Does it spring from the education which during many ages the human race has received from its first instructors? A vast and novel question, but with which I have nothing to do. It is sufficient to observe that as the lover of the wonderful always prefers the most surprising to the most natural account, this last has been too frequently neglected, and is irrevocably lost. Occasionally, however (and we shall cite more than one instance), simple truth has escaped from the power of oblivion.

Credulous man may be deceived once, or more frequently; but his credulity is not a sufficient instrument to govern his whole existence.

The wonderful excites only a transient admiration. In 1798, the French _savans_ remarked with surprise how little the spectacle of balloons affected the indolent Egyptian.... But man is led by his pa.s.sions, and particularly by _hope_ and _fear_."

When parallel rays fall upon a convex mirror, they are scattered and dispersed in all directions, and the image of an object reflected in a convex mirror appears to be very small, being reduced in size because the reflected picture I M is nearer the surface of the mirror than the object A B. No. 1. (Fig. 267.)

[Ill.u.s.tration: Fig. 267. A B, D H. (No. 2) represent two parallel rays incident on the convex surface B H, the one (A B) perpendicularly, the other (D H) obliquely. C is the centre of convexity. H E is the reflected ray of the oblique incident one, D H; whilst C H I is the perpendicular.]

Convex mirrors are not employed in any optical deception on a large scale, although some ingenious delusions are producible from cylindrical and conical mirrors, and are thus described by Sir David Brewster:

"Among the ingenious and beautiful deceptions of the seventeenth century, we must enumerate that of the re-formation of distorted pictures by reflection from cylindrical and conical mirrors. In these representations, the original image from which a perfect picture is produced, [Page 280] is often so completely distorted, that the eye cannot trace in it the resemblance to any regular figure, and the greatest degree of wonder is of course excited, whether the original image is concealed or exposed to view. These distorted pictures may be drawn by strict geometrical rules, and I have shown a simple method of executing them. Let M be an accurate cylinder made of tin-plate or of thick pasteboard. Out of the further side of it cut a small aperture, _a b c d_, and out of the nearer side cut a larger one, A B C D (white letters), the size of the picture to be distorted; having perforated the outline of the picture with small holes, place it in the opening A B C D (white letters), so that its surface may be cylindrical; let a candle or a bright luminous object--the smaller the better--be placed at S, as far behind the picture A B C D (white letters) as the eye is afterwards to be placed before it, and the light pa.s.sing through the small holes will represent on a horizontal plane a distorted image of the picture at A B C D, which, when sketched in outline with a pencil, shaded, and coloured, will be ready for use. If we now subst.i.tute a polished cylindrical mirror of the same size in place of M, then the distorted picture, when laid horizontally at A B C D, will be restored to its original state when seen by reflection at A B C D (white letters) in the polished mirror." The effect of a cylindrical mirror on a distorted picture is shown at No. 2, being copied from an old one seen by Sir D.

Brewster.

[Ill.u.s.tration: Fig. 268.]

By looking at a reflection of the face in a dish-cover or the common surface of a bright silver spoon or of a silver mug, the latter truly becomes ugly as the image is seen reflected from its surface, and [Page 281] a.s.sumes the most absurd form as the mouth is opened or shut, and the face advanced or removed from the silver vessel. (Fig. 269.)

[Ill.u.s.tration: Fig. 269. Distorted image produced by an irregular convex surface.]

In the writings of the ancients there are to be found certain indications of the results of illusions produced by simple optical arrangements, and the sudden and momentary apparition (from the gloom of perfect darkness) of splendid palaces, delightful gardens, &c., with which?-the concurrent voice of antiquity a.s.sures us-?the eyes of the beholders were frequently dazzled in the mysteries, such as the evocation and actual appearance of departed spirits, the occasional images of their _umbrae_, and of the G.o.ds themselves. From a pa.s.sage in "Pausanias," (Boeotic x.x.x.), when, speaking of Orpheus, he says there was anciently at Aornos, a place where the dead were evoked, [Greek: _nekuomanteion_], we learn that in those remote ages there were places set apart for the evocation of the dead. Homer relates, in the eleventh book of the "Odyssey," the admission of Ulysses alone into a place of this kind, when his interview with his departed friend was interrupted by some fearful voice, and the hero, apprehending the wrath of Proserpine, withdrew; the priests who managed these deceptive exhibitions no doubt adopted this method of getting rid of their visitor, who might become too inquisitive, and discover the secret of the mysteries.

