cine at Heidelberg. He edited some of the works of Hippocrates, with notes; but his best known work is an edition of the Sibylline oracles. He died 1596, aged 40.

OPTABLE. a. (optabilis, Lat.) Desirable; to be wished.

OPTATIVE. a. (optativus, Latin.) Expressive of desire (Clarke).

OPTATIVE, in grammar, the third mood in the conjugations of verbs, serving to express an ardent desire or wish for any thing.

Instead of a particular mood, or a particular set of inflexions to express this desire, the English, Latins, &c. express it by an adverb of wishing prefixed to it. The Latins by utinam; the French by plût à Dieu; and the English by would to God, &c.

In these languages, setting aside the adverb, the optative is the same with the subjunctive; the inflexions of the verb, which make what we call the moods, being the same in both. Indeed, in the Greek, the wish is expressed by a particular inflexion, thence called optative; and in the French, Spanish, and Italian, there is something like it; their triple senses serving the same purpose. But the optative mood may be safely retrenched from the Latin and English.

OPTIC. a. (no.) 1. Visual; producing vision; subservient to vision (Newton). 2. Relating to the science of vision (Wotton).

OPTIC. S. An instrument of sight; an organ of sight (Brown).

OPTIC ANGLE. The angle which the optic axes of both eyes make with one another, as they tend to meet at some distance before the eyes.

OPTIC AXES. The axes of the eve, or a line going through the middle of the pupil and the centre of the eye.

OPTIC CHAMBER. See CAMER AOBSCURA. OPTIC NERVES. (nervi optici, from loμa, to see; because they are the organs of sight.) The second pair of nerves of the brain, arise from the thalami nervorum opticorum, perforate the bulb of the eye, and in it form the retina.

OPTIC PENCIL. See PENCIL. O'PTICAL. a. (Tinos.) Relating to the science of optics (Boyle).

OPTICIAN s. (from optic.) One skilled in optics.

OPTICS, (from enroual, to see,) is that science which considers the nature, the composition, and the motion of light ;-the changes which it suffers from the action of bodies;-the phenomena of vision, and the instruments in which light is the chief agent.

HISTORY.

Sect. I. Discoveries concerning the Refraction of Light. Though the ancients made few optical experiments, they nevertheless knew, that when light passed through media of different densities, it did not move in a straight line, but was bent or refracted out of its original direction. This was probably suggested to them by the appearance of a straight rod partly immersed in water; and accordingly we find many questions concerning this

and other optical appearances in the works of Aristotle. Archimedes is said to have written & treatise on the appearance of a ring or circle under water, and therefore could not have been ignorant of the common phenomena of refraction. The ancients, however, were not only acquainted with these more ordinary appearances, but also with the production of colours by refraction. Seneca says, that if the light of the sun shines through an angular piece of glass, it will show all the colours of the rainbow. These colours, he says, are false, such as are seen in a pigeon's neck when it changes its position; and of the same nature, he says, is a speculum, which, without having any colour of its own, assumes that of any other body. It appears also, that the ancients were not ignorant of the magnifying power of glass globes filled with water, though they do not seem to have been acquaint ed with its cause; and the ancient engravers are supposed to have used a glass globe filled with evident, from their lenticular and spherical gems water to magnify their figures. This indeed seems of which, in magnifying at least, could scarcely of rock crystal which are still preserved, the effect have escaped the notice of those who had often occasion to handle them; if indeed, in the spherical or lenticular form, they were not solely intended for the purposes of burning. One of these, of the spherical kind, of about an inch and a half diameter, is preserved among the fossils presented by Dr. Woodward to the university of Cambridge.

The first treatise of any consequence written on the subject of optics was by the celebrated Ptolemy. The treatise is now lost; but from the sccounts of others, we find that he treated of astronomical refractions. The first astronomers were not aware that the intervals between stars appear less near the horizon than near the meridian; but it is evident that Ptolemy was aware of this circumstance, by the caution which he gives to allow something for it, upon every recourse to ancient observations.

