occurrence of clouds far to the west, and were in fact passing to the place whence they seemed to originate; and the circumstances of the case seem to have been as follow:-the atmosphere contained a slight haze, which allowed the sun's beams to pass forward with but little interruption, but was yet in sufficient quantity to reflect a considerable portion of light to the eye. The sun was just setting; the clouds very far to the west, and out of sight from the place where the observer stood, stopped the light wherever they interfered, and cast immense horizontal, or nearly horizontal, shadows along the sky, parallel to each other, and over the head of the observer. The difference between these shadows and the intervening illuminated parts could not be observed over head, or on the right or left hand, i. e. perpendicular to their direction, because of the want of sufficient depth, as it were, in the parts thus circumstanced, to make them visible; but, as they receded from the observer in the direction from the sun, they became fore-shortened, and then from the greater depth of mass, and consequently greater number of particles looked at, became visible. This is at least one reason why they were so visible towards the east; but another is the probable existence of more haze in that direction than towards the west, or to the right or left of the observer's situation: the rays could not be seen between the sun and the observer, though the sun was out of sight, and consequently the general light, it may be supposed, not too great; which seems to imply that less haze existed in that direction; and its presence was fully proved towards the east by the dull red color which the moon assumed upon rising a short time after the appearance had ceased. The convergence of the rays to one spot, and that opposite the sun, was merely an effect of perspective, and requires no explanation here.

319. Although the appearance on this evening was exceedingly beautiful and rare, and the more striking from the absence, to the observer, of the sun or clouds, and the complete insulation of the phenomenon, yet by close observation, upon other evenings, it was found that partial effects of the same kind were very common; and, from the manner in which these could be observed, the explanation above given fully confirmed. On several evenings after, when observing the sun set from the neighbouring hill of St. Catherine, it was found that if the atmosphere was generally clear, but with compact and distinctly-formed clouds floating in it, the effect was produced. The usual appearance of rays at sunset, diverging amongst the clouds in the west, from the sun, is well known; but, even when these were not visible, upon looking to the eastern half of the hemisphere, and especially to the north or south of east towards the horizon, it was rarely that some clouds could not be distinguished, with long shadowy projections behind them, always converging to the spot opposite to the sun. Frequently, clouds could be selected moving more immediately in the neighbourhood of the observer of those which passed overhead the shadows could not be observed close to the clouds; but, carrying the eye onwards

towards the east, the same shadows became visible, when considerably fore-shortened, and could be observed moving on and changing with the clouds themselves. All these phenomena, with their variations, were easily referrible to their causes, and may be observed at almost any sun-set in fine weather; but the effect of the first evening, so similar in kind, though so different in appearance, was not again remarked. It is with a view of guarding persons who may observe the same effect, against any mistake as to its origin, that the appearance, with its nature, has been thus particularly described.

320. The elevation of coasts, ships, and mountains, above their usual level, when seen in the distant horizon, has been long known and described under the name of Looming. The name of mirage has been applied by the French to the same class of phenomena; and the appellation of Fata Morgana has been given by the Italians to the singular appearances of the same kind which have been repeatedly seen in the straits of Messina. See FATA MORGANA.

321. The phenomena of the mirage are most frequently seen in the case of ships when they are just beginning to appear above the visible horizon. Mr. Huddart, Dr. Vince, and captain Scoresby, have described various appearances of this kind, of which the following are the most interesting:

322. On the 1st of August, 1798, Dr. Vince observed at Ramsgate a ship which appeared as at A, fig. 2, the top mast being the only part of it that was seen above the horizon. An inverted image of it was seen at B immediately above the real ship A, and an erect image at C, both of them being complete and well defined. The sea was distinctly seen between them as at v w. As the ship rose to the horizon the image C gradually disappeared, and while this was going on the image B descended, but the mainmast of B did not meet the mainmast of A. The two images BC were perfectly visible when the whole ship was actually below the horizon.

323. While navigating the Greenland Sea on the 28th of June, 1820, captain Scoresby observed about eighteen or nineteen sail of ships at the distance of from ten to fifteen miles. He saw them from the mast head, beginning to change their form. One was drawn out, or elongated, in a vertical plane; another was contracted in the same direction; one had an inverted image immediately above it as at a fig. 3, and two at b and c, had two distinct inverted images in the air: along with these images there appeared images of the ice, as at b and c, in two strata, the highest of which had an altitude of about 15'.