Of all the reflecting surfaces mentioned, none produce more interesting deceptions than the concave mirror, and there is very little doubt that silver mirrors of this form were known to the ancients, and employed in [Page 282] some of their sacred mysteries. Mons. Salverte has industriously collected in his valuable work the most interesting proofs of their use, and quotes the following pa.s.sage of "Damascius," in which the results obtainable from a concave mirror are clearly apparent. (Fig.

270.)

[Ill.u.s.tration: Fig. 270. The picture of a human face, possibly reflected from a concave mirror concealed below the floor of the temple; the opening being hidden by a raised ma.s.s of stone, and the worshippers confined to a certain part of the temple, and not allowed to approach it.]

He says:--"In a manifestation which ought not to be revealed ... there appeared on the wall of the temple a ma.s.s of light which at first seemed very remote; it transformed itself in coming nearer into a face evidently divine and supernatural, of a severe aspect, but mixed with gentleness, and extremely beautiful. According to the inst.i.tution of a mysterious religion, the Alexandrians honoured it as Osiris and Adonis."

Parallel rays thrown upon a concave surface are brought to a focus or converge, and when an object is seen by reflection from a concave surface, the representation of it is various, both with regard to its [Page 283] magnitude and situation, according as the distance of the object from the reflecting surface is greater or less. (Fig. 271.) When the object is placed between the _focus_ of parallel rays and the centre, the image falls on the _opposite_ side of the centre, and is larger than the object, and in an inverted position. The rays which proceed from any remote terrestrial object are nearly parallel at the concave mirror--not strictly so, but come diverging to it in separate pencils, or, as it were, bundles of rays, from each point of the side of the object next the mirror; therefore they will not be converged to a point at the distance of half the radius of the mirror's concavity from its reflecting surface, but in separate points at a little greater distance from the concave mirror. The nearer the object is to the mirror, the further these points will be from it, and an inverted image of the object will be formed in them, which will seem to hang pendant in the air, and will be seen by an eye placed beyond it (with regard to the mirror), in all respects like the object, and as distinct as the object itself. No. 2. (Fig. 271.)

[Ill.u.s.tration: Fig. 271. No. 1. A B, D H represent two parallel rays incident on the concave surface B H, whose centre of concavity is C. B F and H F are the reflected rays meeting each other in F, and A B being perpendicular to the concave surface, is reflected in a straight line.

No. 2. A B. The object. I M. The image.]

[Ill.u.s.tration: Fig. 272. A B represents the object, S V the reflecting surface, F its focus of parallel rays, and C its centre. Through A and B, the extremities of the object, draw the lines C E and C N, which are perpendicular to the surface, and let A R, A G, be a pencil of rays flowing from A. These rays proceeding from a point beyond the focus of parallel rays, will, after reflection, converge towards some point on the opposite side of the centre, which will fall upon the perpendicular, B C, produced, but at a greater distance from C than the radiant A from which they diverged. For the same reason, rays flowing from B will converge to a point in the perpendicular N C produced, which shall be further from C than the radiant B, from whence it is evident that the image I M is larger than the object A B, that it falls on the _contrary_ side of the centre, and that their positions are inverted with respect to each other.]

[Page 284]

It appears, from a circ.u.mstance in the life of Socrates, that the effects of burning-gla.s.ses were known to the ancients; and it is probable that the Romans employed the concave speculum for the purpose of lighting the "sacred fire." This is very likely to be true, considering that the priests who conducted the heathen worship of Osiris and Adonis were acquainted with the use of concave metallic specula, as already described at page 282. The effects that can be produced with the aid of concave mirrors are very impressive, because they are not merely confined to the reflection of inanimate objects, but life and motion can be well displayed by them; thus, if a man place himself directly before a concave mirror, but further from it than its centre of concavity, he will see an inverted image of himself in the air between him and the mirror of a less size than himself; and if he hold out his hand towards the mirror the hand of the image will come out towards his hand and coincide with it, being of an equal bulk when his hand is in the centre of concavity, and he will imagine he may shake hands with his image.

