Читать книгу A History of Inventions, Discoveries, and Origins, Volume II (of 2) - Johann Beckmann - Страница 7

It is highly probable that a limpid brook was the first mirror136, but we have reason to think that artificial mirrors were made as mankind began to exercise their art and ingenuity on metals and stones. Every solid body, capable of receiving a fine polish, would be sufficient for this purpose; and indeed the oldest mirrors mentioned in history were of metal. Those which occur in Job137 are praised on account of their hardness and solidity; and Moses relates138, that the brazen laver, or washing-basin, was made from the mirrors of the women who had assembled at the door of the tabernacle to present them, and which he caused them to deliver up. As the women appeared in full dress at divine worship, it was necessary for them to have looking-glasses after the Egyptian manner. With these the washing-basins, according to the conjecture of most interpretators, were only ornamented, covered, or perhaps hung round; and Michaelis139 himself was once of this opinion. But why should we not rather believe that the mirrors were melted and formed into washing-basins? As soon as mankind began to endeavour to make good mirrors of metal, they must have remarked that every kind of metal was not equally proper for that use, and that the best could be obtained only from a mixture of different metals. In the mirrors however which were collected by Moses, the artists had a sufficient stock of speculum metal, and were not under the necessity of making it themselves; and for this reason they could much more easily give to the whole basin a polished surface, in which the priests, when they washed, might survey themselves at full length. At any rate such a basin would not be the only one employed instead of a mirror. Artemidorus140 says that he who dreams of viewing himself in a basin, will have a son born to him by his maid. Dreams indeed are generally as groundless as this interpretation; but one can hardly conjecture that Artemidorus would have thought of such a dream, had it not been very common for people to contemplate themselves in a basin. There were formerly a kind of fortune-tellers, who pretended to show in polished basins to the simple and ignorant, what they wished to know141. The ancients also had drinking-vessels, the inside of which was cut into mirrors, so disposed that the image of the person who drank from them was seen multiplied142. Vopiscus mentions, among the valuable presents of Valerian to the emperor Probus, when a tribune, a silver cup of great weight, which was covered on the inside with mirrors of this sort143.

Menard and others conjecture that mirrors in the time of Homer were not much used, because he mentions them on no occasion, not even where he describes in so circumstantial a manner the toilet of Juno144. In answer to this, however, I have two things to observe. In the first place, it is not to be expected that Homer should have mentioned every article with which he was acquainted; and secondly, we are assured by Callimachus, where he evidently has imitated the passage of Homer before-quoted145, that neither Juno nor Pallas employed a mirror when they dressed. Mythology therefore did not allow the poet to introduce a mirror upon the toilet of that deity. Polydore Vergilius, Boccace, Menard, and others have all fallen into the error of making Æsculapius the inventor of mirrors, though Cicero146 seems to say the same thing; but the best commentators have long since observed very justly, that the Roman philosopher alludes not to a mirror but to a probe, the invention of which we may allow to the father of medicine, who was at first only a surgeon.

When one reflects upon the use made of metal mirrors, particularly at Rome, to add to magnificence and for other purposes, and how many artists, during many successive centuries, were employed in constructing them, and vied to excel each other in their art, one cannot help conjecturing that this branch of business must at those periods have been carried to a high degree of perfection. It is therefore to be regretted that they have not been particularly described by any writer, and that on this account the art was entirely lost after the invention of glass mirrors, which are much more convenient. No one at that time entertained the least suspicion that circumstances would afterwards occur which would render these metal mirrors again necessary, as has been the case in our days by the invention of the telescope. Our artists then were obliged to make new experiments in order to discover the best mixture for mirrors of metal; and this should be a warning to mankind, never to suffer arts which have been once invented and useful to become again unknown. A circumstantial description of them should at any rate be preserved for the use of posterity, in libraries, the archives of human knowledge.

When we compare metals in regard to their fitness for mirrors, we shall soon perceive that the hardest of a white colour possess in the highest degree the necessary lustre. For this reason platina is preferable to all others, as is proved from the experiments made by the Count von Sickengen. Steel approaches nearest to this new metal, and silver follows steel; but gold, copper, tin and lead, are much less endowed with the requisite property. I have however observed among the ancients no traces of steel mirrors; and it is probable they did not make any of that metal, as it is so liable to become tarnished, or to contract rust. An ancient steel-mirror is indeed said to have been once found, but as some marks of silvering were perceived on it, a question arises whether the silvered side was not properly the face of the mirror147. Besides, every person knows that a steel mirror would not retain its lustre many centuries amidst ruins and rubbish.