324. In a later voyage, performed in 1822, captain Scoresby was able to recognise his father's ship, when below the horizon, from the inverted image of it which appeared in the air. 'It was,' says he,' so well defined, that I could distinguish by a telescope every sail, the general Irig of the ship,' and its particular character; insomuch that I confidently pronounced it to be my father's ship, the Fame, which it afterwards proved to be; though, in comparing notes with my father, I found that our relative position at

the time gave our distance from one another very nearly thirty miles, being about seventeen miles beyond the horizon, and some leagues beyond the limit of direct vision. I was so struck by the peculiarity of the circumstance that I mentioned it to the officer of the watch, stating my full conviction that the Fame was then cruising in the neighbouring inlet.'

325. One of the most curious phenomena of this kind was seen by Dr. Vince on the 6th of August, 1806, at P. M. To an observer at Ramsgate the tops of the four turrets of Dover castle are usually seen over a hill between Ramsgate and Dover. Dr. Vince, however, when at Ramsgate saw the whole of Dover castle as if it had been brought over and placed on the Ramsgate side of the hill. The image of the castle was so very strong and well defined, that the hill itself did not appear through the image.

326. In the sandy plains of Egypt the mirage is seen to great advantage. These plains are often interrupted by small eminences, upon which the inhabitants have built their villages, in order to escape the inundations of the Nile. In the morning and evening objects are seen in their natural form and position, but, when the surface of the sandy ground is heated by the sun, the land seems terminated at a particular distance by a general inundation; the villages which are beyond it appear like so many islands in a great lake, and between each village an inverted image of it is seen.

327. Our limits will not permit us to give any farther examples of these curious phenomena. We shall therefore attempt to give a popular explanation of their cause.

328. Let S H, fig. 4, be a ship in the horizon, and visible to the eye at E, by rays SE, HE, proceeding in straight lines to E, through a tract of the atmosphere in its usual state. If we suppose, what is known to be sometimes the case, that the refractive power of the atmosphere, or air, above the line Sa E varies, so as to be less at c than at a, then rays Sd, Hc, proceeding upwards from the ship, and that never could in the ordinary state of the air reach the eye at E, will be refracted into curve lines H c, Sd; and, if the variation of refractive power is such that these last rays cross each other at a, then the ray Sd, in place of being the uppermost, will now be the undermost, and consequently will enter the eye as if it came from the lower end of the object.

329. If we now draw lines E s, E h, tangents to these curve lines at E, these lines will be the direction in which the ship will be seen by the rays Hc, Sd, and the observer at E will see an inverted image s h of the ship SH considerably elevated above the horizon. The refractive power of the air still continuing to diminish, other rays, Hm, Hn, that never could reach the eye at E in the ordinary state of the atmosphere, may likewise be bent into curves which will not cross each other before they reach the eye at E. In this case the tangent E s to the upper curve Sn E will be uppermost, and the tangent Eh' to the lower curve Sm E lowermost, so that the observer at E will see an erect image s'h' of the ship above the inverted image. It is possible

that a third and even a fourth image may be

seen.

330. If the variation of refractive power takes place only in the tract of air through which the rays Hc, Sd, pass, then there may only be an inverted image; and if it takes place only in the tract through which Sm, Sn, pass, there may only be an erect image. It is also obvious, that if the variation of refractive power commences at the line joining the eye and the horizon, the ordinary image SH will not be seen; and, in like manner, it is clear that the inverted and erect images sh, s'h', may be seen even if the real ship S H is below the visible horizon.

331. In the case of Dover Castle, the rays from the top and bottom of the castle passed above the hill in curve lines, and the top of the hill was seen by the observer at Ramsgate, by means of a curved ray which reached the eye between the rays of the top and bottom of the castle.

332. That the phenomena of the mirage are produced by such variations in the refractive power of the atmosphere as we have mentioned may be proved by actual experiment. All the phenomena may be represented artificially to the eye, and we may even venture to predict new phenomena which have not yet been witnessed. If the variation of the refractive power of the air takes place in a horizontal line perpendicular to the line of vision, that is, from right to left, then we may have a lateral mirage, that is, an image of a ship may be seen on the right or left hand of the real ship, or on both, if the variation of refractive power is the same on each side of the line of vision. If there should happen at the same time both a vertical and a lateral variation of refractive power in the air, and if the variation should be such as to expand or elongate the object in both directions, then the object would be magnified as if seen through a telescope, and might be seen and recognised at a distance at which it would not otherwise have been visible. If the refractive power, on the contrary, varied, so as to contract the object in both directions, the image of it would be diminished as if seen through a concave lens.