(Fig. 273.)

[Ill.u.s.tration: Fig. 273. A concave mirror, showing the appearance of the inverted and reflected image in the air.]

By using a large concave mirror of about three feet in diameter, the author was enabled to show all the results to a large audience that were usually visible to one person only. Whilst experimenting with a concave mirror, by holding out the hand in the manner described, a bystander will see nothing of the image, because none of the reflected rays that form it enter his eyes. This circ.u.mstance is well ill.u.s.trated by placing a concave mirror opposite the fire, and allowing the image of the flames projected from it to fall upon a well-polished mahogany table. If the door of the room opens towards the mirror, and a spectator unacquainted with the properties of concave mirrors should enter the apartment, the person would be greatly startled to see flames apparently playing over the surface of the table, whilst another spectator might enter from another door and see nothing but a long beam of light, rendered visible by the floating particles of dust. To give proper effect to this experiment the concave mirror should be large, and no other light must illuminate the room except that from the fire.

On the same polished table the appearance of a planet with a revolving [Page 285] satellite may be prettily shown by darkening the fire with a screen, and placing a lighted candle before it, which will be reflected by the concave mirror, and appear on the table as a brilliant star of light, and the satellite may be represented by the flame of a small wax taper moved around the large burning candle. The following is the arrangement used by the author at the Polytechnic Inst.i.tution for the purpose of exhibiting the properties of the concave mirror. A lantern enclosing a very brilliant light, such as the electric or lime light, is required for the illumination of the objects which are to be projected on to the screen. The lantern and electric lamp of Duboscq was preferred, although, of course, any bright light enclosed in a box, with a plain convex lens to project the beam of light when required, will answer the purpose. (Fig. 274.)

[Ill.u.s.tration: Fig. 274. A B. Portable screen of light framework, covered with black calico. C C C C. Square aperture just above the shelf, D D, upon which the object--viz., a bottle half full of water--is placed. E. Duboscq lantern to illuminate the object at D D.]

By removing the diaphragm required to project the picture of the charcoal points on to the screen, a very intense beam of light is obtained, which may be focussed or concentrated on any opaque object by another double convex lens, conveniently mounted with a telescope stand, so that it may be raised or lowered at pleasure. This lens is independent of the lantern, and may be used or not at the pleasure of the operator.

The object is now placed on a shelf fixed to the screen, with a square aperture just above it. The object of the screen is to cut off all extraneous rays of light reflected from the mirror, or to increase the sharpness of the outline of the picture of the object. The screen and object being arranged, and the light thrown on from the lantern, the next step is to adjust the concave mirror, and by moving it towards the [Page 286] object, or backwards, as the case requires, a good image, solid and quasi-stereoscopic, is projected on to the screen. (Fig. 275.)

[Ill.u.s.tration: Fig. 275. A. The concave mirror. B. The lantern. C. The portable screen, shelf, and object. D. The inverted image of the bottle filling with water, with the neck downwards, and when thrown on the disc at D producing a most curious illusion.]

The act of filling the bottle with water, or better still with mercury, is one of the most singular effects that can be shown; and if all the apparatus is enclosed in a box, so that the picture on the screen only is apparent, the illusion of a bottle being filled in an inverted position is quite magical, and invariably provokes the inquiry, how can it be done? The study of numismatics, the science of coins and medals, is generally considered to be limited to the taste of a very few persons, and any description of a collection of coins at a lecture would be voted a great bore, unless, of course, the members of the audience happened to be antiquaries; great light, however, may be thrown on history by a study of these interesting remains of bygone times, and a lecture on this subject, ill.u.s.trated with pictures of coins thrown on to the disc by a concave mirror in the manner described, might be made very pleasing and instructive.

Coins, or plaster casts of coins _gilt_, flowers, birds, white mice, the human face and hands, may all, when fully illuminated, be reflected by the concave mirror on to the disc. A Daguerreotype picture at a certain angle appears, when reflected by the concave mirror, to be like any ordinary collodion negative, and all the lights and shadows are reversed, so that the face of the portrait appears black, whilst the black coat is white. On placing the Daguerreotype in another position, easily found by experiment, it is now reflected in the ordinary manner, showing an enlarged and perfect portrait on the disc. In using the Daguerreotype the gla.s.s in front of it must be removed. The pictures from the concave mirror may be also projected on thick smoke procured from [Page 287] smouldering damped brown paper, or from a mixture of pitch and a little chlorate of potash laid on paper, and allowed to burn slowly by wetting it with water.

