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A Table Containing the General Heads of Natural Magick

"Preface To The Reader" 

 The Seventeenth Book of Natural Magick

John Baptista Porta

(Giambattista della Porta)


"Of Strange Glasses"

("Wherein are propounded Burning-glasses, and the wonderful sights to be seen by them.")

"The Proeme"

Chapter I - "Diverse representations made by plain Glasses."

Chapter II - "Other merry sports with plain Looking-Glasses."

Chapter III - "A Looking-glass called a Theatrical Glass."

Chapter IV - "Diverse operations of Concave-glasses."

Chapter V - "Of the mixed operations of the plain Concave-glasses."

Chapter VI - "Other operations of a Concave-Glass."

Chapter VII - "How you may see in the dark what is light without by reason of torches."

Chapter VIII - "How without a Glass or representation of any other thing, an image may seem to hang in the air."

Chapter IX - "Mixtures of Glasses, and diverse apparitions of images."

Chapter X - "Of the effects of a Lenticular Crystal."

Chapter XI - "Of Spectacles whereby one may see very far, beyond imagination."

Chapter XII - "How we may see in a Chamber things that are not."

Chapter XIII - "Of the operations of a crystal pillar."

Chapter XIV - "Of Burning Glasses."

Chapter XV - "Of a Parabolical Section, that is of all glasses the most burning."

Chapter XVI - "How a Parabolical Section may be described, that may burn obliquely, and at a very great distance."

Chapter XVII - "A Parabolical Section that may burn to infinite distance."

Chapter XVIII - "To make a Burning Glass of many Sparical Sections."

Chapter XIX - "Fire is kindled more forcible by refraction."

Chapter XX - "In a hollowed Glass how the image may hang without."

Chapter XXI - "How Spectacles are made."

Chapter XXII - "How upon plain Concave and Convex Glasses, the foils are laid on and they are banded."

Chapter XXIII - "How metal Looking-Glasses are made."

The Proeme

Now I am come to Mathematical Sciences, and this place requires that I show some experiments concerning Catoptrick glasses.  For these shine among geometrical instruments, for ingenuity, wonder and profit.  For what could be invented more ingeniously, then that certain experiments should follow the imaginary conceits of the mind, and the truth of mathematical demonstrations should be made good by ocular experiments?  What could seem more wonderful, then that by reciprocal strokes of reflection, images should appear outwardly, hanging in the air, and yet neither the visible object nor the glass seen?  That they may seem not to be the repercussion of the glasses, but spirits of vain phantasms?  To see burning glasses, not to burn alone where the beams unite, but at a great distance to cast forth terrible fires, and flames, that are most profitable in warlike expeditions, as in many other things.  We read that Archimedes at Syracuse with burning glasses defeated the forces of the Romans.  And that King Ptolomey built a tower in Pharos, where he set a glass, that he could see for six hundred miles, see by it the enemy ships, that invaded his country and plundered it.  I shall add also those spectacles, whereby poor blind people can at great distance, perfectly see all things.  And though venerable antiquity seem to have invented many and great things, yet I shall set down greater, more noble, and more famous things, and that will not a little help to the optic science, that more sublime wits may increase it infinitely.  Lastly, I shall show how to make Crystal and metal glasses, and how to polish them.

Chapter I

"Diverse representations made by plain Glasses."

shall begin with plain glasses, for they are more simple, and the speculations thereof, are not so laborious, though the apparitions of them be almost common, yet they will be useful for what follows.  And we shall add some secret apparitions unto them.  The variety of the images that appear, proceed either from the matter or form of the glass.  Crystal must be clear, transparent, and exactly made plain on both sides.  And if one or both of these be wanting, they will represent diverse and deformed apparitions to our sight.  I shall therefore begin from the matter, and show,

"How apparitions may seem to him that looks upon them, to be pale, yellow, or of diverse colors."

When the glass is melted with heat in the furnace, with any little color it will be tainted.  If you cast in yellow, the face of him that looks into it, will seem to have the Yellow Jaundices.  If black, he will appear wan and deformed.  If you add much of it, like to a Blackmoore.  If red, like a drunkard or furious fellow.  And so will it represent images of any color.  How to mingle the colors, I taught when I spoke of jewels.  I have often made sport with the most fair women, with these glasses.  When they looked, and saw not themselves as they were, but there are many varieties arise from the form.

"That the face of him that looks on the Glass may seem to be divided in the middle,"

Let the superficies of the Looking-glass that you look on, be plain, and exactly polished by rule.  But the backside must have a blunt angle in the middle, that the highest part of it may be in the middle.  In the outward parts it must be sharp and pressed down;  Then lay on the Foil.  Wherefore the image that falls on your sight, where the lines meet in the angle, will seem divided in two.  If you will,

"That he that looks in the Glass, shall seem like an Ass, Dog, or Sow."