The greater part of the ancient mirrors were made of silver, not on account of costliness and magnificence, as many think, but because silver, as has been said, was the fittest and the most durable of all the then known unmixed metals for that use. In the Roman code of laws, when silver plate is mentioned, under the heads of heirship and succession by propinquity, silver mirrors are rarely omitted; and Pliny148, Seneca149, and other writers, who inveigh against luxury, tell us, ridiculing the extravagance of the age, that every young woman in their time must have a silver mirror. These polished silver plates may however have been very slight, for all the ancient mirrors, preserved in collections, which I have ever seen, are only covered with a thin coat of that expensive metal; and in the like manner our artists have at length learned a method of making the cases of gold and silver watches so thin and light, that every footman and soldier can wear one. At first the finest silver only was employed for these mirrors, because it was imagined that they could not be made of that which was standard; but afterwards metal was used of an inferior quality. Pliny tells us so expressly, and I form the same conclusion from a passage of Plautus150. Philematium having taken up a mirror, the prudent Scapha gives her a towel, and desires her to wipe her fingers, lest her lover should suspect by the smell that she had been receiving money. Fine silver however communicates as little smell to the fingers as gold; but it is to be remembered that the ancients understood much better than the moderns how to discover the fineness of the noble metals by the smell, as many modes of proof which we use to find out the alloy, were to them unknown. Money-changers therefore employed their smell when they were desirous of trying the purity of coin151. The witty thought of Vespasian, who, when reproached on account of his tax upon urine, desired those who did so to smell the money it produced, and to tell him whether it had any smell of the article which was the object of it, alludes to this circumstance. In the like manner many savage nations at present can by their smell determine the purity of gold152.

We are informed by Pliny, that Praxiteles, in the time of Pompey the Great, made the first silver mirror, and that mirrors of that metal were preferred to all others. Silver mirrors however were known long before that period, as is proved by the passage of Plautus above-quoted. To reconcile this contradiction, Meursius remarks that Pliny speaks only of his countrymen, and not of the Greeks, who had such articles much earlier, and the scene in Plautus is at Athens. This therefore seems to justify the account of Pliny, but of what he says afterwards I can find no explanation. Hardouin is of opinion, that mirrors, according to the newest invention, at that period, were covered behind with a plate of gold, as our mirrors are with an amalgam. But as the ancient plates of silver were not transparent, how could the gold at the back part of them produce any effect in regard to the image? May not the meaning be, that a thin plate of gold was placed at some distance before the mirror in order to throw more light upon its surface? But whatever may have been the case, Pliny himself seems not to have had much confidence in the invention.

Mirrors of copper, brass and gold, I have found mentioned only by the poets, who perhaps employed the names of these metals because they best suited their measure, or because they wished to use uncommon expressions, and thought a golden mirror the noblest. By the brass ones perhaps are to be understood only such as were made of mixed copper. Did golden mirrors occur oftener, I should be inclined to refer the epithet rather to the frame or ornaments than to the mirror itself; for at present we say a gold watch, though the cases only may be of that metal.

Mirrors seem for a long time to have been made of a mixture of copper and tin, as is expressly said by Pliny153, who adds, that the best were constructed at Brundisium. This mixture, which was known to Aristotle, produces a white metal, which, on account of its colour, may have been extremely proper for the purpose, and even at present the same mixture, according to the careful experiments made by Mr. Mudge, an Englishman154, produces the best metal for specula. It appears that the ancients had not determined the proportion very accurately; for Pliny assures us twice that in his time mirrors of silver were preferred. It is indeed not easy to ascertain the quantity of each metal that ought to be taken, and the most advantageous degree of heat; upon which a great deal depends. One of the principal difficulties is to cast the metal without blisters or air-holes, and without causing any part of the tin to oxidize, which occasions knots and cracks, and prevents it from receiving a fine polish. A passage of Lucian155, which no one as yet has been able to clear up, alludes certainly, in my opinion, to these faults. A mixture of copper and tin is so brittle, that it is very liable to crack; and a mirror formed of it, if not preserved with great care, soon becomes so dim, that it cannot be used till it has been previously cleaned and polished. For this reason a sponge with pounded pumice-stone was generally suspended, from the ancient mirrors, and they were kept likewise in a case or box, as may be seen by the greater part of those still extant. Mirrors of silver were less subject to this inconvenience, and I am inclined to think that the latter on this account made the former be disused, as we are informed by Pliny.

As ancient mirrors of metal are still to be found in collections of antiquities, it might be of some importance to the arts if chemical experiments were made on their composition. Those who have hitherto given us any account of them have contented themselves with describing their external figure and shape. Count Caylus156 is the only person, as far as I know, who caused any chemical experiments to be undertaken on this subject. They were made on a mirror found near Naples, by M. Roux, who asserts that the composition was a mixture of copper and regulus of antimony, with a little lead. Antimony however was not known to the ancients. If that metal was really a component part, the mirror must have been the work of more modern times, or it must be allowed that the artist had metal combined with antimony without knowing it; but the latter is not probable. The experiments made by Roux do not seem to me to have proved in a satisfactory manner the presence of regulus of antimony; moreover, no certain information can be derived from them, for the antiquity of the mirror was not ascertained; nor was it known whether it ought to be reckoned amongst the best or the worst of the period when it was made.

Those mirrors, which were so large that one could see one’s self in them at full length, must, in all probability, have consisted of polished plates of silver; for to cast plates of such a size of copper and tin would have required more art than we can allow to those periods; and I do not know whether our artists even now would succeed in them157.