An image reflected from smoke would be visible to a number of spectators, just as the light from the furnace fires of the locomotive is frequently visible at night, being reflected on the escaping column of steam.

It was probably with the help of some kind of smoke and the concave speculum that the deception practised on the worshippers at the temple of Hercules at Tyre was carried out, as it is mentioned by Pliny that a consecrated stone existed there "from which the G.o.ds easily rose." At the temple of Esculapius at Tarsus, and that of Enguinum in Sicily, the same kind of optical delusions were exhibited as a portion of the religious ceremonies, from which no doubt the priests obtained a very handsome revenue, much more than could be obtained in modern times by the mere exhibition of such wonders at Adelaide Galleries, Polytechnics, or Panopticons.

The smoke from brown paper is very useful in showing the various directions of the rays of light when reflected from plane, convex, and concave surfaces. The equal angles of the incident and reflected rays may be perfectly shown by using the next arrangement of apparatus. (Fig.

276.)

[Ill.u.s.tration: Fig. 276. A. Rays of light slightly divergent issuing from the lantern, and received on a little concave mirror, which brings the rays almost parallel, and reflects them to E, a piece of looking-gla.s.s, from which they are again reflected. C is the incident, and D the reflected rays. F. Smoke from brown paper.]

A very dense white smoke is obtained by boiling in separate flasks (the necks of which are brought close together) solutions of ammonia and hydrochloric acid.

The opposite properties of convex and concave mirrors--the former scattering and the latter collecting the rays of light which fall upon them--are also effectively demonstrated by the help of the same illuminating [Page 288] source and proper mirrors, the smoke tracing out perfectly the direction of the rays of light. (Fig. 277.)

[Ill.u.s.tration: Fig. 277. The smoke shows the rays of light falling on a convex mirror, and rendered still more divergent.]

The smoke developes the cone of rays reflected from a concave mirror in the most beautiful manner, and by producing plenty of [Page 289] smoke, and turning the mirror about--the position of the focus (_focus_, a fire-place), is indicated by a brilliant spot of light, and the reason the images of objects reflected by the concave mirror are reversed, may be better understood by observing how the rays cross each other at that point. (Fig. 278.)

[Ill.u.s.tration: Fig. 278. The smoke shows rays of light falling on the concave mirror. In this experiment attention should be directed to the bright point, E, the focus where the convergent rays meet.]

One of the most perfect applications of the reflection of light is shown in the "Gregorian reflecting telescope," or in that magnificent instrument constructed by Lord Rosse, at Parsonstown, in Ireland. (Fig.

279.)

[Ill.u.s.tration: Fig. 279. Lord Rosse's gigantic telescope.]

The description of nearly all elaborate optical instruments is somewhat tedious, but we venture to give one diagram, with the explanation of the Gregorian reflecting telescope. (Fig. 280.)

[Ill.u.s.tration: Fig. 280. The Gregorian reflecting telescope.]

At the bottom of the great tube T T T T, (Fig. 280), is placed the large concave mirror D U V F, whose princ.i.p.al focus is at M; and in its middle is a round hole P, opposite to which is placed the small mirror L, concave towards the greater one, and so fixed to a strong wire M, that it may be moved farther from the great mirror or nearer to it, by means of a long screw on the outside of the tube, keeping its axis still in the same line P M N with that of the great one. Now since in viewing a very remote object we can scarcely see a point of it but what is at least as broad as the great mirror, we may consider the rays of each pencil, which flow from every point of the object, to be parallel to each other, [Page 290] and to cover the whole reflecting surface D U V F. But to avoid confusion in the figure, we shall only draw two rays of a pencil flowing from each extremity of the object into the great tube, and trace their progress through all their reflections and refractions to the eye f, at the end of the small tube t t, which is joined to the great one.

