catherine, it was found that if the atmo- verted image immediately above it as at a fig. 3, sphere was generally clear, but with compact and two at b and c, had two distinct inverted and distinctly-formed clouds floating in it, the images in the air: along with these images there effect was produced. The usual appearance of appeared images of the ice, as at b and c, in two rays at sunset, diverging amongst the clouds in strata, the highest of which had an altitude of the west, from the sun, is well known; but, even about 15'. when these were not visible, upon looking to the 324. In a later voyage, performed in 1822, eastern half of the hemisphere, and especially captain Scoresby was able to recognise' his to the north or south of east towards the horizon, father's ship, when below the horizon, from the it was rarely that some clouds could not be dis- inverted image of it which appeared in the air. tinguished, with long shadowy projections be- 'It was,' says he,' so well defined, that I could hind them, always converging to the spot oppo. distinguish by a telescope every sail, the general site to the sun. Frequently, clouds could be 'rig of the ship, and its particular character; selected moving more immediately in the neigh- insomuch that I confidently pronounced it to be bourhood of the observer : of those which pass- my father's ship, the Fame, which it afterwards ed overhead shadows could not be observed proved to be; though, in comparing notes with close to the clouds; but, carrying the eye onwards my father, I found that our relative position at
the time gave our distance from one another that a third and even a fourth image may be very nearly thirty miles, being about seventeen miles beyond the horizon, and some leagues 330. If the variation of refractive power takes beyond the limit of direct vision. I was so place only in the tract of air through which the struck by the peculiarity of the circumstance that rays Hc, S d, pass, then there may only be an I mentioned it to the officer of the watch, stating inverted image; and if it takes place only in the my full conviction that the Fame was then tract through which Sm, Sn, pass, there may cruising in the neighbouring inlet.'
only be an erect image. It is also obvious, that 325. One of the most curious phenomena of if the variation of refractive power commences this kind was seen by Dr. Vince on the 6th of at the line joining the eye and the horizon, the August, 1806, at 7 P. M. To an observer at ordinary image SH will not be seen; and, in Ramsgate the tops of the four turrets of Dover like manner, it is clear that the inverted and castle are usually seen over a hill between Rams- erect images sh, s' h', may be seen even if the gate and Dover. Dr. Vince, however, when at real ship S H is below the visible horizon. Ramsgate saw the whole of Dover castle as if it 331. In the case of Dover Castle, the rays bad been brought over and placed on the Rams- from the top and bottom of the castle passed gate side of the hill. The image of the castle above the hill in curve lines, and the top of the was so very strong and well defined, that the hill hill was seen by the observer at Ramsgate, by itself did not appear through the image. means of a curved ray which reached the eye
326. In the sandy plains of Egypt the mi- between the rays of the top and bottom of the rage is seen to great advantage. These plains castle. are often interrupted by small eminences, upon 332. That the phenomena of the '
mirage are which the inhabitants have built their villages, produced by such variations in the refractive in order to escape the inundations of the Nile. power of the atmosphere as we have mentioned In the morning and evening objects are seen in may be proved by actual experiment. All the their natural form and position, but, when the phenomena may be represented artificially to the surface of the sandy ground is heated by the eye, and we may even venture to predict new sun, the land seems terminated at a particular phenomena which have not yet been witnessed. distance by a general inundation; the villages If the variation of the refractive power of the which are beyond it appear like so many islands air takes place in a horizontal line perpendicular in a great lake, and between each village an to the line of vision, that is, from right to left, inverted image of it is seen.
then we may have a lateral mirage, that is, an 327. Our limits will not permit us to give any image of a ship may be seen on the right or left farther examples of these curious phenomena. hand of the real ship, or on both, if the variaWe shall therefore attempt to give a popular ex- tion of refractive power is the same on each side planation of their cause.