We read in various authors, that, besides metals, the ancients formed stones into mirrors, which were likewise in use. It is undoubtedly certain that many stones, particularly of the vitreous kind, which are opake and of a dark colour, would answer exceedingly well for that purpose; but let the choice have been ever so good, they would not, in this respect, have been nearly equal to metals. These of all mineral bodies have the most perfect opacity; and for that reason the greatest lustre: both these properties are produced by their solidity; and hence they reflect more perfectly, and with more regularity, the rays of light that proceed from other bodies. Our glass mirrors, indeed, are properly metallic. Stones, on the other hand, have at any rate some, though often hardly perceptible, transparency; so that many of the rays of light are absorbed, or at least not reflected. Mention of stone mirrors occurs also so seldom in the ancients, that we may conclude they were made rather for ornament than real utility. In general, we find accounts only of polished plates or panels of stone, fixed in the walls of wainscoted apartments, which were celebrated on account of their property of reflection.

Pliny158 praises in this respect the obsidian stone, or, as it is now called, the Icelandic agate. Everything that he says of it will be perfectly intelligible to those who are acquainted with this species of stone or vitrified lava. The image reflected from a box made of it, which I have in my possession, is like a shadow or silhouette; but with this difference, that one sees not only the contour, but also the whole figure distinctly, though the colours are darkened. To form it into images and utensils, which Pliny speaks of, must have been exceedingly difficult, on account of its brittleness. I saw at Copenhagen, among other things made of it, a drinking-cup and cover, on which the artist had been employed four years.

Domitian, when he suspected that plots were formed against him, caused a gallery, in which he used to walk, to be lined with phengites, which by its reflection showed everything that was done behind his back159. Under that appellation we are undoubtedly to understand a calcareous or gypseous spar, or selenite, which is indeed capable of reflecting an image; but we cannot therefore pretend to say that the ancients formed mirrors of it; nor do I explain what Pliny says, where he speaks of the phengites, as if whole buildings had been once constructed of it160. That kind of stone, for various reasons, and particularly on account of its brittleness, is altogether unfit for such a purpose. At those periods, the windows of houses were open, and not filled up with any transparent substance, but only covered, sometimes by lattices or curtains. It is probable, therefore, that those openings of the walls of the building mentioned by Pliny, where the windows used to be, were filled up with phengites, which, by admitting a faint light, prevented the place from being dark even when the doors were shut; so that Pliny might say, “It appeared as if the light did not fall into the building, but as if it were inclosed in it.”

I might be accused of omission did I not here mention also a passage of Pliny161, where he seems to speak of a mirror made of an emerald, which Nero used to assist him to see the combats of the gladiators. Cary asserts that Nero was short-sighted, and that his emerald was formed like a concave lens. The former is expressly said by Pliny162, but the latter, though by Abat considered not improbable163, I can scarcely allow myself to believe, because such an interpretation of Pliny’s words is too forced, and because they can be explained much better in another manner. As no mention of such an excellent help to short-sighted people is to be found in any other ancient author, we must allow, if Cary’s opinion be adopted, that this property of the concave emerald was casually remarked, and that no experiments were made to cut any other natural or artificial glass in the same form for the like use, because people imagined that this property was peculiar to the emerald alone, which was then commonly supposed to be endowed with the power of greatly strengthening the eye-sight. Much more probable to me is the explanation of an Italian, which Abat also does not entirely reject, that the emerald had a smooth polished surface, and served Nero as a mirror164; and the passage of Pliny alluded to seems to have been thus understood by Isidore165 and Marbodæus. It may here be objected, that real emeralds are too small to admit of being used as mirrors; but the ancients speak of some sufficiently large for that purpose, and also of artificial ones166; so that we may with certainty conclude, that they classed among the emeralds fluor-spar green vitrified lava, or the green Icelandic agate as it is called, green jasper, and also green glass. The piece of green glass in the monastery of Reichenau, which is seven inches in length, three inches in thickness, and weighs twenty-eight pounds three-quarters; and the large cup at Genoa, which is however full of flaws167, have been given out to be emeralds even to the present time.

Mirrors were made also of rubies, as we are assured by Pliny168, who refers to Theophrastus for his authority; but this precious stone is never found now of such a size as to render this use possible; and Gary and the anonymous Italian before-mentioned have proved very properly that Pliny has committed a gross mistake, which has not been observed by Hardouin. Theophrastus, in the passage alluded to169, does not speak of a ruby, but of the well-known black marble of Chio, though he calls both carbunculus, a name given to the ruby on account of its likeness to a burning coal, and to the black marble on account of its likeness to a quenched coal or cinder; and the latter, as well as the obsidian stone, was used sometimes for mirrors.