Ptolemy also advances a very sensible hypothesis to account for the 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 be greater when a number of objects intervene;

distance from us: and this distance is fancied to

which is the case when we see the heavenly bodies near the horizon. In his Almagest, however, he ascribes this appearance to a refraction of the rays by vapours, which actually enlarge the angle subtended by the luminaries.

The nature of refraction was afterwards considered by Alhazen an Arabian writer; insomuch that, having made experiments upon it at the com mon surface between air and water, air and glass, water and glass; and, being prepossessed with the ancient opinion of crystalline orbs in the regions above the atmosphere, he even suspected a refrac tion there also, and fancied he could prove it by astronomical observations. Hence this author concludes, that refraction increases the altitudes of all objects in the heavens; and he first advanced, that the stars are sometimes seen above the horizon by means of refraction, when they are really below it.

This observation was confirmed by Vitellio, B. Waltherus, and by the excellent obser vations of Tycho Brahe. Alhazen observed, that refraction contracts the vertical diameters and distances of the heavenly bodies, and that it is the cause of the twinkling of the stars. But we de

Lot find that either he, or his follower Vitellio, ubjected it to mensuration. Indeed it is too mall to be determined except by very accurate nstruments, and therefore we hear little more of till about the year 1500; when great attention as paid to the subject by Bernard Walther, læstlin, and Tycho Brahe.

Alhazen supposed that the refraction of the atosphere did not depend upon the vapours, but 1 the different transparency; by which, as Moncla conjectures, he meant the density of the oss air contiguous to the earth, and the ether subtile air that lies beyond it. We judge of stance, he says, by comparing the angle under hich objects appear, with their supposed distance; that if these angles be nearly equal, and the stance of one object be conceived greater than at of the other, it will be imagined to be larger. e also observes, that the sky near the horizon is ways imagined to be further from us than any ber part of the concave surface. Roger Bacon cribes this account of the horizontal moon to olemy; and as such it is examined, and ob ted to by B. Porta,

In the writings of Roger Bacon, we find the first stinct account of the magnifying power of isses; and it is not improbable, that what he ote upon this subject gave rise to the useful inntion of spectacles. He says, that if an object applied close to the base of the larger seg ent of a sphere of glass, it will appear magfied. He also treats of the appearance of an ject through a globe, and says that he was the st who observed the refraction of rays into it. Vitellio, a native of Poland, published a treatise optics, about 1270, containing all that was luable in Alhazen. He observes, that light is ways lost by refraction; but he does not pretend estimate the quantity of this loss. He reduced to a table the result of his experiments on the fractive powers of air, water, and glass, correonding to different angles of incidence. In his count of the horizontal moon he agrees exactly ith Alhazen. He ascribes the twinkling of the ars to the motion of the air in which the light is fracted; and to illustrate this hypothesis, he obrves that they twinkle still more when viewed in ater put in motion. He also shows, that refracon is necessary as well as reflection, to form the inbow; because the body which the rays fall pon is a transparent substance, at the surface f which one part of the light is always reflected nd another refracted. But he seems to consider efraction as serving only to condense the light, hereby enabling it to make a stronger impression pon the eye. This writer also makes many atempts to ascertain the law of refraction. He likevise considers the foci of glass spheres, and the pparent size of objects seen through them: though pon these subjects his observations are inaccuate. It is sufficient indeed to show the state of snowledge, at that time, to observe, that both Vitellip, and his master Albazen, account for bjects appearing larger when seen under water, of the circular figure of its surface; since, being Quid, it conforms to the figure of the earth. Contemporary with Vitellio was Roger Bacon, man of extensive genius, who wrote upon al most every branch of science; yet in optics he does not seem to have made any considerable advances. Even some of the most absurd of the opinions of the ancients have had the sanction of bis authority. He believed that visual rays proVOL. VIII.

ceed from the eye; because 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 Mathe matica, he added some observations of little im portance on the refraction of the light of the stars; the apparent size of objects; the enlarge ments of the sun and moon in the horizon. In his Opus Majus he demonstrates, what Alhazen had done before, that if a transparent body inter posed between the eye and an object be convex towards the eye, the object will appear magnified,

From this time, to that of the revival of learning in Europe, we have no treatise on optics, One of the first who distinguished himself in this way was Maurolycus, teacher of mathematics at Messina, about 1575. In two works, entitled Theoremata Lucis et Umbræ, and Diaphanorum Partes, &c. he demonstrates that the crystalline humour of the eye is a lens that collects the rays of light issuing from the object, and throws them upon the retina, where is the focus of each pencil. From this principle he discovered the reason why some people were short-sighted and others long-sighted; and why the former are relieved by concave, and the others by convex glasses.

