Plate attentively at a filver feal, on which a cipher is engraCLXII. ved. It will at firft appear cut in, as to the naked eye; but if you continue to obferve it fome time, without changing your fituation, it wlll feem to be in relief, and the lights and shades will appear the fame as they did before. If y you regard it with the fame attention ftill longer, it will again appear to be engraved and fo on alternately.

Fig. 6.

If you look off the feal for a few moments, when you view it again, instead of seeing it, as at firft, engraved, it will appear in relief. If, while you are turned toward the light, you fuddenly incline the feal, while you continue to regard it, thofe parts that feem ed to be engraved will immediately appear in relief: and if, when you are regarding these feeming prominent parts, you turn yourself fo that the light may fall on the right hand, you will fee the fhadows on the fame fide from whence the light comes, which will appear not a little extraordinary. In like manner the fhadows will appear on the left, if the light fall on that fide. If, inflead of a feal, you look at a piece of money, thefe alterations. will not be vifible, in whatever fituation you place yourself.

It has been fufpected that this illufion arifes from the fituation of the light and in fact, "I have obferved (fays M. Guyot, from whom this article is taken), that when I have viewed it with a candle on the right, it has appeared engraved; but by changing the light to the left fide, it has immediately appeared in relief." It ftill, however, remains to be explained, why we fee it alternately hollow and prominent, without changing either the fituation or the light. Perhaps it is in the fight itself that we must look for the caufe of this phenomenon; and this feems the more probable, as all these appearances are not difcernible by all perfons.

Mr William Jones of Holborn, has remarked to us, that this illufion is ftill more extraordinary and permanent, when you look at a cavity in a feal or other object through the three eye-glaffes of a common four glafs refracting telescope: all cavities viewed thro' thefe glaffes appear conftantly reliefs, in almost all fituations of the light you fee them with.

V. The Dioptrical Paradox.

A NEW and curious optical, or what may be called properly a dioptrical, deception, has been made by Mr W. Jones. Its effect is, that a print, or an ornamented drawing, with any object, fuch as an ace of diamonds, &c. in the centre F, will be feen as the ace of clubs when it is placed in the machine ABDC, and viewed

through a fingle glafs only contained in the tube E. The conftruction of this machine is truly fimple. The glafs in the tube F, which brings about this furprifing change, is fomewhat on the principle of the common multiplying glass, as represented at G, which by the number of its inclined furfaces, and from the refractive power of the rays proceeding from the objects placed before it, fhows it in a multiplied state or quantity. Its only difference is, that the fides of this glafs are flat, and diverge upwards from the bafe to a point in the axis of the glafs like a cone: the number of the fides is fix; and each fide, from its angular pofition to the eye, has the property of refracting from the border of the print F fuch a portion of it (defignedly there placed), as will make a part in the compofition of the figure to be represented: for the hexagonal and conical figure of this glass prevents any fight of the ace of diamonds in the centre being feen; confequently the ace of clubs being previously and mechanically drawn in the circle of refraction in fix different parts of the border, at 1, 2, 3, 4, 5, 6, and artfully difguifed in the ornamental border by blending them with it, the glafs in the tube at E will change the appearance of the ace of diamonds F into the ace of clubs G. In the fame manner may other prints undergo fimilar changes, according to the will of an inge nious draughtfman who may defign them. The figure of the glafs is clearly fhown at H.

VI. The Camera Obfcura, or Dark Chamber.

MAKE a circular hole in the fhutter of a window, from whence there is a profpect of the fields, or any other object not too near; and in this hole place a convex glass, either double or fingle, whofe focus is at the distance of five or fix feet (A). Take care that no light enter the room but by this glass: at a distance from it, equal to that of its focus, place a pafteboard, covered with the whiteft paper; which should have a black border, to prevent any of the fide rays from diturbing the picture. Let it be two feet and a half long, and 18 or 20 inches high: bend the length of it inwards, to the form of part of a circle, whofe diameter is equal to double the focal diftance of the glass. Then fix it on a frame of the fame figure, and put it on a moveable foot, that it may be eafily fixed at that exact distance from the glass where the objects paint themselves to the greateft perfection. When it is thus placed, all the objects that are in the front of the window will be painted on the paper, in an inverted pofition (B), with the greatest regularity and in the most natural colours. E 2

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If you place a moveable mirror without the winCLXII. dow; by turning it more or lefs, you will have on the paper all the objects that are on each fide of the window (c).

five, will, by magnifying both the image and the fpots, Plate make them appear to greater advantage.

Rays let into a Dark Chamber.

Nothing can be more pleafing than this experiment, efpecially when the objects are frongly enlightened by the fun and not only land- profpects, but a fea-port, when the water is fomewhat agitated, or at the fetting of the fun, prefents a very delightful appearance.