The account how mirrors were formed by the native Americans, before they had the misfortune to become acquainted with the Europeans, is of considerable importance in the history of this art. These people had indeed mirrors which the Europeans could not help admiring. Some of them were made of black, somewhat transparent, vitrified lava, called by the Spaniards gallinazo, and which is of the same kind as the obsidian stone employed by the Romans for the like purpose. Of this substance the Americans had plane, concave, and convex mirrors. They had others also made of a mineral called the Inca’s stone170, which, as has been already said by Bomare, Sage, Wallerius, and other mineralogists, was a compact pyrites or marcasite, susceptible of a fine polish; and on that account often brought to Europe, and worn formerly in rings under the name of the stone of health. Ulloa says the Inca’s stone is brittle, opake, and of a somewhat bluish colour; it has often veins which cannot be polished, and where these veins are it frequently breaks. The mirrors formed of it, which he saw, were from two to three inches in diameter; but he saw one which was a foot and a half. The opinion which some have entertained, that these mirrors were cast, has no other foundation than the likeness of polished marcasite to cast brass. This mineral is very proper for reflecting images; and I am inclined to think that the Peruvians had better mirrors than the Greeks or the Romans, among whom we find no traces of marcasite being employed in that manner. It appears, however, that the Indians had mirrors also of silver, copper, and brass171.

I come now to the question in what century were invented our glass mirrors, which consist of a glass plate covered at the back with a thin leaf of metal. This question has been answered by some with so much confidence, that one might almost consider the point to be determined; but instead of real proofs, we find only conjectures or probabilities; and I must here remark, that I cannot help thinking that they are older than has hitherto been supposed, however desirous I may be to separate historical truth from conjecture. When I have brought together everything which I know on the subject, I would say, that attempts were even made at Sidon to form mirrors of glass; but that they must have been inferior to those of metal, because they did not banish the use of the latter. The first glass mirrors appear to me to have been of black-coloured glass, or an imitation of the obsidian stone; and to have been formed afterwards of a glass plate with some black foil placed behind it172. At a much later period, blown glass, while hot, was covered in the inside with lead or some metallic mixture; and still later, and, as appears, first at Murano, artists began to cover plates of glass with an amalgam of tin and quicksilver. The newest improvements are, the casting of glass-plates, and the art of making plates equally large by blowing and stretching, without the expensive and uncertain process which is required for casting.

That glass mirrors were made at the celebrated glass-houses of Sidon, is mentioned so clearly by Pliny that it cannot be doubted173. When I read the passage, however, without prejudice, without taking into consideration what others have said on it, and compare it with what certain information the ancients, in my opinion, give on the same subject, I can understand it no otherwise than as if the author said, that the art of manufacturing glass various ways was invented, principally, at Sidon, where attempts had been made to form mirrors of it. He appears therefore to allude to experiments which had not completely succeeded; and to say that such attempts, at the time when he wrote, had been entirely abandoned and were almost forgotten. Had this circumstance formed an epoch in the art, Pliny, in another place, where he describes the various improvements of it so fully, would not have omitted it; but of those experiments he makes no further mention174. All the inventions which he speaks of, evidently relate to metal mirrors only, of which the silver, at that time, were the newest. Had the Sidonian mirrors consisted of glass plates covered at the back, those of metal, the making of which was, at any rate, attended with no less trouble, which were more inconvenient for use on account of their aptness to break, their requiring to be frequently cleaned and preserved in a case, and which were more unpleasant on account of the faint, dull image which they reflected, could not possibly have continued so long in use as they really did; and circumstances and expressions relative to glass mirrors must certainly have occurred. Though glass continued long to be held in high estimation, particularly at Rome; and though many kinds of glass-ware are mentioned in ancient authors, among costly pieces of furniture, mirrors are mentioned only among articles of silver plate. I am acquainted with no certain trace of glass mirrors from the time of Pliny to the thirteenth century; but after that period, at which they are spoken of in the clearest manner, we find them often mentioned in every century; and mirrors of metal at length entirely disappear.

How the Sidonian mirrors were made, is not known; but if I may be allowed a conjecture, I am of opinion that they consisted of dark-coloured glass, which had a resemblance to the obsidian stone. Such is the usual progress of inventions. At those periods one had no other representation of glass mirrors than that afforded by natural glass or vitreous stones. When artists wished to make mirrors of glass, they would try to imitate the latter. After the invention of printing, people endeavoured to render printed books as like as possible to manuscripts; because they imagined that this invention was to be approved only so far as it enabled them to imitate these, without observing that it could far excel the art of writing. But the Sidonian glass mirrors were so much surpassed by the silver or brass ones, which perhaps were invented about the same time, that on this account they were never brought into use. Glass mirrors, perhaps, would have been invented sooner, had mankind employed at an earlier period glass-windows, which often, when they are shut on the outside so that no light can pass through them, reflect images in a much better manner than the best mirrors of metal. This observation, which may be made daily, would then, in all probability, have been sooner turned to advantage.

