former they appeared so distinct that I could read readily, and thought I saw them as distinct as when I viewed them with my naked eye: in both cases, I viewed the same objects, through the same prism, at the same distance from me, and in the same situation. There was no difference but in the lights by which the objects were illuminated, and which in one case was simple, in the other compound; and therefore the distinct vision in the former case, and confused in the latter, could arise from nothing else than from that difference in the lights, which proves the whole proposition.

the prism, was by no means calculated to give definitive results, and therefore left the question in all its uncertainty.

In order to obtain precise indications, which were not capable of being misinterpreted when applied to practical purposes, Sir David Brewster formed a spectrum from a luminous disk, by means of a prism of a highly dispersive power, and with a large reflecting angle. He then examined this spectrum through a great variety of coloured media, both solid and fluid, and marked the size and shape of the image into which it was converted. The perfection of this image, or its narrowness in the direction of the length of the spectrum, became a precise and unbelonged to the light out of which it was formed.

"In these three experiments, it is further very remarkable that the colour of homogeneal light was never changed by the refraction. And as these co-equivocal test of the fitness for distinct vision which lours were not changed by refractions, so neither were they by reflections. For all white, grey, red, yellow, green, blue, or violet-coloured bodies, as paper, ashes, red lead, orpiment, indigo, bice, gold, silver, copper, grass, blue flowers, violets, bubbles of water tinged with various colours, peacocks' feathers, the tincture of lignum nephriticum, and such like, in red homogeneal light appeared totally red, in blue light totally blue, in green light totally green, and so of other colours. In the homogeneal light of any colour they all appeared totally of that same colour; with this only difference, that some of them reflected that light more strongly, others more faintly. I never yet found any body which by reflecting homogeneal light could sensibly change its colour.

"From all which it is manifest that, if the sun's light consisted of but one sort of rays, there would be but one colour in the world, nor would it be possible to produce any new colour by reflections and refractions; and, by consequence, that the variety of colours depends upon the composition of light."

By this method of observation he found that a distinct image of the luminous disk could not be obtained either by producing a blue or a green im ge, and that it was only in the red portion of the spectrum that such an effect was likely to be obtained. In the use of purple glasses, it was observed that the middle portion of the red space was absorbed before the two extreme portions, so that instead of one red image there were two quite separate, and tolerably distinct. By increasing, however, the thickness of the plate, the most refrangible red image was ab sorbed, and the least refrangible one left in a state of the most perfect distinctness. Although he had now determined the part of the spectrum that was best fitted for giving perfect vision, yet the quantity of light extinguished before the insulation of the extreme red ray was affected was so great as to render the determination of little practical utility, excepting in cases where the outline of an object was to be observed. Had it been possible to insulate the most luminous rays of the spectrum as perfectly as the extreme red ones, the advantage would have been of very considerable amount; but he found it quite impracticable.

Abandoning, therefore, all hopes of obtaining from coloured media any further improvement upon the microscope than what had been formerly announced, it occurred to Sir D. Brewster that the object which he had in view might be obtained if he could procure from the combustion of inflammable substances a homogeneous flame for illuminating microscopic objects.

The value of a coloured homogeneous medium, thus simply though satisfactorily pointed out by Newton, has been felt by subsequent experimentalists. The monochromatic lamp, a very important instrument suggested by Sir David Brewster with reference to the same views, may now be described, and this may be best done by a slight abridgment of the learned author's own account of the instrument. Sir David Brewster observes that in a paper on vision through coloured glasses, which he submitted to the Royal Society, he pointed out the advantages of coloured media in microscopical and telescopical observations. Having experienced the great utility of It had long been known that a great quantity of green and red lenses, in developing vegetable struc-homogeneous yellow light was created by placing tures that required to be examined with high powers, he was anxious to derive from this new principle all the advantages which it appeared to possess. In attempting to do this, it became necessary to ascertain the power of giving distinct vision which belonged to each separate colour of the spectrum; and though he had stated in a former paper that it was difficult to discover any reason why one coloured medium should be preferred to another, provided each of them transmit equal quantities of homogeneous light, yet it was desirable to put this theoretical opinion to the test of direct experiment. Sir William Herschel had before investigated this point, in reference to the use of coloured media for solar observations, and had concluded that every colour of the spectrum possessed the same power of giving distinct vision; but his method of observation, which consisted in viewing through a microscope a nail illuminated in succession with each of the colours of