of the line of vision. If there should happen at 328. Let S H, fig. 4, be a ship in the horizon, the same time both a vertical and a lateral variaand visible to the eye at E, by rays S E, HE, tion of refractive power in the air, and if the proceeding in straight lines to E, through a tract variation should be such as to expand or elonof the atmosphere in its usual state. If we gate the object in both directions, ihen the object suppose, what is known to be sometimes the would be magnified as if seen through a telecase, that the refractive power of the atmosphere, scope, and might be seen and recognised at a or air, above the line S a E varies, so as to be distance at which it would not otherwise have less at c than at a, then rays Sd, H c, proceeding been visible. If the refractive power, on the upwards from the ship, and that never could in contrary, varied, so as to contract the object in the ordinary state of the air reach the eye at E, both directions, the image of it would be diwill be refracted into curve lines H c, Sd; and, minished as if seen through a concave lens. if the variation of refractive power is such that 333. In order to represent artificially the effects these last rays cross each other at x, then the ray of the mirage, Dr. Wollaston views an object Sd, in place of being the uppermost, will now through a stratum of spirit of wine lying above be the undermost, and consequently will enter water, or a stratum of water laid above one of the eye as if it came from the lower end of the syrup. These substances, by their gradual inobject.
corporation, produce a refractive power dimin329. If we now draw lines E s, E h, tangents ishing from the spirit of wine to the water, or 10 these curve lines at E, these lines will be the from the syrup to the water; so that by looking direction in which the ship will be seen by the through the mixed, or the intermediate stratum rays Hc, Sd, and the observer at E will see an at a word or object held behind the bottle which inverted image s h of the ship SH considerably contains the fluids, an inverted image will be elevated above the horizon. The refractive seen. The same effect Dr. Wollaston has shown power of the air still continuing to diminish, may be produced by looking along the side of a other rays, Hm, H n, that never could reach the red-hot poker at a word or object ten or twelve eye at É in the ordinary state of the atmosphere, feet distant. At a distance less than three-eighths may likewise be bent into curves which will not of an inch from the line of the poker, an inverted cross each other before they reach the eye at E. image was seen, and within and without that an In this case the tangent E s' to the upper curve erect image. Sn E will be uppermost, and the tangent El' to 334. The method employed by Dr. Brewster the lower curve Sm E lowermost, so that the to illustraʻe these phenomena consists in holding observer at E will see an erect image s' h' of the a heated iron above a mass of water bounded by ship above the inverted image. it is possible parallel plates of glass; as the heat descends
slowly through the fluid, we have a regular va minute grooves are scarcely distinguishable, unriation of density which gradually diminishes less at the boundary between a dark and a lufrom the bottom to the surface. If we minous object. In sharp lights, however, and withdraw the heated iron, and put a cold body particularly in that of the sun, the colors shine in its place, or even allow the air to act alone, with extraordinary brilliancy, and the beautiful the superficial stratum of water will give out its tints which accompany every luminous image can heat, so as to produce a decrease of density from only be equalled by their matchless exhibition on the surface to a certain depth below it. Through the reflections of the diamond. the medium thus constituted the phenomena of 340. The colors transmitted by plates of glass the mirage may be seen in the finest manner. containing a film of air or water are well worth
335. We have no doubt that some of the facts an attentive examination. The plates of reflecascribed in the Western Highlands of Scotland. tion or transmission of the several colors in a to second sight have been owing to the unusual series are so near each other that the colors dilute refraction of the atmosphere, and that the same each other by mixture, whence the number of cause will explain some of those wonders which series in the open daylight seldom exceeds seven sceptics discredit, and which superstitious minds or eight: but if the system be viewed through a attribute to supernatural causes. The beacon prism, by which means the rings of various colors keeper of the Isle of France, who saw ships in are separated, according to their refrangibilities, the air before they rose above the visible horizon, they may be seen on that side towards which the may now recover his good character in the eyes refraction is made, so numerous that it is imposof the former, while the latter may cease to re sible to count them. Or if, in a dark chamber, gard him as a magician.