333. In order to represent artificially the effects of the mirage, Dr. Wollaston views an object through a stratum of spirit of wine lying above water, or a stratum of water laid above one of syrup. These substances, by their gradual incorporation, produce a refractive power diminishing from the spirit of wine to the water, or from the syrup to the water; so that by looking through the mixed, or the intermediate stratum at a word or object held behind the bottle which contains the fluids, an inverted image will be seen. The same effect Dr. Wollaston has shown may be produced by looking along the side of a red-hot poker at a word or object ten or twelve feet distant. At a distance less than three-eighths of an inch from the line of the poker, an inverted image was seen, and within and without that an erect image.

334. The method employed by Dr. Brewster to illustrate these phenomena consists in holding a heated iron above a mass of water bounded by parallel plates of glass; as the heat descends

slowly through the fluid, we have a regular variation of density which gradually diminishes from the bottom to the surface. If we now withdraw the heated iron, and put a cold body in its place, or even allow the air to act alone, the superficial stratum of water will give out its heat, so as to produce a decrease of density from the surface to a certain depth below it. Through the medium thus constituted the phenomena of the mirage may be seen in the finest manner.

335. We have no doubt that some of the facts ascribed in the Western Highlands of Scotland. to second sight have been owing to the unusual refraction of the atmosphere, and that the same cause will explain some of those wonders which sceptics discredit, and which superstitious minds attribute to supernatural causes. The beacon keeper of the Isle of France, who saw ships in the air before they rose above the visible horizon, may now recover his good character in the eyes of the former, while the latter may cease to regard him as a magician.

336. Very beautiful effects resulting from the decomposition of white light are visible on the surface of the feathers of birds; and, as we have stated in a previous section, are also exhibited on the surface of mother-of-pearl. Dr. Brewster, while pursuing a series of experimental investigations on this subject, found, by the aid of the microscope, that they arose from grooves in the striated surfaces; that they were produced when the flat surface was unpolished, and that they could be communicated to wax, gum-arabic, tin-foil, the fusible metal, and even to lead, by hard pressure or the blow of a hammer. He determined also that the mottled colors upon all bodies with an imperfect polish, and the scratches or grooves upon polished metals, could be communicated to wax and other substances. The same structure which gives these communicable colors he succeeded in producing artificially on the surface of calvesfeet jelly, that had been boiled a considerable time; and he discovered, with a powerful microscope, the same minute grooves which exist in mother-of-pearl, and they were so near one another that some thousands of them must have been contained in a single inch. These grooves were completely visible to, the unassisted eye, but they gave in a very distinct manner the colors of mother-of-pearl.

337. Mr. Barton, of the mint, has succeeded in ornamenting steel and other articles, with the colors of striated surfaces, and of applying this principle to practical purposes.

338. In applying the principle of striated colors to ornament steel, the effect, or pattern, is produced upon the polished surface by the point of the diamond, so that either the whole or a part of the surface is covered with lines or grooves, whose distance may vary from the 1000th to the 10,000th of an inch. When these lines are most distant the prismatic images of the candle, or any luminous body, seen by reflection from the polished surface, are nearest one another, and the common colorless image: and, when the lines are least distant, the colored images are farthest from one another, and the colors are most vivid.

339. In daylight the colors produced by these

minute grooves are scarcely distinguishable, unless at the boundary between a dark and a luminous object. In sharp lights, however, and particularly in that of the sun, the colors shine with extraordinary brilliancy, and the beautiful tints which accompany every luminous image can only be equalled by their matchless exhibition on the reflections of the diamond.

340. The colors transmitted by plates of glass containing a film of air or water are well worth an attentive examination. The plates of reflection or transmission of the several colors in a series are so near each other that the colors dilute each other by mixture, whence the number of series in the open daylight seldom exceeds seven or eight but if the system be viewed through a prism, by which means the rings of various colors are separated, according to their refrangibilities, they may be seen on that side towards which the refraction is made, so numerous that it is impossible to count them. Or if, in a dark chamber, the sun's light be separated into its original rays by a prism, and a ray of one uncompounded color be received upon two glasses, the number of circles will become very numerous. In this experiment it is also seen that, in any series, the circles formed by the less refrangible rays exceed in magnitude those which are formed by the more refrangible rays, and consequently that, in any series, the more refrangible rays are reflected at less thicknesses than those which are less refrangible. If the light be incident obliquely, the rings of colors dilate and enlarge themselves; whence it follows that the thickness required to reflect the colors of any series is different in different obliquities.

