THE BRITISH CYCLOPÆDIA. DIVISION I.-ARTS AND SCIENCES. quantity: indeed it is too small to be determined, except by very accurate instruments, and therefore we hear little more of it till about the year 1500, at which time great attention was paid to it by Bernard Walther, Mæstlin, and others, but chiefly by Tycho Brahe. OPTICS; the science which treats of the nature of with the ancient opinion of crystalline orbs in the light and the phenomena of vision. Under this head regions above the atmosphere, he even suspected a we purpose in the first instance giving a history of refraction there also, and fancied he could prove it the progress of optical knowledge, and on account by astronomical observations. This author deduces of the great importance of the subject it will be from hence several properties of atmospherical readvisable to follow somewhat in detail the labours fraction, as that it increases the altitude of all the of each successive experimentalist. The application objects in the heavens; and he states that the stars of the doctrines thus laid down will then be readily are sometimes seen above the horizon by means of understood. The first treatise of any importance refraction, when in reality they are below it. This written on this subject was by the celebrated astro- observation was confirmed by Vitellio, B. Walther, nomer Claudius Ptolemæus, who lived about the and especially by the excellent observations of Tycho middle of the second century. The treatise is lost; Brahe. Alhazen observed that refraction must conbut, from the accounts of others, we find that he tract the vertical diameters and distances of the treated of astronomical refraction. Though refrac-heavenly bodies. But we do not find that either he tion in general had not been minutely observed or his follower, Vitellio, knew any thing of its just very early, it might have occurred to any philosopher much before his time that the light of the sun, moon, and stars, must undergo a degree of refraction in consequence of falling obliquely upon the atmosphere that surrounds the earth, and that they must, by that means, be turned out of their Alhazen supposed that the refraction of the atmorectilinear course, so as to cause those luminaries to sphere did not depend upon the vapours in it, as appear higher in the heavens than they would other-was the opinion of philosophers before his time, but wise do. The first astronomers were not aware that on the different degrees of transparency; by which, the intervals between the stars appear less near the as Montucla conjectures, he meant the density of the horizon than near the meridian; and, on this account, air contiguous to the earth, and the lighter air that they must have been much embarrassed in their ob- lies beyond it. In examining the effects of refraction, servations. But it is evident that Ptolemy was aware he endeavours to prove that, so far from its being of this circumstance by the caution that he gives to the cause of the heavenly bodies appearing larger allow something for it upon every reference being near the horizon, it would make them appear less, made to ancient observations. "two stars," he says, "appearing nearer together in the horizon than near the meridian." This phenomenon he ranks among optical deceptions. "We judge of distance," he says, "by comparing the angle under which objects appear with their supposed distance; so that if these angles be nearly equal, and the distance of one object be conceived greater than that of the other, it will be imagined to be larger." He also observes that the sky near the horizon, is always imagined to be further from us than any other part of the concave surface. This philosopher also advances a very sensible hypothesis to account for the remarkably greater apparent size of the sun and moon when seen near the horizon. The mind, he says, judges of the size of objects by means of a preconceived idea of their distance from us: and this distance is fancied to be greater when a number of objects are interposed between the eye and the body we are viewing, which is the case when we see the heavenly bodies near the horizon. In his Almagest, however, he ascribes this appearance to the refraction of the rays by vapours, which actually enlarge the angle under which the luminaries appear, just as the angle is enlarged by which an object is seen from under water. In the writings of Roger Bacon, whose genius perhaps almost equalled that of his great namesake Lord Verulam, we find the first distinct account of the magnifying power of glasses; and it is not improbaIn the twelfth century, the nature of refraction was ble that what he wrote upon this subject gave rise to largely treated of by Alhazen, an Arabian writer; the invention of spectacles. He says that if an obindeed, having made experiments upon it at the com-ject be applied close to the base of the larger segment mon surface between air and water, air and glass, of a sphere of glass it will appear magnified. He water and glass, or crystal, and being prepossessed also treats of the appearance of an object through a ARTS & SCIENCES.