No one has employed a greater profusion of words to maintain an opinion opposite to mine, than Abat; but when his proofs are divested of their ornaments, they appear so weak that one has very little inclination to agree with him. “The observation,” says he, “that a plate of glass is the best mirror, when all other rays of light, except those reflected back from the glass, are prevented, by a metallic covering placed behind it, from falling on the eye, is so easy, that it must have been made immediately after the invention of glass.” Who does not think here of Columbus and his egg? Instances occur in history of many having approached so near an invention, that we are astonished how they could have missed it; so that we may exclaim with a certain emperor, “Taurum toties non ferire difficile est175.” “The Sidonian invention,” continues he, “would not have been worth mentioning, had it not produced better mirrors than those which the ancients had before of the obsidian stone. But these even are mentioned only once, in so short and abrupt a manner, and as it were out of ridicule, that one may easily perceive they were not much esteemed.” “If the Sidonians,” adds he, “were not the inventors, let some other inventor be mentioned;” and he assures us that he had sought information on this subject, in Neri, Kunkel, and Merret, but without success. That I believe; but Abat does not remark that by the same manner of reasoning we may ascribe to the Sidonians the invention of watches, and many other articles, the inventors of which are not to be found in books where they ought as much to be expected as the inventor of glass in Neri. The grounds on which many old commentators of the Bible, Nicholas de Lyra and others, have supposed that glass mirrors were known so early as the time of Moses, are still weaker. If quoting the names of writers who entertain a like opinion be of any weight, I could produce a much greater number of learned men, who, after an express examination of the question, deny altogether that glass mirrors were used by the ancients.

Dr. Watson176 also has endeavoured to support the opinion of Abat, but with less confidence and with more critical acumen. His grounds, I think, I have weakened already; but one observation here deserves not to be overlooked, because it suggests an idea that may serve to illustrate a passage of Pliny, which, as I before remarked, has never yet been explained. “If we admit,” says he, “that Pliny was acquainted with glass mirrors, we may thus understand what he says respecting an invention, which was then new, of applying gold behind a mirror.” Instead of an amalgam of tin, some one had proposed to cover the back of the mirror with an amalgam of gold, with which the ancients were certainly acquainted, and which they employed in gilding177. He mentions, also, on this occasion, that a thought had once occurred to Buffon, that an amalgam of gold might be much better for mirrors than that used at present178. This conjecture appears, at any rate, to be ingenious; but when I read the passage again, without prejudice, I can hardly believe that Pliny alludes to a plate of glass in a place where he speaks only of metallic mirrors; and the overlaying with amalgam requires too much art to allow me to ascribe it to such a period without sufficient proof. I consider it more probable that some person had tried, by means of a polished plate of gold, to collect the rays of light, and to throw them either on the mirror or the object, in order to render the image brighter.

Professor Heeren showed me a passage in the Ecloga of Stobæus, which, on the first view, seems to allude to a glass mirror179. It is there said, Philolaus the Pythagorean believed that the sun was a vitreous body, which only received the rays of the æthereal fire and reflected them to us like a mirror. When we compare, however, the words of Stobæus with those by which Plutarch180, Achilles Tatius181, Eusebius182, and others, express the same thing, that meaning cannot be drawn from them. It appears, at first, as if Philolaus had considered the sun to be transparent, and supposed that the rays passed through it, and came condensed to our earth, in the same manner as they are brought to a focus by a glass globe. Some commentators have explained the passage in this manner; and on account of the affinity of the Greek words have thought also of a funnel. In that case, however, the comparison of the sun with a mirror would not have been just; and if it be admitted that Philolaus considered the sun as a bright body endowed with the property of reflection, what he says of rays passing or transmitted through it, and of the pores of the sun’s body, will become unintelligible. But even if we adopt the last explanation, that Philolaus imagined the sun to be a mirror, it does not follow that he had any idea of a glass one183; and besides, he only speaks of a body capable of reflecting a strong light; and that glass, under certain circumstances, is fit for that purpose, may have been remarked as soon as it was invented, though men might not find out the art of forming it into proper mirrors by placing some opake substance behind it184. Empedocles also said, that the sun was a mirror, and that the light received by our earth was the reflection of the æthereal fire, which Eusebius compares to the reflection made by water185.

In the problems ascribed to Alexander of Aphrodisias, glass mirrors, covered on the back with tin, are clearly mentioned; but this information does not lead us one step further in the history of the art; as it is proved that the above Alexander, who lived in the beginning of the third century, could not have written that work. The author, who must have been a physician, maintains the immortality of the soul, which Alexander of Aphrodisias, with Aristotle, denies. Some therefore have ascribed these problems to Alexander Trallianus, who practised physic in the middle of the sixth century; but this is only a conjecture which no one has as yet rendered probable, especially as there have been many physicians of the name of Alexander. The problem to which I allude is not to be found in every manuscript and edition; so that it is doubtful whether it may not be the production of a later author than that of the rest of the book, particularly as it is certain that many who had it in their possession added problems of various kinds according to their pleasure. However this may be, it is evident that the author of this problem was acquainted with mirrors covered at the back; and the expression which he uses does not merely imply that a leaf of tin was placed behind the glass plate, but that the tin in a liquid state was rubbed over it. The old French translator thinks that the author speaks of windows; but that opinion is undoubtedly false186.