Let us then suppose the object A B to be at such a distance, that the rays E flow from its lower extremity B, and the rays C from its upper extremity A. Then the rays C falling parallel upon the great mirror at D, will be thence reflected by converging in the direction D G; and by crossing at i in the princ.i.p.al focus of the mirror, they will form the upper extremity i of the inverted image i K, similar to the lower extremity B of the object A B; and pa.s.sing on the concave mirror L (whose focus is at N) they will fall upon it at g and be thence reflected, converging in the direction N, because g m is longer than g n; and pa.s.sing through the hole P in the large mirror, they would meet somewhere about r, and form the lower extremity d of the erect image a d, similar to the lower extremity B of the object A B. But by pa.s.sing through the plano-convex gla.s.s R in their way they form that extremity of the image at b. In like manner the rays E which come from the top of the object A B and fall parallel upon the great mirror at F, are thence reflected converging to its focus, where they form the lower extremity K of the inverted image i K, similar to the upper extremity A, of the object A B; and pa.s.sing on to the smaller mirror L and falling upon it at h, they are thence reflected in the converging state h o; and going on through the hole P of the great mirror, they would meet somewhere about q, and form there the upper extremity a of the erect image a d, similar to the upper extremity A of the object A B; but by pa.s.sing through the convex gla.s.s R, in their way, they meet and cross sooner, as at a, where that point of the erect image is formed. The like being understood of all those rays which flow from the intermediate points of the object, between A and B, and enter the tube T T, all the intermediate points of the image between a and b will be formed; and the rays pa.s.sing on from the image through the eye-gla.s.s S, and through a small hole e in the end of the lesser tube t t, they enter the eye f which sees the [Page 291] image a d (by means of the eye-gla.s.s), under the large angle c e d, and magnified in length, under that angle, from c to d.

To find the magnifying power of this telescope, multiply the focal distance of the great mirror by the distance of the small mirror, from the image next the eye, and multiply the focal distance of the small mirror by the focal distance of the eye-gla.s.s; then divide the product of the latter, and the quotient will express the magnifying power. (Fig.

280.)

We now come to that much disputed and often quoted experiment of Archimedes, who is stated to have employed metallic concave specula or some other reflecting surface by which he was enabled to set fire to the Roman fleet anch.o.r.ed in the harbour of Syracuse, and at that time besieging their city, in which the great and learned philosopher was shut up with the other inhabitants. The story handed down to posterity was not disputed till about the seventeenth century, when Descartes boldly attacked the truth of it on philosophical grounds, and for the time silenced those who supported the veracity of this ancient Joe Miller. Nearly a hundred years after this time, the neglected Archimedes fiction was again examined by the celebrated naturalist Buffon, and the account of his experiments detailed by the author of "Adversaria," in Chambers' Journal, is so logical and conclusive, that we give a portion of it verbatim.

"For some years prior to 1747, the French naturalist Buffon had been engaged in the prosecution of those researches upon heat which he afterwards published in the first volume of the Supplement to his 'Natural History.' Without any previous knowledge, as it would seem, of the mathematical treatise of Anthemius ([Greek: _peri paradoxon mechanematon_]), in which a similar invention of the sixth century is described,[G] Buffon was led, in spite of the reasonings of Descartes, to conclude that a speculum or series of specula might be constructed sufficient to obtain results little, if at all, inferior to those attributed to the invention of Archimedes.

[Footnote G: See Gibbon's "Decline and Fall," chap. xl., section v., note _g_.]

"This, after encountering many difficulties, which he had foreseen with great acuteness, and obviated with equal ingenuity, he at length succeeded in effecting. In the spring of 1747, he laid before the French Academy a memoir which, in his collected works, extends over upwards of eighty pages. In this paper, he describes himself as in possession of an apparatus by means of which he could set fire to planks at the distance of 200, and even 210 feet, and melt metals and metallic minerals at distances varying from twenty-five to forty feet. This apparatus he describes as composed of 168 plain gla.s.ses, silvered on the back, each six inches broad by eight inches long. These, he says, were ranged in a large wooden frame, at intervals not exceeding the third of an inch; so that, by means of an adjustment behind, each should be moveable in all directions independently of the rest--the s.p.a.ces between the gla.s.ses being further of use in allowing the operator to see from behind the point on which it behoved the various disks to be converged.