-VOL. II. A globe, and says that he was the first who observed | vision, Joannes Baptista Porta, of Naples, discothe refraction of the rays of light in passing through it. vered the camera obscura, which throws still more In 1270, Vitellio, a native of Poland, published a light on the same subject. His house was constantly treatise on optics, containing all that was valuable in resorted to by all the ingenious persons at Naples, Alhazen, and arranged in a much more intelligible whom he formed into what he called an academy of and methodical manner. He observes that light is secrets, each member being obliged to contribute always lost by refraction, in consequence of which something that was not generally known and might the objects seen by refracted light always appear less be useful. By this means he was furnished with Juminous; but he does not estimate the quantity of materials for his Magia Naturalis, which contains this loss. He reduced into a table the result of his his account of the camera obscura, and the first ediexperiments on the refractive powers of air, water, tion of which was published, as he informs us, when and glass, corresponding to different angles of inci- he was not quite fifteen years old. He also gave the dence. In his account of the horizontal moon he first hint for the construction of the magic lantern, agrees exactly with Alhazen, observing that in the which Kircher afterwards followed and improved. horizon she seems to touch the earth and appears His experiments with the camera obscura convinced much more distant from us than in the zenith, on him that vision must be performed by the passage account of the intermediate space containing a greater of light into the eye, and not by visual rays proceedvariety of objects upon the visible surface of the earth. ing from the eye, as had been formerly imagined; He ascribes the twinkling of the stars to the motion and he was the first who fully satisfied himself and of the air in which the light is refracted; and, to others upon this subject. Indeed the resemblance illustrate this hypothesis, he observes that they between experiments with the camera obscura and twinkle still more when viewed in water put in the manner in which vision is performed in the eye motion. He also shows that refraction is necessary, was too striking to escape the observation of a less as well as reflection, to form the rainbow, because ingenious person. But when he says that the eye the body which the rays fall upon is a transparent is a cameru obscura, and the pupil the hole in the substance, at the surface of which one part of the window-shutter, he was so far mistaken as to sup light is always reflected and another refracted. This pose that it was the crystalline humour which cor. writer makes some ingenious attempts to explain responded to the wall which received the images refraction, or to ascertain the law of it. He also nor was it discovered till the year 1604 that this considers the foci of glass spheres, and the apparent office was performed by the retina. size of objects seen through them; though he is not very accurate in his remarks. We have already adverted to Roger Bacon, a man of very extensive genius, and who wrote upon almost every branch of science. He does not hesitate to assent to an opinion adopted by many of the ancients, and indeed by most philosophers till his times, that visual rays proceed from the eye; giving this reason for it, that every thing in nature is qualified to discharge its proper functions by its own powers, in the same manner as the sun and other celestial bodies. In his Specula Mathematica, he added some observations on the refraction of the light of the stars, the apparent size of objects, the extraordinary size of the sun and moon in the horizon: but in all this he is not very exact, and advances little that is new. In his Opus Majus he demonstrates that if a transparent body, interposed between the eye and an object, be convex towards the eye, the object will appear magnified. This observation, however, he certainly had from Alhazen; the only difference between them is that Bacon prefers the smaller segment of a sphere and Alhazen the larger. The great problem concerning the measuring of refraction, however, remained still unsolved. Alhazen and Vitellio, indeed, had attempted it; but failed, by attempting to measure the angle itself instead of its sine. At last it was discovered by Snellius, professor of mathematics at Leyden; but this philosopher did not perfectly understand his own discovery, nor did he live to publish any account of it himself. It was afterwards explained by professor Hortensius, both publicly and privately, before it appeared in the writings of Descartes, who published it under a different form, without making any acknowledgment of his obligations to Snellius, whose papers Huygens assures us Descartes had seen. Before this time Kepler had published a new table of refracted angles, determined by his own experiments for every degree of incidence. Kircher had done the same, and attempted a rational or physical theory of refraction, on a principle, and by a mode of investigation, which, if conducted with precision, would have led him to the law assumed or discovered by Snellius. Descartes undertook to