salt or nitre in the white flame of a candle, or in the blue and white flame of burning alcohol. A light, however, generated in this manner, was more fitted for a casual experiment than for a permanent source of illumination; and, as insalubrious vapours are disengaged during the combustion of these salts, he did not avail himself of this method of obtaining yellow light.

After numerous experiments, attended with much trouble and disappointment, he found that almost all bodies in which the combustion was imperfect, such as paper, linen, cotton, &c., gave a light in which the homogeneous yellow rays predominated; that the quantity of yellow light increased with the humidity of these bodies; and that a great portion of the same light was generated when various flames were urged mechanically by a blow-pipe or a pair of bellows.

As the yellow rays seem to be the product of an

imperfect combustion, Sir David Brewster conceived | tints, which obliterate the principal image. In illuthat alcohol diluted with water would produce them minating the wires of transit instruments and microin greater abundance than when it was in a state of meters; in graduating the limbs of divided instrupurity; and, upon making the experiment, found it ments, which is generally done by candle-light; in to succeed beyond his most sanguine expectations. reading off the same divisions in fixed observations; The whole of the flame, with the exception of a in forming signals in trigonometrical surveys; in small portion of blue light, was a fine homogeneous obtaining correct and uniform measures of refractive yellow, which, when analysed with the prism, ex-power; in measuring the separation of the two pencils hibited faint traces of green and blue, but not a single in doubly-refracting crystals; in determining the focal ray of red or orange light. The green and blue rays, lengths of lenses; in observing various optical phewhich accompanied the yellow flame, had compara-nomena, where the light is decomposed; in these, tively so little intensity that they disappeared in the and in general in all delicate works, where correct processes of illuminating and magnifying the object vision is essential, the employment of a homogeneous under examination; and, even if they had existed in flame will be found to confer the most signal benegreater abundance, it was quite easy to absorb them fits. at once by the intervention of a plate of the palest yellow glass, thus rendering the lamp perfectly monochromatic.

From many experiments on the combustion of diluted alcohol he found that the discharge of yellow light depended greatly on the nature of the wick, and on the rapidity with which the fluid was converted into vapour. A piece of sponge, with a number of projecting points, answered the purpose of a wick better than any other substance, and the extrication of the yellow light became more copious by placing a common spirit-lamp below the burner of the other. In order to obtain a very strong light, for occasional purposes, he connected with the top of the burner a frame of wire-gauze, which by moving vertically round a hinge, or by a motion to one side, could be placed in a horizontal position about half an inch above the wick. As soon as it became red-hot it was made to descend into contact with the sponge, when it converted the alcohol quickly into vapour, and produced an abundant discharge of yellow light.

If a permanently strong light is required, it is found preferable to dispense entirely with the use of the wick, and to allow the diluted alcohol to descend slowly from the rim into the bottom of a concave dish of platinum, kept very hot by a spiritlamp placed beneath it. The bottom of the dish is made with a number of projecting eminences, in order that the film of the fluid which rests upon it may be exposed at many points to the action of the heated surface. After the lamp has burned for some time, a portion of unevaporated water, mixed with a small quantity of alcohol, will remain at the bottom of the dish, in a state unfit for combustion. This water may be taken up by a sponge, or it might be prevented from accumulating by having a fountain of pure alcohol, from which the exhausted strength of the diluted fluid could be renewed.