the sun's light be separated into its original rays 336. Very beautiful effects resulting from the hy a prism, and a ray of one uncompounded codecomposition of white light are visible on the lor be received upon two glasses, the number of surface of the feathers of birds; and, as we have circles will become very numerous. In this exstated in a previous section, are also exhibited periment it is also seen that, in any series, the on the surface of mother-of-pearl. Dr. Brew- circles formed by the less refrangible rays exceed ster, while pursuing a series of experimental in magnitude those which are formed by the investigations on this subject, found, by the more refrangible rays, and consequently that, in aid of the microscope, that they arose from any series, the more refrangible rays are reflected grooves in the striated surfaces; that they at less thicknesses than those which are less rewere produced when the flat surface was un- frangible. If the light be incident obliquely, the polished, and that they could be communicated rings of colors dilate and enlarge themselves ; to wax, gum-arabic, tin-foil
, the fusible metal, whence it follows that the thickness required to and even to lead, by hard pressure or the reflect the colors of any series is different in difblow of a hammer. He determined also that ferent obliquities. the mottled colors upon all bodies with an im 341. When the solar rays are passed through perfect polish, and the scratches or grooves upon
a convex lens, or reflected from a concave, a very polished metals, could be communicated to wax intense heat is produced by the concentration of and other substances. The same structure which the rays. Count Rumford has shown that when gives these communicable colors he succeeded in the rays of the sun are made to pass through a producing artificially on the surface of calves- certain aperture, and fall upon any substance to feet jelly, that had been boiled a considerable be heated, while the same area of light is made
and he discovered, with a powerful micro to pass through a lens, in the focus of which the scope, the same minute grooves which exist in same quantity of matter is to be heated, they bemother-of-pearl, and they were so near one an come heated in the same time to the same degree. other that some thousands of them must have Nothing is better known, in short, than that the been contained in a single inch. These grooves rays of the sun are capable of exciting sensible were completely visible to the unassisted eye, heat. Newton, and the philosophers of his age, but they gave in a very distinct manner the colors accounted for heat by the motion excited in the of mother-of-pearl.
parts of the body by the agitating power of the 337. Mr. Barton, of the mint, has succeeded absorbed light. Melville supposed that the heat in ornamenting steel and other articles, with the was expelled from the terrestrial matter by the colors of striated surfaces, and of applying this light. At present it is generally admitted, on the principle to practical purposes.
strength of some valuable experiments to which 338. In applying the princip
ot striated co we have already alluded, that the rays of light lors to ornament steel, the effect, or pattern, is and caloric are separately emitted from the sun, produced upon the polished surface by the point the luminous rays producing light, and the caof the diamond, so that either the whole or a loric, heat. part of the surface is covered with lines or 342. It was originally remarked by Newton, grooves, whose distance may vary from the and the fact has since been confirmed by the ex1000th to the 10,000th of an inch. When these periments of Dr. Herschel, that the differentlines are most distant the prismatic images of the colored rays have not the same illuminating candle, or any luminous body, seen by reflection power. The violet rays appear to have the from the polished surface, are
least luminous effect: the indigo more; and the another, and the common colorless image: and, effect increases in the order of the colors, the when the lines are least distant, the colored the green being very great; between the green images are farthest from one another, and the and yellow the greatest of all; the yellow the colors are most vivid.
same as the green; but the red less than the 339. In daylight the colors produced by these yellow.
343. Sir William Herschel introduced a beam place, they might try whether two of these small of light into a dark room, which was decompos- segments of a sphere placed together, or a glass ed by a prism, and then exposed a very sensible convex on both sides, would not magnify more thermometer to all the rays in succession, and than one of them. They would then find that observed the heights to which it rose in a given two of these glasses, one for each eye, would time. He thus determined that the heating answer the purpose of reading better than one; power of the red, to that of the green rays, was and, lastly, they might find that different degrees two and three-quarters to one; and three and a of convexity suited different persons. half to one in red to violet.
349. It is certain that spectacles were well 344. On repeating these experiments he found known in the thirteenth century, and not long that the greatest quantity of calorific rays were before. It is said that Alexander Spina, a native even beyond the colored spectrum at about half of Pisa, who died in 1313, and who was very an inch from the commencement of the red rays. ingenious in executing whatever he saw or heard At a greater distance from this point it began to of as having been done by others, happened to diminish, but was very perceptible even at the see a pair of spectacles in the hands of a person distance of an inch and a half.