Of as little importance as the above passage of Alexander, is another of Isidore, often quoted in support of the antiquity of glass mirrors. On the first view it appears to be a testimony of great weight; but when closely examined it becomes reduced to very little. “Nothing,” says he, “is so fit for mirrors as glass187.” Abat and others, who have considered these words as decisive, make less hesitation to ascribe to the sixth century, in which Isidore lived, a knowledge of mirrors covered on the back with tin and quicksilver, as the same writer, in another place, observes, that quicksilver can be kept in no vessel but one of glass188. It is very true that a glass filled with that metal will form a very good mirror; but I am of opinion that this may have been long known, before people thought of making an amalgam of tin and quicksilver in order to cover the backs of mirrors. The first passage, which is properly the one of any consequence, loses its force when we see that it is taken from Pliny and copied incorrectly. The latter says, that one can give to glass every kind of shape and colour, and that no substance is more ductile, or fitter to be moulded into any form189. Isidore, as is usual, says the same thing, and in the same words, except, that instead of sequacior he substitutes speculis aptior; so that the mention of a mirror is altogether unexpected, and so little suited to what goes before and what follows, that one must believe that this alteration, occasioned perhaps by the similitude of the words, or by an abbreviation, was not made by Isidore, but by some transcriber. But even if we believe that Isidore himself spoke of glass being used at that period for mirrors, we are not able to comprehend, from what he says, how glass mirrors were made in the sixth century.

I have met with no information respecting this subject in the whole period between the age of Isidore and the eleventh century. About the year 1100, at least as is supposed not without probability, Alhazen the Arabian wrote his well-known treatise on Optics, in which I conjectured that I should find mention made of glass mirrors; but I searched that work in vain, though I must confess I did not read it through entirely. Where he begins his catoptrical lessons, he however often speaks of iron mirrors, by which we may understand mirrors of the best steel. In explaining a certain phænomenon, he says, that the cause of it cannot be in the darkness of the iron mirror, because if a mirror of silver be used, the same effects will be produced. Would he not on this occasion have introduced glass mirrors, had he been as well-acquainted with them as with those already mentioned? At first, he never speaks of mirrors without adding of iron, of silver; but he mentions them afterwards without any epithet of the kind.

All these mirrors I find also in the Optics of Vitello, who wrote in the middle of the thirteenth century, in Italy, a country which was at that time almost the only one where the arts flourished190. That author has, indeed, borrowed a great deal from Alhazen, though there are many things of his own, and he gives an account of some experiments on the refracting power of glass; but he never, as far as I have observed, mentions glass mirrors. Whether Jordanus Nemorarius, or Nemoratius, who also wrote, in the thirteenth century, a book De Speculorum Natura, makes mention of them, I do not know, because I have never had an opportunity of seeing that work. I am of opinion it was never printed.

It is in the thirteenth century, that I find the first undoubted mention of glass mirrors covered at the back with tin or lead. Johannes Peckham, or Peccam, an English Franciscan monk, who taught at Oxford, Paris, and Rome, and who died in 1292, wrote about the year 1279 a treatise of optics, which was once printed, with the title of Johannis Pisani Perspectiva Communis191. In this work, besides mirrors made of iron, steel, and polished marble, the author not only speaks often of glass mirrors, but says also that they were covered on the back with lead, and that no image was reflected when the lead was scraped off. Vincentius Bellovacensis192 speaks in a manner still clearer, for he tells us that lead was poured over the glass plate while hot. To the same century also belong the concurrent testimonies of Raimundus Lullius193, Roger Bacon194, Antonius di Padua195, and Nicephorus Gregoras196, who died after the year 1360197.

That this invention cannot be much older we have reason to conclude, because glass mirrors were extremely scarce in France even in the fourteenth century, while mirrors of metal were in common use; and we are told that the mirror of Anne de Bretagne, consort of Louis XII., was of the latter kind198. Metal mirrors also were made and employed in Persia and the East, where indeed ancient usages continued longest, and glass mirrors were not known there till the commencement of the European trade with these remote regions. The former are still preferred in those countries, because they are not so liable to break, and can be preserved better in a dry hot climate than the amalgam of the latter.

Respecting the progress of this art, I know nothing more than what follows: – At first, melted lead, or perhaps tin, was poured over the glass plate while yet hot as it came from the furnace. This process agrees with that which, since very early periods, has been employed in or around Nuremberg for making convex mirrors by blowing with the pipe into the glass-bubble whilst still hot a metallic mixture, with a little resin or salt of tartar, which prevents oxidation and assists the fusion. When the bubble is covered all over in the inside, and after it has cooled, it is cut into small round mirrors. This art is an old German invention, for it is described by Porta and Garzoni, who both lived in the beginning of the sixteenth century, and who both expressly say, that it was then common in Germany. Curious foreigners often attempted to learn it, and imagined that the Germans kept it a secret. Boyle made various experiments in order to discover the process; and the secretary of the Royal Society endeavoured, by means of the ambassador from Charles II., who, perhaps about 1670, resided at Frankfort, to obtain a knowledge of it; but did not succeed, as we are told by Leibnitz199. It was called the art of preparing mirrors without foil; and it was highly esteemed, because it was supposed that it might be useful to those fond of catoptrics, by enabling them to form convex and concave mirrors themselves. This account of Leibnitz seems to have led Von Murr into a slight error, and induced him to believe that the art of making convex mirrors without foil was first found out at Nuremberg in 1670. I introduce this remark because I flatter myself he will not be displeased that I make the above service, rendered by his native city, to be a century and a half older. These small convex mirrors, which reflect a diminished, but a clearer image than our usual mirrors, are perhaps made still, though they are not now carried round so frequently for sale in Germany as they were thirty years ago, at which time, if I remember right, they were called (Ochsen-augen) ox-eyes. They were set in a round painted board, and had a very broad border or margin. One of them, in my possession, is two inches and a half in diameter. It is probable that the low price of plane mirrors, when glass-houses began to be more numerous, occasioned these convex ones to be little sought after. The mixture employed in making them was, according to Porta, antimony, lead, and colophonium; but according to Garzoni, it was una mistura di piombo, stagno, marchesita d’argento, e tartaro, which in the German edition is translated very badly, “lead, tin, flint, silver, and tartar.” The following observation perhaps is not altogether useless: Colophonium, which is employed on many other occasions for soldering, was formerly called mirror-resin, and was sold under that name even in the beginning of the present century. Frisch assigns no reason for this appellation, and Jacobson gives a wrong one, viz. its having a bright shining surface when broken. The true reason was the above-mentioned use; and as that is now very little known, it is called from that to which it is principally applied, violin-resin.