The above engraving represents one form of the monochromatic lamp, in which the diluted alcohol is burned in a flat platinum or metallic dish mn, which may be made to have a slight spontaneous oscillatory motion, for the purpose of bringing the fluid over the heated projections of the platinum. A common spirit-lamp, op, enclosed in a case, is placed below the platinum dish m n, in order to procure sufficient heat for throwing off the vapour from the diluted alcohol. A chimney, or a cylinder, of pale yellow glass may be placed round the flame, if it should be thought of any consequence to absorb the small portion of blue light which accompanies the yellow flame.

In the preceding portion of this article we have given a complete history of all the early optical discoveries, thus embracing the most essential features of the science. In the articles REFRACTION, REFLECTION, POLARIZATION, PHOTOMETER, and VISION, we purpose adverting to the experimental labours of those distinguished philosophers Young, Brewster, and Herschel. Before, however, we close this part of our form of the lens as applicable to optical purposes.

The monochromatic lamp being thus completed, Sir David Brewster lost no time in applying it to the illumination of microscopic objects. The effect which it produced far exceeded his expectations. The images of the most minute vegetable structures were precise and distinct, and the vision in every respect more perfect than it could have been had all the lenses of the microscope been made perfectly achro-subject, it will be necessary to point out the ordinary matic by the most skilful artist.

Independently of its use in microscopical observations, the monochromatic lamp will find an extensive application in various branches of arts and sciences. In certain cases of imperfect vision, where a number of coloured images are formed by the separation of the fibres of the crystalline lens, a homogeneous light will improve the vision, by removing the prismatic

A lens is a transparent body of a different density from the medium in which it is intended to be employed. It is commonly of glass, and used by opticians to collect or disperse the rays of light. Lenses are in general convex, that is, thicker in the middle than at the edges (these collect, and, by the force of refraction, converge the rays, and consequently

magnify), or concave, that is, thinner in the middle | seen on a paper placed at the meeting of the rays. than at the edges: these by refraction disperse the Hence if an object, a b c, in the subjoined diagram, rays of light, and diminish the objects that are seen hrough them.

In the above figure are given sections of the most common forms of lenses. The plano-convex lens, shown at a, is bounded by a plane surface on one side and by a convex one on the other. The planoconcave lens, shown at b, is bounded by a plane surface on one side and a concave one on the other. At e is shown a double-convex lens, bounded by two convex spherical surfaces, whose centres are on opposite sides of the lens. It is equally convex when the radii of both sufaces (that is, the distances from the centres to the circumferences of the circles they belong to) are equal, and unequally convex when their radii or distances are unequal: d is a double-concave lens; it is bounded by two concave spherical surfaces, whose centres are on opposite sides of the lens: e is a meniscus-lens, bounded by a concave and a convex spherical surface, and these two surfaces meet if continued.

9

be placed beyond the focus of the convex lens d e f, some of the rays which flow from every point of the object, on the side next the glass, will fall upon it, and after passing through it they will be converged into as many points on the opposite side of the glass, where the image of every point will be formed, and consequently the image of the whole object, which will be inverted. Thus the rays a d, a e, af, flowing from the point a, will converge in the space def and by meeting will form the image of the point of the arrow a on the opposite side. The rays bd, be, bf, flowing from the point b, will be united at the centre of the image. And the rays c d, ce, cf, flowing from the point c, will, when united, form the image of c.

The axis of these lenses is a straight line f, g, in We have in the above diagram selected three which are situated the centres of their spherical sur- pencils of rays only; but it will be obvious that, faces, and to which their plane surfaces are perpen-whatever the form of the figure, similar pencils must dicular. If we suppose these sections to revolve round the line f g they will generate the different solids which they are intended to represent.