This representation affords the most perfect model for painters, as well for the tone of colours, as that degradation of fhades, occafioned by the interpofition of the air, which has been fo juftly expreffed by fome modern painters.

It is neceffary that the paper have a circular form; for otherwise, when the centre of it was in the focus of the glafs, the two fides would be beyond it, and confequently the images would be confufed. If the frame were contrived of a spherical figure, and the glafs were in its centre, the reprefentation would be ftill more accurate. If the object without be at the diftance of twice the focal length of the glass, the image in the room will be of the fame magnitude with the object.

The lights, fhades, and colours, in the camera obfcura, appear not only juft, but, by the images being reduced to a fmaller compafs, much stronger than in nature. Add to this, that these pictures exceed all others, by representing the motion of the feveral objects: thus we fee the animals walk, run, or fly; the clouds float in the air; the leaves quiver; the waves roll, &c.; and all in ftrict conformity to the laws of nature. The beft fituation for a dark chamber is directly north, and the beft time of the day is noon.

VII. To show the Spots on the Sun's Difk, by its Image in the Camera Obfcura.

PUT the object glafs of a 10 or 12 feet telescope into the fcioptric ball, and turn it about till it be directly oppofite to the fun (D). Then place the pafteboard, mentioned in the laft experiment, in the focus of the lens; and you will fee a clear bright image of the fun, of about an inch diameter, in which the spots on the fun's furface will be exactly defcribed.

As this image is too bright to be feen with pleasure by the naked eye, you may view it through a lens whofe focus is at fix or eight inches diftance; which at the fame time that it prevents the light from being offen

LET the rays of light that pass through the lens in the fhutter be thrown on a large concave mirror, pro perly fixed in a frame. Then take a flip or thin plate of glafs; and flicking any fmall object on it, hold it in the incident rays, at a little more than the focal di ftance from the mirror: and you will fee, on the oppofite wall, amidst the reflected rays, the image of that object, very large, and extremely clear and bright.. This experiment never fails to give the fpectator the higheft fatisfaction.

IX. The Portable Camera Obfcura.

THE great pleasure produced by the camera obfcura in the common form, has excited feveral to render it more univerfally ufeful by making it portable; eafily fixed on any spot, and adapted to every profpect. We fhall not here examine the merits of the various forts that have been invented; but content ourfelves with defcribing two of late improved conftructions, as made and fold by the opticians of the prefent time, and that appear in their conftruction the moft convenient and advantageous of any yet contrived.

CLXII

The pocket or portable camera obfcura, with a drawer to draw out in the front, is reprefented in fig. 7. Fig. 7. The images of the objects before the inftrument are reflected upon a glass ground rough on its upper fide, and that is placed at top of the hinder part of the box, under the moveable cover reprefented in the figure. The images reprefented thereon will afford a moft beautiful and perfect piece of perfpective or landfcape of whatever is before the camera, and more particularly fo if the fun fhines upon the objects. The outlines of them may easily be traced on the glass by a black-lead pencil. There is fometimes a fcale of proportions placed in the upper furface of the drawer, by which any particular building or other object may be drawn in a given proportion or magnitude, and according to the figures inferted on the fcale, which are adapted to the focus or foci of the lenfes made ufe of in the camera. The glaffes that are made ufe of in this camera are only three, and are reprefented in fig. 8. The convex Fig. 8. glafs A is placed in the front of the drawer of the camera, and is of a focus agreeable to the length of the box. The mirror CE reclines in the box in an angle of 45 degrees from a perpendicular fituation. The rays flowing from the object F through the con vex glafs A to the plane mirror CE, will be reflected from it, and meet in points on the glafs placed horizontally

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zontally in the direction CD, and will form thereon CLXII. the aforementioned images. If on this glafs an oiled paper or any other tranfparent fubitance be placed, the images will be clearly reprefented, and fufficiently fo to delineate them by a black-lead pencil or crayon. Inftead of the glafs CD, or fometimes underneath it, is often placed a double convex lens of a focus fomewhat fhorter than the length of the box: this alteration confiderably brightens the appearance of the images, and renders them as vivid as the objects themselves, though not quite fo accurate in their contours or outlines as by the preceding method.

Fig. 9.

Another kind of portable camera obfcura is, where the images are formed upon white paper, and the feveral parts of the camera fold up out of a box fhaped like a book or cheft. This way of the images being formed on paper is a much preferable one to the preceding method, and admits of their being traced on the paper with the utmost readinefs. This inftrument, as open out of its cafe and ready for ufe, is reprefented in fig. 9. The front and fides fold up to the height of about two feet from the cafe EFG, by means of hinges placed at P, H, &c. The head ABCD, about The head ABCD, about five inches fquare and high, containing the mirror L and the convex lens beneath it, fits on at CD, and the inner fquare tube of it is moved up and down by rackwork and a pinion NM. This motion ferves to adjust the convex lens d to its proper focal distance from the white paper placed within fide at the bottom of the box EFG, fo that the images may be formed with the greateft poffible diftin&tnefs. In tracing thefe images the face is applied clofe to the hole in the front at K, and the hand in the fleeve in the front at the bottom of FG. When the fides and front are unhooked and folded down, they all lie close in the box EFG, and the lid O folds down as a top on them close, and the box remains then the fize of a common folio book, and is covered with calf leather and lettered on the back in perfect imitation of one.