It appears that, instead of pouring melted metal over plates of glass, artists for some time applied to them the before-mentioned amalgam of tin, or covered them in some other manner, perhaps in the same way as Boyle covered concave glasses in the inside. Porta however saw almost the same process employed at Murano as that which is still followed at present. The tin, hammered to thin leaves, was spread out very smoothly; and quicksilver was poured over it, and rubbed into it, either with the hand or a hare’s foot; and when the tin was saturated it was covered with paper. The glass, wiped exceedingly clean, was then laid above it; and while the workman pressed it down with his left hand, he drew out very carefully with his right the paper that lay between the tin and the glass, over which weights were afterwards placed. This much at any rate is certain, that the method of covering with tin foil was known at Murano so early as the sixteenth century200, and therefore it is much older than J. M. Hoffmann supposes. To conclude, whether this ingenious invention belongs to the Venetians, as several later, and particularly Italian, writers assert, I can neither prove nor contradict; but it is well known that till about the end of the seventeenth century their mirrors were sold all over Europe and in both the Indies. After that period the glass-houses in other countries were improved, and new ones established; and the discovery made in France, that glass, like metal, could be cast into much larger plates than had been before prepared by blowing and rolling, was in more than one respect prejudicial to the sale of those made at Venice.

So early as the year 1634, attempts were made in France to establish glass-houses for manufacturing mirrors, and Eustache Grandmont obtained a patent for that purpose; but his undertaking was not attended with success. As Colbert exerted himself very much to promote manufactures of every kind, Nicholas de Noyer proposed to make mirrors according to the Venetian method. This plan was adopted by Charles Rivière, sieur du Freni, valet-de-chambre to the king; and having procured the royal permission, he sold it afterwards for a large sum to De Noyer, who, in 1665, received a confirmation of the patent, and an advance of 12,000 livres for four years, on condition of his procuring workmen from Venice, who, after serving eight years in the kingdom, were to be naturalized. De Noyer was joined by several more, who entered into partnership with him, and particularly by one Poquelin, who had hitherto carried on the greatest trade in Venetian mirrors, and who engaged workmen from Murano. The glass-houses were erected at the village of Tourlaville, near Cherbourg, in Lower Normandy. After the death of Colbert, who was succeeded by Louvois, the charter of the company was in 1684 renewed for thirty years longer, and at that period Pierre de Bagneux was at the head of it.

Scarcely had five years of this period elapsed, when, in 1688, Abraham Thevart made a proposal to the court for casting glass mirrors of a much larger size than any ever before made. This plan, after an accurate investigation, was approved; and in the same year he received the royal permission to use his invention for thirty years, but it was not registered till 1693 or 1694. The first plates were cast at Paris, and astonished every artist who saw them. They were eighty-four inches in height, and fifty in breadth. In order to lessen the excessive expense, the glass-houses were erected at St. Gobin, in Picardy; and to prevent all dispute with the old privileged company, Thevart was expressly bound to make plates at least sixty inches in length and forty in breadth, whereas the largest of those made before had never exceeded forty-five or fifty inches in length. On the other hand, the old company were allowed to make plates of a smaller size, and were prohibited from employing any of the instruments or apparatus invented by Thevart. These however had not been so accurately defined as to remove all cause of litigation between the companies, and for that reason permission was at length granted, in 1695, for both to be united into one, under the inspection of François Plastrier, to whom the king, in 1699, sold the palace of St. Gobin. After this they declined so rapidly, that in 1701 they were not able to pay their debts, and were obliged to abandon several of the furnaces. To add to their misfortune, some of the workmen whom they had discharged retired to other countries, which were already jealous of the French invention, and wished to turn it to their advantage. The French writers assert that their attempts never succeeded, and that most of the workmen returned again to France, when a new company was formed in 1702, under the management of Antoine d’Agincourt, who by prudent œconomy improved the establishment, so as to render the profit very considerable. At present mirrors are cast as well as blown, both at St. Gobin and at Cherbourg; and in 1758 the price of them was greatly reduced, in order probably to weaken the competition of the foreign glass-houses, among which there are many not inferior to the French.