While Maurolycus made such advances towards. the discovery of the nature of vision, Baptista Porta of Naples, born 1445,died1515, invented the camera obscura, which throws still more light on the same subject. His house was resorted to by all the ingenious persons at Naples, whom he formed into an academy of secrets; each member being obliged to contribute something useful and not generally known. By this means he was furnished with materials for his Magia Naturalis, which contains his account of the camera obscura, and which was published, as he informs us, when he was not quite fifteen years old. He also gave the first hint of the magic lantern; which Kircher afterwards improved. His experiments with the camera obscura convinced him, that vision, as Aristotle supposed, is performed by the intromission of something into the eye, and not by visual rays proceeding from the eye, as had been formerly imagined by Empedocles; and he was the first who fully satisfied himself and others upon this subject. The resemblance indeed between experiments with the camera obscura and the manner in which vision is performed in the eye, was too striking to escape the observation of a less ingenious person. But when he says that the eye is a camera obscura, and the pupil the hole in the window shutter, he was so far mistaken as to suppose that it was the crystalline humour that corresponds to the wall which receives the images; nor was it discovered till the year 1604, that this office is performed by the retina. He makes a variety of just observations on vision; and explains several cases in which we imagine things to be without the eye, when the appearances are occasioned by some affection of the organ itself, or some motion within it. He remarks also, that, in certain circumstances, vision will be assisted by convex or concave glasses; and he seems also to have made some small advances towards the discovery of telescopes. He observes, that a round and flat surface plunged into water, will appear hollow as well as magnified to an eye above it; and be explains by a figure the manner in which this effect is produced."

The great problem concerning the measure of PP

refractions was still unsolved. Alhazen and Vitellio, indeed, had attempted it; but failed, by trying to measure the angle instead of its sine. At last it was discovered by Snellius, professor of mathematics at Leyden, about 1637. This philosopher, however, did not perfectly understand his own discovery, nor did he live to publish any account of it. It was afterwards explained by professor Hortensius 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, were seen by Descartes. Before this time Kepler had published a New Table of Angles of Refraction, determined by his own experiments, for every degree of incidence. Kircher had done the same, and attempted a theory of refraction, ou principles, which, if conducted with precision, would have led him to the law discovered by Snellius.

Descartes undertook to explain the cause of refraction by the resolution of forces. Hence he was obliged to suppose that light passes with more ease through a dense medium, than through a rare one. The truth of this explanation was first questioned by M. Fermat, who asserted, contrary to the opinion of Descartes, that light suffers more resistance in water than air, and more in glass than in water; and maintained, that the resistance of different media with respect to light is in proportion to their densities. M. Leibnitz adopted the same general idea, upon the principle that nature accomplishes her ends by the shortest methods, and that light therefore ought to pass from one point to another, either by the shortest road, or that in which the least time is required.

At a meeting of the Royal Society, August 31, 1664, it was found, with a new instrument prepared for that purpose, that the angle of incidence being 40 degrees, that of refraction is 30. About this time also we find the first mention of media 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 4 to 3; 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 November 9, the same year, Dr. Hooke mentioned, that pure and clear salad oil produced a much greater refraction than any liquor which he had tried; the angle of refraction that answered to an angle of incidence of 30° being no less than 40° 30', and the angle of refraction that answered to an angle of incidence of 20° being 29° 47'.-M. de la Hire also made several experiments to ascertain the refractive power of oil, and found the sine of the angle of incidence to that of refraction as 60 to 42; 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 farther with aqueous solutions of vitriol, saltpetre, and alum. They found the refraction of the solution of vitriol and saltpetre a little more, but that of alum a little less, than common water.