who would not explain them to him; but that 345. It will appear, from what has been stated, he succeeded in making a pair for himself, and that these calorific rays are less refrangible than immediately made the construction public, for the the rays of light; hence the calorific focus will good of others. It is also inscribed on the tomb fall beyond that of the luminous. Dr. Herschel of Salvinus Armatus, a nobleman of Florence, made an experiment to verify this inference, but who died in 1317, that he was the inventor of did not come at any thing conclusive. He after- spectacles. wards made experiments to collect these invisible 350. The use of concave glasses to help those calorific rays, and caused them to act independ- persons who are shortsighted was probably a ently of the light; by which he concludes that discovery that followed not long after that of they are sufficient to account for all the effects convex ones for the relief of those whose sight is produced by the solar rays in exciting heat; that defective in the contrary extreme, though we find ihey are capable of passing through glass, and no trace of this improvement. From this time, of being refracted and reflected, after they have, though both convex and concave lenses were been finally detached from the solar beam. sufficiently common, yet no attempt was made to
346. Dr. Morichini appears to have been the form a telescope, by a combination of them, till first person to point out the connexion between the end of the sixteenth century. Descartes light and magnetism. To experimentally illus- considers James Metius, a person who was no trate this important fact, he employed a prism, mathematician, though his father and brother had and caused the decomposed rays to fall on a se- applied to those sciences, as the first constructor ries of small needles, and the needle intersected of a telescope; and says that, as he was amusing by the violet ray was soon found to acquire per- himself with making mirrors and burning-glasses, manent polarity. It has since been ascertained he casually thought of looking through two of his that this property is not peculiar to the violet lenses at a time, and that happening to take one ray, but extends, though in a less degree, through that was convex and another that was concave, several other rays in the series.
and happening also to hit upon a pretty good ad347. Our present article was commenced with justment of them, he found that, by looking through a few facts illustrative of the history of optics, them, distant objects appeared very large and when viewed in connexion with the theory of distinct. In fact, without knowing it, he had made light; it may now be advisable to furnish similar a telescope. data for a right understanding of the progress 351. Others say that this great discovery was first that has been made in the construction of optical made by John Lippershein), a maker of spectamachines.
cles at Middleburg, or rather by his children; 348. The ancients were so little acquainted who, like Metius, were diverting themselves with with the science of optics that they seem to have looking through two glasses at a time, and plachad no instruments of the optical kind, excepting ing them at different distances from one another. the glass globes and speculums formerly men But Borellus, the author of a book entitled De tioned, which they used in some cases for magnify- vero Telescopii Inventore, gives this honor to ing and burning. Alhazen gave the first hint of Zacharias Joannides or Jansen, another maker the invention of spectacles, and it is probable that of spectacles at the same place, who made the they were found out soon after his time. From first telescope in 1590 ; and it seems now to be the writings of Alhazen, together with the ob- the general opinion that this account of Borellus servations and experiments of Roger Bacon, it is is the most probable. Indeed his account of the not improbable that some monks gradually hit discovery of telescopes is so circumstantial, and upon the construction of spectacles ; to which so well authenticated, that it does not seem posBacon's smaller segment, notwithstanding his sible to call it in question. It is not true, he mistake concerning it, was a nearer approach says, that this great discovery was made by a than Alhazen's larger one. Whoever they were person who was no philosopher; for Zacharias that pursued the discoveries of Bacon, they pro- Jansen was a diligent enquirer into nature; and, bably observed that a very small convex glass, being engaged in these pursuits, he was trying when held at a greater distance from the book, what uses could be made of lenses for those purwould magnify the letters more than when it poses, when he fortunately hit upon the construcwas placed close to them, in which position only tion. Bacon seems to have used it. In the next 352. This ingenious mechanic and philosopher
had no sooner found the arrangement of glasses account, Galileo instantly returned to Padua, that produced the effect he desired than he en- considering what kind of an instrument this closed them in a tube, and ran with his instru must be. The night following the construction ment to prince Maurice, who, immediately con- occurred to him; and the day after, putting the ceiving that it might be of use to him in his wars, parts of the instrument together, as he had predesired the author to keep it a secret. But this viously conceived of it, and notwithstanding the was impossible; and several persons in that city imperfection of the glasses that he could then immediately began to make and sell telescopes. procure, the effect answered his expectation, as One of the most distinguished of these was John he acquainted his friends at Venice, to which Laprey, called Lippersheim by Sirturus. By place he six days afterwards carried another and him some person in Holland being very early a better instrument, and where, from several supplied with a telescope, he passed with many eminences, he showed to some of the principal for the inventor; but both Metius above men senators of that republic a variety of distant obtioned, and Cornelius Drebell of Alcmaer, in jects, to their very great astonishment. When he Holland, applied to the inventor himself in had made farther improvements in the instru1620; as did also Galileo, and many others. ment, he made a present of one of them to the The first telescope made by Jansen did not ex- doge, Leonard Donati, and of one to each of the ceed fifteen or sixteen inches in length; but Sir- senators of Venice; giving along with the inturus, who says that he had seen it and made use strument a written paper, in which he explained of it, thought it the best that he had ever the structure and wonderful uses that might be examined.
made of it both by land and at sea.