By the diagonal pofition of a plane mirror the curious opera-glafs is conftructed, by which any person may be viewed in a theatre or public company, and yet know nothing of it. It confifts only in placing a concave glass near the plane mirror, in the end of a fhort round tube, and a convex glafs in a hole in the fide of the tube. Then holding the end of the tube with the glafs to the eye, all objects next to the hole in the fide will be reflected fo as to appear in a direct line forward, or in a pofition at right angles to the perfon's fituation who is looked at. Plane glaffes inftead of a concave and convex may be used; but in this cafe there will be no magnitude of the object, but it will appear brighter. It is called by opticians the diagonal opera-glass.

X. The Magic Lantern.

THIS very remarkable machine, which is now known over all the world, caufed great aftonishment at its origin. It is ftill beheld with pleafing admiration; and pleafing admiration; and the fpectator very frequently contents himfelf with wondering at its effects, without endeavouring to in veftigate their caufe. The invention of this ingenious illufion is attributed to the celebrated P. Kercher, who has published on various fciences, works equally learned, curious, and entertaining. Its defign is to repre

fent at large, on a cloth or board, placed in the dark, Plate the images of fmall objects, painted with tranfparent colours on plates of glafs.

The conftruction is as follows. Let ABCD be the fide of a tin box, eight inches high, eight inches long, Fig. 10. and ten broad (or any other fimilar dimenfions), the top of which must have a funnel, with a cover, as reprefented in fig. 11; which at the fame time it gives a paffage to the fmoke, prevents the light from coming out of the box. In the middle of the bottom of the box must be placed a low tin lamp E, which is to be moveable. It should have three or four lights, that must be at the height of the centre of the glaffes in the tubes N and O. In the largest of these tubes must be placed a glafs femiglobular lens N, about four inches diameter; and in the fmaller one a double convex lens o, about 24 inches diameter, and fix inches focus, the length of the tubes holding them about 4 inches each: the inner tube containing the fmall lens o must be a fliding one, in order to adjuft it at a proper diftance from the painted fliders, fo that the objects thereon may be diftinctly reprefented on the cloth or white wall. wall. A flit or opening between the glass N and the front fide BGDH of the box must be made large enough to admit the fliders to be paffed through, (as in fig. 11.) The clearness of the light, and the ob jects upon the cloth, will depend much upon the light of the lamp: it will therefore be proved beft, to place, inftead of the common lamp E, a kind of the new or Argant's Patent Lamp, which will be found confiderably to improve the effect of the lantern by its fuperior ftrength of light.

From the conftruction of this lantern it is evident, that when the glafs fliders, with the painted figures, are placed in the groove or flit in the lantern for that purpofe, and the room darkened, a quantity of light from the lamp at E will be collected by the lens N, and refracted upon the cloth placed oppofite; and that by moving the fliding tube containing the small lens o gradually in or out as occafion may require, this lens will form images of the figures on the fliders in their diftinct colours and proportions, with the ap pearance of life itself, and of any fize from fix inches. to 7 feet, according to the distance of the lantern from the cloth. The lantern, with one of the fliders ready for ufe, is clearly reprefented in fig. 11. By the aid of the new patent lamp aforementioned, confiderable ufeful improvements are made to this lantern. Mr Jones optician of Holborn has contrived an apparatus. to be applied to it, that converts it into a microscope by night; and it fhows all the variety of tranfparent and many of the opaque objects magnified upon a cloth. or fkreen oppofite, fimilar to the figures above mentioned, but not in fo large a degree; about one or two. feet diameter is the utmoft that can at prefent be ob tained.

Method of Painting the Glaffes for the Lantern. Draw on a paper the fubject you defire to paint, and fix it at each end to the glafs. Provide a varnish with which you have mixed fome black paint; and with a fine pencil draw on the other fide of the glafs, with very light touches, the defign drawn on the paper. If you are defirous of making the painting as perfect as poffible, you should draw fome of the outlines in their

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proper colours, provided they are the ftrongest tints of CLXII. thele colours that are used. When the outlines are dry, you colour the figures with their proper tints or degradations. Tranfparent colours are molt proper for this purpofe, fuch as carmine, lake, Pruffian blue, verdigris, &c. and thefe must be tempered with a ftrong white varnish, to prevent their peeling off. You are then to shade them with black mixed with the fame varnish, or with biftre, as you find convenient. You may also leave strong lights in fome parts, without any colours, in order to produce a more ftriking effect. Obferve, in particular, not to use more than four or five colours, fuch as blue, red, green, and yellow. You fhould employ, however, a great variety of tints, to give your painting a more natural air; without which they will reprefent vulgar objects, which are by no means the more pleafing because they are gawdy.