Dr. Hooke made an experiment before the

Royal Society, February 11, 1663, which clearly proves that ice refracts the light less than water. M. de la Hire also took a good deal of pains to determine whether the refractive powers of jen and water were the same; and be found as Dr. Hooke had done before, that ice refracts les tart water.

By a most accurate experiment made in 1988, in which a ray of light was transmitted thread a Torticellian vacuum, Mr. Lowthorp fouad, ts the refractive power of air is to that of water a 36 to 34.400. He observes that the refracta power of bodies is not proportioned to the derct, at least not to the specific gravity, of the refrac ing medium. For the refractive power of gast that of water is as 55 to 34, whereas its speci gravity is as 87 to 34; that is, the squares of their refractive powers are very nearly as the respective gravities. And there are some Ar which, though they are lighter than water, s have a greater power of refraction. Thus the the fractive power of spirit of wine, according to Dr. Hooke's experiment, is to that of water as 36 to 24 and its gravity reciprocally as 33 to 36 or But the refractive powers of air and water set to observe the simple direct proportion of ther gravities.

The Royal Academy of Sciences at Paris deavoured to repeat this experiment in 1700; = they did not succeed.-For, as they said, bras of light passed through the vacuum without fering any refraction. The Royal Society being informed of this, ordered Mr. Hawksbee to cal an instrument for the purpose, under the dre tion of Dr. Halley, for the purposes of repeat, the experiment. It consisted of a strong br prism, two sides of which had sockets to recent two plane glasses, whereby the air in the prim might either be exhausted or condensed. Th prism had also a mercurial gaze fixed to it, t discover the density of the contained air; turned upon its axis, in order to make the fractions equal on each side when it was fired a the end of a telescope. The refracting angle v near 64°; and the length of the telescope, har a fine hair in its focus, was about 10 feet. The event of this accurate experiment was as fa Having chosen a proper object, whose discer was 2588 feet, June 15, O. S. 1708, in the mi ing, the barometer being then at 29.73, and L thermometer at 60, they first exhausted the pr and then applying it to the telescope, the home hair in the focus covered a mark on the distinctly seen through the vacuum, the two gasu being equally inclined to the visual ray. Then mitting the air into the prism, the object was r to rise above the hair gradually as the air cate and when the prism was full, the hair was obs to hide a mark 10 inches below the former merk

After this they applied the condensing u to the prism; and having forced in another at sphere, so that the density of the included air double to that of the outward, they again ph it before the telescope, and, letting out te the object which before seemed to rise, a gradually to descend, and the hair at length re on an object higher than before by the same > terval of 104 inches. They then forced in art atmosphere; and upon discharging the coarned air, the object was seen near 21 inches lower than before.

Now the radius in this case being 2788 feet, 10 inches will subtend an angle of 18,

the angle of incidence of the visual ray being 32 degrees (because the angle of the glass planes was 64°), it follows from the known laws of refraction, that as the sine of 39° is to that of 31° 59′ 26′′, differing from 32° by 34" the half of 1'8"; so is the sine of any other angle of incidence, to the sine of its angle of refraction; and so is radius, or 1000000, to 999736; which, therefore, is the proportion between the sine of incidence in vacuo and the sine of refraction from thence into common air, It appears, by these experiments, that the refractive power of the air is proportional to its density, And since the density of the atmosphere is as its weight directly, and its temperature inversely, the ratio of its density, at any given ime, may be had 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. De l'Isle the younger to repeat the former experiment with more care. He presently found, that their operators had never made any vacuum at all, there being chinks in their instrument, through which the air had insinuated itself, He therefore annexed a gage to his instrument, by which means he was sure of his vacuum; and then the result of the experiment was the same with that of the Royal Society. The refraction was always proportional 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 atmosphere, observed at Paris, with the result of his experiment, he found, that the best vacuum he could make was far short of that of the regions above the atmosphere.