explain the cause of refracFrom this time to that of the revival of learning tion by the resolution of forces, on the principles of in Europe we have no treatise on the subject of re-mechanics. In consequence of this, he was obliged fraction, or indeed on any other part of optics. One to suppose that light passed with more ease through of the first who distinguished himself in this way a dense medium than through a rare one. The truth was Maurolycus, teacher of mathematics at Messina. In a treatise De Lumine et Umbra, published in 1575, he demonstrates that the crystalline humour of the eye is a lens that collects the rays of light issuing from the surrounding objects, and throws them upon the retina, where is the focus of each pencil. From this principle he inferred the reason why some people were shortsighted and others long-sighted, and why the former are relieved by concave and the others by convex glasses. About the same time that Maurolycus made such advances towards the discovery of the nature of of this explanation was first questioned by M. Fermat, counsellor to the parliament of Thoulouse, and an able mathematician. He asserted, contrary to the opinion of Descartes, that light suffers more resistance in water than air, and more in glass than in water; and he maintained that the resistance of different mediums with respect to light is in proportion to their densities. M. Leibnitz adopted the same general idea; and these gentlemen argued upon the subject in the following manner : Nature, say they, accomplishes her ends by the shortest methods. Light, therefore, ought to pass ; from one point to another either by the shortest | density, at least not to the gravity of the refracting road, or that in which the least time is required. But medium. For, he observes, the refractive power it is plain that the line in which light passes, when of glass to that of water is as fifty-five to thirty-four, it falls obliquely upon a denser medium, is not the whereas its gravity is as eighty-seven to thirty-four most direct or the shortest; so that it must be that that is, the squares of their refractive power are very in which the least time is spent. And whereas nearly as their respective gravities. And there are it is demonstrable that the light falling obliquely some fluids which, though they are lighter than upon a denser medium (in order to take up the least water, yet have a greater power of refraction. Thus time possible in passing from a point in one medium the refractive power of spirit of wine, according to Dr. to a point in the other) must be refracted in such a Hooke's experiment, is to that of water as thirty-six manner that the sines of the angles of incidence and to thirty-three, and its gravity reciprocally as thirtyof refraction must be to one another as the different three to thirty-six or thirty-six and a half. But the facilities with which light is transmitted in those me- refractive powers of air and water seem to observe diams, it follows that (since light approaches the the simple proportion of their gravities directly. And, perpendicular when it passes obliquely from air into if this should be confirmed by succeeding experiwater, so that the sine of the angle of refraction is ments, it is probable, he says, that the refractive ess than that of the angle of incidence) the facility powers of the atmosphere are every where, and at all with which water suffers light to pass through it heights above the earth, proportioned to its density s less than that of the air; so that light meets with and expansion; and then "it would be no difficult more resistance in water than air. matter to trace the light through it, so as to termi proper expedients for treasuring the quantity of light illuminating an opaque body, to examine at what distances the moon must be from the earth to suffer eclipses of the observed durations." Cassini happened to be present when Mr. Lowthorp made the above-mentioned experiment before the Royal Society; and, upon his return home, having made a report of it to the members of the Royal Academy of Sciences, those gentlemen endeavoured to repeat the experiment in 1700; but they did not succeed, for, as they said, beams of light passed through the vacuum without suffering any refraction. The Royal Society, being informed of this, were desirous that it might be put past dispute, by repeated and well-attested trials; and ordered Mr. Hauksbee to make an instrument for the purpose, under the direction of Dr. Halley. It consisted of a strong brass prism, two sides of which had sockets to receive two plane glasses, by which the air in the prism might either be exhausted or condensed. The prism had also a mercurial gauge fixed to it, to discover the density of the contained air, and was contrived to turn upon its axis, in order to make the refractions equal on each side when it was fixed to the end of a telescope. The refracting angle was near 64°; and the length of the telescope was about ten feet, having a fine micrometer wire hair in its focus. result of this experiment was as follows: Arguments of this kind could not prove very satis-nate the shadow of the earth, and, together with factory; and the