Fig. 12.

Fig. 13.

When the lamp in this lantern is lighted, and, by drawing out the tube to a proper length, the figures painted on the glass appear bright and well defined, the fpectator cannot fail of being highly entertained by the fucceffion of natural or grotefque figures that are painted on the glaffes. This piece of optics may be rendered much more amufing, and at the fame time more marvellous, by preparing figures to which different natural motions may be given (E), which every one may perform according to his own tafle; either by movements in the figures themselves, or by painting the fubject on two glaffes, and paffing them at the fame time through the groove, as will be seen in the next experiment.

XI. To reprefent a Tempeft by the Magic Lantern. PROVIDE two plates of glass, whofe frames are fo thin that they may both pals freely through the flit or groove of the common magic lanterns at the fame time.

On one of thefe glaffes you are to paint the appear. ance of the fea, from the flighteft agitation to the most violent commotion. Representing from A to B a calm; from B to C a fmall agitation, with fome clouds; and fo on to F and G, which should exhibit a furious ftorm. Obferve, that these representations are not to be diftinct, but run into each other, that they may form a natural gradation remember alfo, that great part of the effect depends on the perfection of the painting, and the picturefque appearance of the defign.

On the other glafs you are to paint veffels of different forms and dimenfions, and in different directions, together with the appearance of clouds in the tempeftuous parts.

You are then to pafs the glafs flowly through the groove; and when you come to that part where the form begins, you are to move the glafs gently up and down, which will give it the appearance of a fea that begins to be agitated: and fo increase the motion till you come to the height of the ftorm. At the fame time you are to introduce the other glafs with the fhips, and moving that in like manner, you will have a natural reprefentation of the fea, and of fhips in a calm and in a ftorm. As you draw the glaffes flowly back,

the tempeft will feem to fubfide, the sky grow clear, and the fhips glide gently over the waves.-By means CLXIL of two glaffes difpofed in this manner you may likewife reprefent a battle, or fea-fight, and numberless other fubjects, that every one will contrive according to his own tafte. They may also be made to reprefent fome remarkable or ludicrous action between different perfons, and many other amusements that a lively imagination will eafily fuggeft.

XII. The Nebulous Magic Lantern.

THE light of the magic lantern, and the colour of images, may not only be painted on a cloth, but also reflected by a cloud of fmoke.

Provide a box of wood or pafteboard (fig. 14.) of about four feet high, and of feven or eight inches fquare at bottom, but diminishing as it afcends, fo that its aperture at top is but fix inches long, and half an inch wide. At the bottom of this box there must be a door that shuts quite clofe, by which you are to place in Fig. 14. the box a chafing-dish with hot coals, on which is to be thrown incenfe, whofe fmoke goes out in a cloud at the top of the box. It is on this cloud that you are to throw the light that comes out of the lantern, and which you bring into a smaller compafs by drawing out the moveable tube. The common figures will here ferve. It is remarkable in this representation, that the motion of the fmoke does not at all change the figures; which appear fo confpicuous, that the fpectator thinks he can grafp them with his hand.

Note, In this experiment fome of the rays paffing through the fmoke, the representation will be much lefs vivid than on the cloth; and if care be not taken to reduce the light to its smallest focus, it will be ftill more imperfect.

XIII. To produce the Appearance of a Phantom upon a

Pedefal placed on the middle of a Table.

INCLOSE a common fmall magic lantern in a box ABCD, that is large enough to contain also an inclined mirror M; which must be moveable, that it may reflect the cone of light thrown on it by the lantern, in fuch a manner that it may pafs out at the aperture made in the top of the box. There fhould be a flap with hinges to cover the opening, that the infide of the box may not be feen when the experiment is making. This aperture fhould likewife be oval, and of a fize adapted to the cone of light that is to pass thro' it. There must be holes made in that part of the box which is over the lantern, to let out the fmoke; and over that part must be placed a chafing-dish of an oblong figure, and large enough to hold feveral lighted coals. This chafing-difh may be inclofed in a painted tin box of about a foot high, and with an aperture at top fomething like fig. 14. It should stand on four short feet, to give room for the smoke of the lamp to pafs There muft alfo be a glass that will afcend and defcend at pleasure in a vertical groove ab. To this glass let there be fixed a cord, that, going over a pulley

out.

Fig. 15.