Dr. Hooke first suggested the idea of making allowance for the effect of the refraction of light, in passing from the rarer to the denser regions of the atmosphere, in the computed height of mountains. To this he ascribes the different opinions of authors concerning the height of several very high hills. He could not account for the appearance of very high mountains, at so great a distance as that at which they are actually seen, but upon the supposition of the curvature of the visual ray, that is made by its passing obliquely through a medium of such different density, from the top of them to the eye, very far distant in the horizon. All calculations of the height of mountains that are made upon the supposition that the rays of light come from the tops of them, to our eyes, in straight lines, he considers very erroneous.

Dr. Hooke ascribes the twinking of the stars to the irregular and unequal refraction of the rays of light, which is also the reason why the limbs of the sun, moon, and planets, appear to wave or dance. That there is such an unequal distribution of the atmosphere, he says, will be evident by looking upon distant objects, over a piece of hot glass, which cannot be supposed to throw out any kind of exhalation from itseif, as well as through ascending steams of water.

About this time Grimaldi first observed that the coloured image of the sun refracted through

a prism is always oblong, and that colours pras ceeded from refraction.-The way in which he first discovered this was by Vitellio's experiment already mentioned, 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 refracted medium were exactly parallel to each other, no colours were produced. But of the true cause of those colours, he had not the least suspicion. This dis, covery was reserved for sir Isaac Newton, in 1666. Having procured a triangular glass prisin to satisfy himself concerning the phenomena of colours, he was surprised at the oblong figure of the coloured spectrum, and the great disproportion be twixt its length and breadth; the former being about five times the measure of the latter. After various conjectures respecting the cause of these appearances, he suspected that the colours might arise from the light being dilated by some uneven, ness in the glass, or some other accidental irregu larity; and to try this, he took another prism like the former, and placed it in such a manner, that the light, passing through them both, might be refracted in opposite directions, and thus be res turned 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 augmented by the multipli, city of refractions. The event was, that the light, diffused by the first prism 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. He then hit upon what he calls the experimentum crucis, and found that light is not similar, or homogeneous; but that it consists of rays, some of which are more refrangible than others: so that, without any dif ference 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 different parts of the opposite wall.

Since it appears from these experiments that different rays of light have different degrees of refrangibility, it follows, that the rules laid down by preceding philosophers concerning the refrac tive power of water, glass, &c. must be limited to the mean rays of the spectrum. Sir Isaac, however, proves, both geometrically and by experiment, that the sine of the incidence of every kind of light, considered apart, is to its sine of refrac tion in a given ratio.

The most important discovery concerning re, fraction since the time of sir Isaac Newton is that of Mr. Dollend, who found out a method of reme, dying the defects of refracting telescopes arising from the different refrangibility of light. Sir Isaac Newton imagined that the different rays were refracted in the same proportion by every medium, so that the refrangibility of the extreme rays might be determined if that of the mean ones were given. From this it followed, as Mr. Dollond observes, that equal and contrary refractions must not only destroy each other, but that the divergency of the colours from one refraction would likewise be corrected by the other, and that there could be no possibility of producing any such thing as refraction without colour. Hence it was natural to infer, that all object glasses of telescopes must be equally affected by the dif

ferent refrangibility of light, in proportion to their apertures, of whatever materials they may be formed.

For this reason, philosophers despaired of bringing refracting telescopes to perfection. They therefore applied themselves chiefly to the improvement of the reflecting telescope; till 1747, when M. Euler, improving upon a hint of sir Isaac Newton's, proposed to make object glasses of water and glass; hoping, that by their difference of refractive powers, the refractions would balance one another, and thereby prevent the dispersion of the rays that is occasioned by their difference of refrangibility. This memoir of M. Euler excited the attention of Mr. Dollond. He went over all M. Euler's calculations, substituting for his hypothetical laws of refraction those which had been ascertained by Newton; and found, that, it folJowed from Euler's own principles, that there could be no union of the foci of all kinds of colours, but in a lens infinitely large.