This short history of the glass manufactories in France is collected from Savary201 and Expilly202. A more particular account perhaps may be expected of the inventor, of his first experiments, and of their success; but notwithstanding a strict search, I have not been able to find any further information on the subject. We are told only that his name was sieur Abraham Thevart, though the historians who record that circumstance have filled their pages with uninteresting anecdotes, and even with the vices of many of the courtiers of the same period.

The principal benefit which has arisen to the art from this invention, properly is, that much larger mirrors can be obtained than formerly; for when attempts were made to blow very large plates, they were always too thin. Casting, however, besides great expense in apparatus203, requires so many expert workmen, and so tedious and severe labour, and is accompanied with so much danger, that it is only seldom that plates of an extraordinary size succeed, and the greater part of them must be cut into smaller plates which might have been blown. Those cast are never so even and smooth as those that have been blown; they require therefore a great deal of polishing, and on that account must be very thick. The monstrous mass requisite for a mirror of the largest size, stands ready melted in a very frail red-hot earthen pot, which is taken from the furnace and placed upon an iron plate, strongly heated, that the mass may be cast upon it into a glass plate. The latter must then be speedily conveyed to the cooling-furnace, and if it be found free from faults, it is ground, polished and silvered; but the last part of the process is generally done at the place where a purchaser can be found for so expensive an article, in order that less loss may be sustained in case it should happen to break by the way.

These great difficulties, which have excited the astonishment of every one who has seen the process, and that of finding sale for so expensive and magnificent wares, have obliged artists to return to the old method of blowing; and many have been so fortunate in improving this branch of manufacture, that plates are formed now by blowing, sixty-four Flemish inches in height and twenty-three in breadth, which it was impossible to make before but by casting.

The mass of matter necessary for this purpose, weighing more than a hundred pounds, is by the workman blown into the shape of a large bag; it is then reduced to the form of a cylinder, and being cut up, is, by stretching, rolling it with a smooth iron, and other means, transformed into an even plane.

[All but the very commonest mirrors are now made of plate-glass; which is also used to a great extent for window-panes, and is manufactured by casting, rolling and polishing. The enormous plates of glass which are seen in many of the large shops of this city are well-calculated to excite the astonishment of those who are not yet aware of the late improvements in this branch of manufacture. An idea of what may be accomplished by blowing was given in 1845, at the Exhibition at Vienna, where a blown glass 7 feet in length and 3½ in breadth was exhibited; and which was of sufficient thickness to admit of polishing. Nevertheless, the casting of plate-glass is now managed with such comparative ease, that there appears to be no limit to the size to which the plates can be brought, so that the blowing of large panes of glass is given up in this country. Private houses may now be seen decorated with single sheets of glass upwards of 20 feet in height and 10 in width.

A patent for a very ingenious process for silvering glass was taken out in November 1843 by Mr. Drayton. It consists in depositing silver, from a solution, upon glass, by deoxidizing the oxide of silver in solution, so that the precipitate will adhere to the glass, without the latter having been coated with metallic or other substances. This is effected by mixing 1 oz. of coarsely powdered nitrate of silver with ½ oz. of spirits of hartshorn and 2 oz. of water; after standing for 24 hours, the mixture is filtered (the deposit on the filter, which contains silver, being preserved), and an addition is made thereto of 3 oz. of spirit (by preference, spirit of wine) at 60° above proof, or naphtha; from 20 to 30 drops of oil of cassia are then added, and after remaining for about 6 hours longer, the solution is ready for use. The glass to be silvered must have a clean and polished surface; it is to be placed in a horizontal position, and a wall of putty formed around it, so that the solution may cover the surface of the glass to the depth of from ⅛th to ¼th of an inch. After the solution has been poured on the glass, from 6 to 12 drops of a mixture of oil of cloves and spirit of wine (in the proportion of 1 part by measure of the oil to 3 of spirit of wine) are dropped into it at different places; or the diluted oil of cloves may be mixed with the solution before it is poured upon the glass; the more oil of cloves used, the more rapid will be the deposition of the silver, but the patentee prefers that it should occupy about two hours. When the required deposit has been obtained, the solution is poured off; and as soon as the silver on the glass is perfectly dry, it is varnished with a composition, formed by melting together equal quantities of bees’ wax and tallow. The patentee states that, by experiment, he has ascertained that about 18 grs. of nitrate of silver are used for each square foot of glass.

It has been urged as an objection to this process, that in the course of a few weeks the surfaces of the mirrors formed by it become dotted over with small brownish-red spots, which greatly injures their appearance. Dr. Stenhouse states that these spots are caused by the metallic silver, whilst being deposited on the surface of the glass, carrying down with it mechanically small quantities of a resinous matter, resulting most probably from the oxidation of the oil. This subsequently acts upon the metallic surface with which it is in contact, and produces the small brown spots already mentioned. Mr. Drayton, however, states that the brown spots only occur when the oil employed is old and unfit for use.]