fallacy of the hypothesis was soon shown. At a meeting of the Royal Society, Aug. 31, 1664, an experiment for measuring the refraction of common water was made with a new instrument which they had prepared for that purpose; and, the angle of incidence being forty degrees, that of refraction was found to be thirty. About this time also we find the first mention of mediums not refracting the light in an exact proportion to their densities. For Mr. Boyle, in a letter to Mr. Oldenburgh, dated Nov. 3, 1664, observes that in spirit of wine the proportion of the sines of the angles of incidence to the sines of the angles of refraction was nearly the same as four to three; and that, as spirit of wine occasions a greater refraction than common water, so oil of turpentine, which is lighter than spirit of wine, produces not only a greater refraction than common water, but a much greater than salt water. And at a meeting held Nov. 9, the same year, Dr. Hooke (who had been ordered to prosecute the experiment) read an account of one that he had made with pure and clear salad oil, which was found to have produced a much greater refraction than any fluid which he had then tried, the angle of refraction that answered to an angle of incidence of 30° being found no less than 40° 30′, and the angle of refraction that answered to an angle of incidence of 20° being 32° 47. M. De la Hire also made several experiments to ascertain the refractive power of oil with respect to that of water and air, and found the sine of the angle of incidence to that of refraction to be as sixty to forty-two; which, he observes, is a little nearer to that of glass than to that of water, though oil is much lighter than water, and glass much heavier. The members of the Royal Society, finding that the refraction of salt water exceeded that of fresh, pursued the experiment further with solutions of vitriol, saltpetre, and alum, in water, when they found the refraction of the solution of vitriol and saltpetre a little more, but that of alum a little less, than common water. By a most elaborate experiment made in the year 1698, in which a ray of light was transmitted through a Torricellian vacuum, Mr. Lowthorp states that he found the refractive power of air to be to that of water as thirty-six to 34.400. He concludes his account of the experiment by observing that the refractive power of bodies is not proportioned to the The Having chosen a very distinct object, whose distance was 2588 feet, the barometer being then at 29.7, and the thermometer at 60°, they first exhausted the prism, and then applying it to the telescope, the horizontal hair in the focus covered a mark on the object distinctly seen through the vacuum, the two glasses being equally inclined to the visual ray. Then admitting the air into the prism the object was seen to rise above the mark gradually as the air entered, and in the end the hair was observed to cover a mark ten inches and a quarter below the former mark. This they often repeated, and with the same success. After this they applied the condensing engine to the prism; and having forced in another atmosphere, so that the density of the included air was double to that of the outward, they again placed it before the telescope, and, letting out the air, the object which before seemed to rise appeared gradually to descend, and the hair at length rested on an object higher than before by the same interval of ten inches and a quarter. This experiment they likewise frequently repeated | without any variation in the event. They then forced in another atmosphere; and, upon discharging the condensed air, the object was seen nearly twenty-one inches lower than before. It appears, by these experiments, that the refrac- | tive power of the air is proportionable to its density. And, since the density of the atmosphere is as its weight directly and its heat inversely, the ratio of its density, at any given time, may be ascertained by comparing the heights of the barometer and thermometer; and thence he concludes that this will also be the ratio of the refraction of the air. But Dr. Smith observes that, before we can depend upon the accuracy of this conclusion, we ought to examine whether heat and cold alone may not alter the refractive power of air, while its density continues the same. The French academicians, being informed of the result of the above-mentioned experiment, employed M. Delisle the younger to repeat their former experiment with more care; and he presently found that their operators had never produced any rarefaction, as the apparatus had not been air-tight. He therefore annexed a gauge to his instrument, by which means he was sure of his vacuum; and then the result of the experiment was the same with that in England. The refraction was always in proportion to the density of the air, excepting when the mercury was very low and consequently the air very rare, in which case, the whole quantity being very small, he could not perceive much difference in them. Comparing, however, the refractive power of the air with the result of his experiment, he found that the best vacuum he could make was far short of that of the higher regions of the atmosphere. He