Euler did not mean to controvert the experiments of Newton: but asserted, that, if they were admitted in all their extent, it would be impossible to correct the difference of refrangibility occasioned by the transmission of the rays from one medium into another of different density; a correction which he thought was very possible, since he supposed it to be effected in the eye, which be considered as an achromatic instrument. To this reasoning Mr. Dollond made no reply, but by appealing to the experiments of Newton, and the circumspection with which it was known that he conducted all his inquiries.

This paper of Euler's was particularly noticed by M. Klingenstierna of Sweden, who found that, from Newton's own principles, the result of his 8th experiment could not answer his description of it. Newton found, that when light passes out of air through several media, and thence goes out again into air, whether the refracting surfaces be parallel or inclined to one another, this light, as often as by contrary refractions it is so corrected as to emerge in lines parallel to those in which it was incident, continues ever after to be white; but if the emergent rays be inclined to the incident, the whiteness of the emerging light will, by degrees, become tinged at its edges with colours. This he tried by refracting light with prisms of glass, placed within a prismatic vessel of water.

By theorems deduced from this experiment he infers, that the refractions of the rays of every sort made out of any medium into air, are known by having the refraction of the rays of any one sort; and also that the refraction out of one medium into another is found as often as we have the refractions out of them both into any third medium.

On the contrary, the Swedish philosopher observes, that in this experiment, the rays of light, after passing through the water and the glass, though they come out parallel to the incident rays, will be coloured; but that the smaller the glass prism is, the nearer will the result of it approach to Newton's description.

This paper of M. Klingenstierna being communicated to Dollond, made him entertain doubts concerning Newton's report, and induced him to have recourse to experiment.

He therefore cemented together two plates of glass at their edges, so as to form a prismatic vessel, when stopped at the ends; and the edge being turned downwards, he placed in it a glass

prism, with one of its edges upwards, and filled up the vacancy with clear water; so that the refraction of the prism was contrary to that of the water, in order that a ray of light, transmitted through both these refracting media, might be affected by the difference only between the two refractions. As he found the water to refract more or less than the glass prism, he diminished or increased the angle between the glass plates, till he found the two contrary refractions to be equal; which he discovered by viewing an object through this double prism. For when it appeared neither raised or depressed, he was satisfied that the refractions were equal, and that the emergent and incident rays were parallel.

But according to the prevailing opinion, the object should have appeared of its natural colour; for if the difference of refrangibility had been equal in the two equal refractions, they would have rectified each other. This experiment, therefore, fully proved the fallacy of the received opinion, by showing the divergency of the light by the glass prism to be almost double of that by the water; for the image of the object was as much infected with the prismatic colours, as if it had been seen through a glass wedge only, whose refracting angle was near 30 degrees.

Mr. Dollond was convinced that if the refract ing angle of the water vessel could have admitted of a sufficient increase, the divergeney of the coloured rays would have been greatly diminished, or entirely rectified; and that there would have been a very great refraction without colour; but the inconvenience of so large an augle as that of the prismatic vessel must have been, to bring the light to an equal divergency with that of the glass prism whose angle was about 60 degrees, made it necessary to try some experiments of the same kind with smaller angles.

He, therefore, got a wedge of plate glass, the angle of which was only nine degrees; and using it in the same circumstances, he increased the angle of the water wedge, in which it was placed, till the divergency of the light by the water was equal to that by the glass; that is, till the image of the object, though considerably refracted by the excess of the refraction of the water, appeared quite free from any colours proceeding from the different refrangibility of the light; and as near as he could then measure, the refraction by the water was about of that by the glass.

As these experiments proved, that different substances caused the light to diverge very dif ferently in proportion to their general refractive power, Mr. Dollond began to suspect that such a variety might possibly be found in different kinds of glass.

His next object, therefore, was to grind wedges of different kinds of glass, and apply them toge ther; so that the refractions might be made in contrary directions, in order to discover whether the refraction and the divergency of the colours would vanish together.

From these experiments, which were not made till 1757, he discovered a difference far beyond his hopes in the refractive qualities of different kinds of glass, with respect to the divergency of colours. The yellow or straw-coloured kind, commonly called Venice glass, and the English crown glass, proved to be nearly alike in that respect; though, in general, the crown glass seemed to make light diverge less than the other. The common English plate glass made the light