In return, 353. Jansen, having a philosophical turn, ap- the republic, on the 25th of August, the same plied his instrument to such purposes as he had year, more than tripled his salary as professor. in view when he hit upon the construction. Di 356. Galileo, directing his tube towards the recting it towards celestial objects, he distinctly moon, found that the surface of it was diversified viewed the spots on the surface of the moon, and with hills and valleys, like the earth. He also discovered many new stars, particularly seven discovered that the via lactea and nebulæ conpretty considerable ones in the Great Bear. His sisted of a collection of fixed stars, which, on son, John Zacharias, noted the lucid circle near account either of their vast distance or extreme the limb of the moon, whence several bright smallness, were invisible to the naked eye. He rays seem to dart in different directions; and he likewise observed innumerable fixed stars dissays that the full moon, viewed through this in- persed over the face of the heavens, which had strument, did not appear flat, but was evidently been unknown to all the ancients : and, spherical, the middle part being prominent. Ju- examining Jupiter with a better instrument piter also, he says, appeared round, and rather than any he had made before, he found that spherical; and sometimes he perceived two, he was accompanied by four stars, which sometimes three or four small stars, a little above performed periodical revolutions round him, or below him; and, as far as he could observe, and which, in honor of the Medici, he they performed revolutions found him. This was called Medicean planets. This discovery he probably the first observation of the satellites of made in January 1610, N. S., and in March he Jupiter.
published an account of all his discoveries, in 354. It may be proper to add, that Francis his Nuncius Sidereus, printed at Venice, and deFontana, an Italian, also claims the invention ; dicated to Cosmo, great duke of Tuscany, who, but he did not pretend to have made it before by a letter dated 10th of July 1610, invited him 1608, and it is well known that the instruments to quit Padua, and assigned him an ample stiwere made and sold in Holland some time be- pend, as primate and extraordinary professor at fore.
Pisa, but without any obligation to read lectures, 355. Some say that Galileo was the inventor or to reside. The extraordinary discoveries conof telescopes ; but he himself acknowledges that tained in the Nuncius Sidereus, which was quickhe first heard of the instrument from a German; ly reprinted in Germany and France, were the but he says that being informed of nothing more cause of much debate among the astronomers ; than the effects of it, first by common report many of whom could not credit Galileo's account, and a few days after by a French nobleman, J. while others ridiculed his discoveries as fictions Badovere, at Paris, he himself discovered the or illusions. Some could not be prevailed upon construction, by considering the nature of refrac- even to look through a telescope; so devoted tion : and thus he had much more real merit were they to the system of Aristotle. But, when than the inventor himself. The account of what it was found vain to oppose the evidence of Galileo actually did in this business is circum- sense, some affirmed that the invention was taken stantially related by the author of his life, pre- from Aristotle; and quoting his writings, in fixed to the 4to. edition of his works, printed at which he mentions stars seen in the day-time Venice in 1744. About April or May, 1609, it from the bottom of a deep well, said, that the well was reported at Venice, where Galileo (who was corresponded to the tube of the telescopes, and professor of mathematics in the university of Pa- that the vapors which arose from it gave the hint dua) then happened to be, that a Dutchman had of putting glasses into it; and that in both cases presented 10 prince Maurice of Nassau a certain the sight is strengthened by the transmission of optical instrument by means of which distant ob- the rays through a thick and dark medium. Gajects appeared as if they were near; but no far- lileo himself, who tells this story, humorously comther account of the discovery had reached that pared such men to alchymists, who pretend that place, though this was nearly twenty years after the art of making gold was known to the ancients, the first discovery. Struck, however, with this but lay concealed under the fables of the poets.