341. When the solar rays are passed through a convex lens, or reflected from a concave, a very intense heat is produced by the concentration of the rays. Count Rumford has shown that when the rays of the sun are made to pass through a certain aperture, and fall upon any substance to be heated, while the same area of light is made to pass through a lens, in the focus of which the same quantity of matter is to be heated, they become heated in the same time to the same degree. Nothing is better known, in short, than that the rays of the sun are capable of exciting sensible heat. Newton, and the philosophers of his age, accounted for heat by the motion excited in the parts of the body by the agitating power of the absorbed light. Melville supposed that the heat was expelled from the terrestrial matter by the light. At present it is generally admitted, on the strength of some valuable experiments to which we have already alluded, that the rays of light and caloric are separately emitted from the sun, the luminous rays producing light, and the caloric, heat.

342. It was originally remarked by Newton, and the fact has since been confirmed by the experiments of Dr. Herschel, that the differentcolored rays have not the same illuminating power. The violet rays appear to have the least luminous effect: the indigo more; and the effect increases in the order of the colors, the the green being very great; between the green and yellow the greatest of all; the yellow the same as the green; but the red less than the yellow.

343. Sir William Herschel introduced a beam of light into a dark room, which was decomposed by a prism, and then exposed a very sensible thermometer to all the rays in succession, and observed the heights to which it rose in a given time. He thus determined that the heating power of the red, to that of the green rays, was two and three-quarters to one; and three and a half to one in red to violet.

344. On repeating these experiments he found that the greatest quantity of calorific rays were even beyond the colored spectrum at about half an inch from the commencement of the red rays. At a greater distance from this point it began to diminish, but was very perceptible even at the distance of an inch and a half.

345. It will appear, from what has been stated, that these calorific rays are less refrangible than the rays of light; hence the calorific focus will fall beyond that of the luminous. Dr. Herschel made an experiment to verify this inference, but did not come at any thing conclusive. He afterwards made experiments to collect these invisible calorific rays, and caused them to act independently of the light; by which he concludes that they are sufficient to account for all the effects produced by the solar rays in exciting heat; that they are capable of passing through glass, and of being refracted and reflected, after they have, been finally detached from the solar beam.

346. Dr. Morichini appears to have been the first person to point out the connexion between light and magnetism. To experimentally illustrate this important fact, he employed a prism, and caused the decomposed rays to fall on a series of small needles, and the needle intersected by the violet ray was soon found to acquire permanent polarity. It has since been ascertained that this property is not peculiar to the violet ray, but extends, though in a less degree, through several other rays in the series.

347. Our present article was commenced with a few facts illustrative of the history of optics, when viewed in connexion with the theory of light; it may now be advisable to furnish similar data for a right understanding of the progress that has been made in the construction of optical

machines.

348. The ancients were so little acquainted with the science of optics that they seem to have had no instruments of the optical kind, excepting the glass globes and speculums formerly mentioned, which they used in some cases for magnifying and burning. Alhazen gave the first hint of the invention of spectacles, and it is probable that they were found out soon after his time. From the writings of Alhazen, together with the observations and experiments of Roger Bacon, it is not improbable that some monks gradually hit upon the construction of spectacles; to which Bacon's smaller segment, notwithstanding his mistake concerning it, was a nearer approach than Alhazen's larger one. Whoever they were that pursued the discoveries of Bacon, they probably observed that a very small convex glass, when held at a greater distance from the book, would magnify the letters more than when it was placed close to them, in which position only Bacon seems to have used it. In the next

place, they might try whether two of these small segments of a sphere placed together, or a glass convex on both sides, would not magnify more than one of them. They would then find that two of these glasses, one for each eye, would answer the purpose of reading better than one; and, lastly, they might find that different degrees of convexity suited different persons.

349. It is certain that spectacles were well known in the thirteenth century, and not long before. It is said that Alexander Spina, a native of Pisa, who died in 1313, and who was very ingenious in executing whatever he saw or heard of as having been done by others, happened to see a pair of spectacles in the hands of a person who would not explain them to him; but that he succeeded in making a pair for himself, and immediately made the construction public, for the good of others. It is also inscribed on the tomb of Salvinus Armatus, a nobleman of Florence, who died in 1317, that he was the inventor of spectacles.

350. The use of concave glasses to help those persons who are shortsighted was probably a discovery that followed not long after that of convex ones for the relief of those whose sight is defective in the contrary extreme, though we find no trace of this improvement. From this time, though both convex and concave lenses were sufficiently common, yet no attempt was made to form a telescope, by a combination of them, till the end of the sixteenth century. Descartes considers James Metius, a person who was no mathematician, though his father and brother had applied to those sciences, as the first constructor of a telescope; and says that, as he was amusing himself with making mirrors and burning-glasses, he casually thought of looking through two of his lenses at a time, and that happening to take one that was convex and another that was concave, and happening also to hit upon a pretty good adjustment of them, he found that, by looking through them, distant objects appeared very large and distinct. In fact, without knowing it, he had made a telescope.