It will be observed that in a double-convex lens the rays of light are twice refracted: first, on entering the convex surface of the dense medium, the glass; and, secondly, on going out of the same dense medium and entering the rare medium, or the air, which from the form of the glass we know must present a concave surface. Now rays are equally converged by entering a convex surface of a dense medium and a concave surface of a rare medium. When parallel rays fall upon a double-convex lens they will be refracted so as to meet in a focus whose distance is equal to the radius or semi-diameter of the sphere of the lens. But, if a lens be more convex on one side than the other, the rule for finding the focal distance is this: a the sum of the semidiameters of both convexities is to the semi-diameter of either, so is double the semi-diameter of the other to the distance of the focus; or divide the double product of the radii by their sums, and the quotient will be the distance sought.

If a candle be placed in the focus of a convex lens, the diverging rays after passing through it will become parallel; but, if it be placed nearer the glass than its focal distance, the rays will diverge more or less as the candle is more or less distant from the focus. And, if on the other hand the candle be placed further from the glass than its focal distance, the rays will converge and meet in a point, which will be more or less distant from the glass as the candle is nearer to or further from its focus; and where the rays meet they will form an inverted image of the flame of the candle, which may be

flow from every part of the surface, and one of the most curious phenomena connected with their transmission is that they never impede each other.

If the object a b c be brought nearer to the glass, the image of it will be removed to a greater distance; for then more rays flowing from every single point will fall more diverging upon the glass, and therefore cannot be so soon collected into the corresponding points behind it. Consequently, if the distance of the object a b c, in the diagram beneath, be equal

to the distance of the focus of the glass, the rays of each pencil will be so refracted by passing through the glass that they will go out of it parallel to each other; and therefore there will be no picture formed behind the glass. If the focal distance of the glass, and the distance of the object from the glass, be known, the distance of the picture from the glass may be found by this rule: viz. multiply the distance of the focus by the distance of the object, and divide the product by their difference; the quotient will be the distance of the picture.

Rays that have been rendered convergent by a double-convex lens, become parallel again by passing through another placed parallel to it, and at double their focal distance. Thus, in the left-hand

figure beneath, k is the focus of both the lenses fg and | lens so that its axis may point as near as possible to de. The parallel rays hi, having passed through the the sun; then holding a paper opposite thereto, the

Under the heads MICROSCOPE, SPECTACLES, and | who first brought it from the East Indies. 4. The TELESCOPE, the theory and construction of those forbidden-fruit-tree, in trunk, leaves, and flowers, valuable optical instruments are fully entered into. very much resembles the common orange-tree; but OR, the heraldic term for gold. In engraving it is the fruit, when ripe, is larger and longer than the denoted by small points covering the field or bearing. orange. It has somewhat the taste of shaddock; It may be supposed to signify of itself generosity, but far exceeds it, as well as the orange, in point of splendour, or solidity; according to G. Leigh, if it is flavour. 5. The horned-orange is a tree of moderate compounded withsize, producing fruit which divides, and the rind runs out into divisions like horns. 6. The hermaphrodite-orange is a moderate-sized tree, producing fruit shaped partly like an orange and partly like a citron. 7. The dwarf-orange, or nutmeg-orange, has a long stem and small head, growing two or three feet high, with very small fruit.

Gul.

Courage.

Azu.

Trust.

Ver.

Joy.

Pur.
Sab.

Charity.
Constancy.