then suspected that these colours might arise from the light being dilated by some unevenness in the glass, or some other accidental irregularity; and, to try this, he took another prism, like the former. and placed it in such a manner as that the light, passing through them both, might be refracted contrariwise, and so be returned by the latter into the same course from which it had been diverted by the former. In this manner he thought that the regular effects of the first prism would be destroyed by the second, but that the irregular ones would be augmented by the multiplicity of refractions. The event was, that the light which by the first prism was diffused into an oblong form was by the second reduced into a circular one, with as much regularity as if it had not passed through either of them. At last, after various experiments and conjectures, he hit upon what he calls the experimentum crucis, which completed this great discovery. He took two boards, and placed one of them close behind the prism at the windows, so that the light might pass through a small hole made in it for the purpose, and fall on the other board, which he placed at the distance of about twelve feet, having first made a small hole in it also, for some of the incident light to pass through. He then placed another prism behind the second board, so that the light which was transmitted through both the boards might pass through that also, and be again refracted before it arrived at the wall. This being done, he took the first prism in his hand, and turned it about its axis so as to make the several parts of the image cast on the second board successively to pass through the hole in it, that he might observe to what places on the wall the second prism would refract them; and he saw, by the change of those places, that the light tending to that end of the image towards which the refraction of the first prism was made did in the second prism suffer a refraction considerably greater than the light which tended to the other end. The true cause, therefore, of the length of the image was discovered to be no other than that a ray of light is not similar or homogeneal, but that it consists of a series of rays, some of which are more refrangible than others: so that, without any difference in their incidence on the same medium, some of them shall be more refracted than others; and therefore that, according to their particular degrees of refrangibility, they will be transmitted through the prism to differ About this time Grimaldi first observed that the coloured image of the sun refracted through a prism must be always oblong, and that colours proceeded from refraction. The way in which he first discovered this was by an experiment in which a piece of white paper, placed at the bottom of a glass vessel filled with water, and exposed to the light of the sun, appears coloured. However, he observed that in case the two surfaces of the refracting medium were exactly parallel to each other no colours were produced. But of the true cause of those colours, viz. the different refrangibility of the rays of light, he had not the least suspicion. This discovery was reserved for Sir Isaac Newton, and it occurred to him in the year 1566. At that time he was en-ent parts of the opposite wall. gaged in grinding optical glasses, and procured a triangular glass prism to satisfy himself concerning the phenomena of colours. While he amused himself with this, the oblong figure of the coloured spectrum first struck him. He was surprised at the great disproportion betwixt its length and breadth, the former being about five times the measure of the latter. He could hardly think that any difference in the thickness of the glass, or in the composition of it, could have such an influence on the light. However, without concluding any thing à priori, he proceeded to examine the effects of these circumstances, and particularly tried what would be the consequence of transmitting the light through parts of the glass that were of different thicknesses, or through holes in the window-shutter of different sizes; or by placing the prism on the outside of the shutter that the light might pass through it, and be refracted before it was terminated by the hole. Since it appears from these experiments that different rays of light have different degrees of refrangibility, it necessarily follows that the rules laid down by preceding philosophers concerning the refractive power of water, glass, &c., must be limited to the middle kind of rays. Sir Isaac, however, proved that the sine of the incidence of every kind of light, considered apart, is to its sine of refraction in a given ratio. This he deduced both by experiment and also geometrically, from the supposition that bodies refract the light by acting upon its rays in lines perpendicular to their surfaces. The most important discovery with regard to refraction since the time of Sir Isaac Newton is that of Mr. Dollond, who found out a method of correcting the faults of refracting telescopes arising from the different refrangibility of the rays, and which had been generally thought to be irremediable. Notwithstanding the great discovery of Sir Isaac New |