351. Others say that this great discovery was first made by John Lippersheim, a maker of spectacles at Middleburg, or rather by his children; who, like Metius, were diverting themselves with looking through two glasses at a time, and placing them at different distances from one another. But Borellus, the author of a book entitled De vero Telescopii Inventore, gives this honor to Zacharias Joannides or Jansen, another maker of spectacles at the same place, who made the first telescope in 1590; and it seems now to be the general opinion that this account of Borellus is the most probable. Indeed his account of the discovery of telescopes is so circumstantial, and so well authenticated, that it does not seem possible to call it in question. It is not true, he says, that this great discovery was made by a person who was no philosopher; for Zacharias Jansen was a diligent enquirer into nature; and, being engaged in these pursuits, he was trying what uses could be made of lenses for those purposes, when he fortunately hit upon the construction.

352. This ingenious mechanic and philosopher

353. Jansen, having a philosophical turn, applied his instrument to such purposes as he had in view when he hit upon the construction. Directing it towards celestial objects, he distinctly viewed the spots on the surface of the moon, and discovered many new stars, particularly seven pretty considerable ones in the Great Bear. His son, John Zacharias, noted the lucid circle near the limb of the moon, whence several bright rays seem to dart in different directions; and he says that the full moon, viewed through this instrument, did not appear flat, but was evidently spherical, the middle part being prominent. Jupiter also, he says, appeared round, and rather spherical; and sometimes he perceived two, sometimes three or four small stars, a little above or below him; and, as far as he could observe, they performed revolutions round him. This was probably the first observation of the satellites of Jupiter.

354. It may be proper to add, that Francis Fontana, an Italian, also claims the invention; but he did not pretend to have made it before 1608, and it is well known that the instruments were made and sold in Holland some time before.

355. Some say that Galileo was the inventor of telescopes; but he himself acknowledges that he first heard of the instrument from a German; but he says that being informed of nothing more than the effects of it, first by common report and a few days after by a French nobleman, J. Badovere, at Paris, he himself discovered the construction, by considering the nature of refraction and thus he had much more real merit than the inventor himself. The account of what Galileo actually did in this business is circumstantially related by the author of his life, prefixed to the 4to. edition of his works, printed at Venice in 1744. About April or May, 1609, it was reported at Venice, where Galileo (who was professor of mathematics in the university of Padua) then happened to be, that a Dutchman had presented to prince Maurice of Nassau a certain optical instrument by means of which distant objects appeared as if they were near; but no farther account of the discovery had reached that place, though this was nearly twenty years after the first discovery. Struck, however, with this

356. Galileo, directing his tube towards the moon, found that the surface of it was diversified with hills and valleys, like the earth. He also discovered that the via lactea and nebulæ consisted of a collection of fixed stars, which, on account either of their vast distance or extreme smallness, were invisible to the naked eye. He likewise observed innumerable fixed stars dispersed over the face of the heavens, which had been unknown to all the ancients: and, examining Jupiter with a better instrument than any he had made before, he found that he was accompanied by four stars, which performed periodical revolutions round him, and which, in honor of the Medici, he called Medicean planets. This discovery he made in January 1610, N. S., and in March he published an account of all his discoveries, in his Nuncius Sidereus, printed at Venice, and dedicated to Cosmo, great duke of Tuscany, who, by a letter dated 10th of July 1610, invited him to quit Padua, and assigned him an ample stipend, as primate and extraordinary professor at Pisa, but without any obligation to read lectures, or to reside. The extraordinary discoveries contained in the Nuncius Sidereus, which was quickly reprinted in Germany and France, were the cause of much debate among the astronomers; many of whom could not credit Galileo's account, while others ridiculed his discoveries as fictions or illusions. Some could not be prevailed upon even to look through a telescope; so devoted were they to the system of Aristotle. But, when it was found vain to oppose the evidence of sense, some affirmed that the invention was taken from Aristotle; and quoting his writings, in which he mentions stars seen in the day-time from the bottom of a deep well, said, that the well corresponded to the tube of the telescopes, and that the vapors which arose from it gave the hint of putting glasses into it; and that in both cases the sight is strengthened by the transmission of the rays through a thick and dark medium. Galileo himself, who tells this story, humorously compared such men to alchymists, who pretend that the art of making gold was known to the ancients, but lay concealed under the fables of the poets.