ORA, in antiquity, was a term equivalent to an ounce; but it has been much debated among modern antiquaries whether the ora the mention of which so often occurs was a coin, or only money of account. Dr. Hickes observes that the mode of reckoning money by marks and oras was never known in England till after the Danish settlements and, by examining the old numismatic estimates among the principal Gothic states upon the Baltic, it appears that the ora and solidus were of the same value and that the ora was the eighth part of the mark. From several of the Danish laws, it likewise appears that the Danish ora, derived by corruption from aureus, was a coin of common occurrence. As a weight the ora was regarded as the uncia or unit, by which the Danish mark was divided; and in Doomsday-book the ora is used for the ounce, or the twelfth part of the Saxon pound, and the fifteenth of the commercial: as a coin it was equal to the Frank solidus of twelve pence. And from the accidental coincidence of the Frank aureus with the eighth part of their mark, the Danes probably took occasion to give it the new name of ora. There was another ora mentioned in the rolls of the twenty-seventh of Henry III. the value of which was sixteen pence; and this was probably derived from the half mancus of the Saxons. Such was the original of these two oras, as there were no aurei of that period to which these two denominations of money of sixteen and twelve pence can possibly be ascribed. It is observed, further, that the name ora distinguishes the gold coins in several parts of Europe to this day. The Portuguese moidore is nothing else but moeda d'oro, from the Latin moneta de auro; the French Louis d'ores come from the same use of the word, and owe their appellation to the ora. See Clarke on Coins.

The fruit of the orange-tree yields a very agreeable acid juice, which, besides the purposes to which it is commonly applied, is much used in medicine. The peel of the orange is an agreeable aromatic, and in cold phlegmatic constitutions proves an excellent stomachic and carminative, promoting appetite, warming the habit, and strengthening the tone of the viscera. The young fruits of the Seville orange dried are used in medicine under the name of aurantia curaflaventia. They are a moderately warm, bitterish aromatic, of an agreeable flavour. The flowers of the orange-tree have been for some time in great esteem as a perfume. They are highly odoriferous, of a somewhat warm and bitter taste. They yield their flavour by infusion to rectified spirit, and in distillation both to spirit and water. The bitter matter is dissolved in water, and on evaporating the decoction remains entire in the extract. The distilled water was formerly kept in the shops, but on account of the great scarcity of the flowers is now laid aside. An oil distilled from these flowers and much employed in medicine is brought from Italy under the name of essentia neroli.

This delicious fruit is sometimes employed as a basis in the manufacture of wine. The following proportions of the various ingredients may be relied on :-To ten gallons of water put 28 lbs. of loaf sugar, and the whites of six eggs; boil them together for three-quarters of an hour, keeping the liquor well scummed all the time, and pour it hot into a tub or large pan, over the peels of fifty Seville oranges. When it is nearly cold take three spoonfuls of yeast, spread over a piece of toasted bread, and put it into the liquor to make it ferment. After it has stood two or three days, pour it from the peels into a cask with a gallon of orange-juice, which takes about 120 Seville oranges; let it remain in the cask until the fermentation ceases. When the fermentation is over, draw off as much of the wine as will admit one quart of brandy for every five gallons. This, when kept four or five years, is found to surpass some of the best foreign wines sold in this country.

ORANGE; the fruit of the citrus aurantium. This fruit, both in its natural and prepared state, has long formed a considerable article of commerce. The orange-tree has an upright trunk, dividing upward into a branched, regular head, from five to ten or twelve feet high. The most celebrated varieties are, 1. the Seville orange. This is a very handsome tree and the hardiest of any, as in this country it shoots ORATORIO; a musical drama of a dignified chafreely, producing large and beautiful leaves and racter, which is better fitted for musical execution flowers. The fruit is large, rough-rinded, and sour, than for theatrical action. Hence, on the part of the of excellent quality for household uses. 2. The poetry, it requires, though not in the strict sense of China orange. This tree has moderately-sized leaves, the theatrical drama, the representation of an action and a smooth, thin-rinded, sweet fruit, of which there or event, either immediately by the persons concerned are several varieties in warm countries, where they in the action or event, or mediately by those who grow in the open grounds. 3. The great Shaddock-narrate the circumstances, and by the chorus at inorange grows larger and stronger than the foregoing, tervals, in which the whole body of individuals conwith large, thick, and somewhat serrated leaves, and very large fruit, having a reddish pulp. It derives the name of Shaddock from a captain of that name,

cerned express their feelings in music. The subject should be of a noble character (as, for example, the Creation), and the music adapted to express various