By variation of the place, the angles, and the representation of the form beheld, will seem various.  If that part of the glass, that is set against your mouth, shall stick forth before like a wreathed band or a Boss-buckler, you mouth will appear to come forth like an ass's or sows snout.  But if it swell forth against your eyes, your eyes will seem to be put forth like shrimps eyes.  If the angle be stretched forth by the length of the glass, your forehead, nose, and chin, will seem to be sharp, as the mouth of a Dog.

That the whole face may seem various and deformed."

Let a plain glass not be exactly plain and even.  Which that it may be done, when the glass is once made plain, put it into the furnace again, and let it be turned by the skillful hand of an artist, till it lose its right position, then soil it.  Then the image on the hollow part of the glass, will represent the opposite part hollow.  So it will hold forth one lying along on his face, or crooked, and swelling outwardly and inwardly.  Then if when the glass is polished, one side be rubbed, the face will seem long and broad.  Wherefore it must be rubbed, and fashioned on all sides, that it may every way represent a perfect face.  I shall show you also,

"How to make a Glass to represent many images."

That it may show diverse images one after another, and of diverse colors, make the solid body of the Looking-glass, or glass that is half a finger thick, and let it be so planed, that upon one side, the thickness may not be touched, but on the other side, the lines of the two superficies may meet, as the sharp edge of a knife.  Make also another table of glass the same way.  Or else more, lay a Foil of Tin upon the last, and place one of them upon the thick part of the other.  So will the face of one that looks into it, appear to be two, one behind the other, and the nethermost will always appear darkest.  So if by the same artifice, you fit three tables of glass, the image will appear to be three, and the farther he that looks, stands with his face from the glass, the farther will those images or faces stand asunder.  But as you come very near, they seem to join all in one.  If you hold a candle lighted against it, there will be many seen together, which comes by the mutual reciprocation of the sight and the glass.  And if the polishers of glasses, putting one aptly above another, but let one be distant from the other by certain courses, then shut them in a frame, that the art may not be discovered.  Nor will I omit,

"How letters may be cast out and read, on a wall that is far distant."

Which we shall do with the same plain glass.  And lovers that are far asunder, may so hold commerce one with another.  On the superficies of a plain glass, make letters with black ink, or with wax, that they may be solid to hinder the light of the glass, and shadow it.  Then hold the glass against the sunbeams, so that the beams reflecting on the glass, may be cast upon the opposite wall of a chamber, it is no doubt but the light and letters will be seen in the chamber, the suns light will be clearest, and letters not so bright, so that they will be clearly discovered, as they are sent in.

Chapter II

"Other merry sports with plain Looking-glasses."

Now I shall annex some other operations of a plain glass, described by our ancestors, that I may seem to leave out nothing.  And I will so augment them, and bring them to a rule, that they may be easily made.  I shall begin with this,

"How by plain Looking-glasses, the head may appear to be downwards, and the heels upwards."

If any man by plain glasses, desires to see his head downward, and his feet upward (though it is proper for Concave glasses to represent that) yet I will endeavour to do it by plain glasses.  Place two glasses longways, that they may stick together and cannot easily come asunder, or move here and there, and that they make a right angle.  When this is so done, according to coherence the long way, set this against your face, that in one, half the face, in the other the other half may be seen.  Then incline the Looking-glass to the right or left hand, looking right into it, and your head will seem to be turned, for according to their latitude, they will cut the face into two, and the image will appear so, as if the head were under, and the heels upwards.  And if the glass be large, the whole body will seem to be inverted.  But this happens from the mutual and manifold reflection, for it flies from one to the other, that it seems to be turned.  We may,

"Make a plain glass that shall represent the Image manifold."

A glass is made that will make many representations, that is, that many things may be seen at once.  For by opening and shutting it, you shall see twenty fingers for one, and more.  You shall make it thus;  Raise two Brass Looking-glasses , or of Crystal, at right angles upon the same Brass, and let them be in a proportion called Sesquialtera, that is, one and half, or some other proportion, and let them be joined together longways, that they may be shut and opened like to a book.  And the angles be diverse, such as are made at Venice.  For one face being objected, you shall see many in them both, and this by so much the straighter, as you put them together, and the angles are less.  But they will be diminished by opening them, and the angles being more obtuse, you shall see the fewer.  So showing one figure, there will be more seen.  And farther, the right parts will show right, and the left to be the left, which is contrary to Looking-glasses.  And this is done by mutual reflection and pulsation, whence arises the variety of images interchangeably.  We may,

"Make a Glass of Plain glasses, wherein one image coming, is seen going back in another."

Take two plain glasses, the length whereof shall be double, or one and half to the latitude, and that for greater convenience.  For the proportion is not material, but let them be of the same length, and equal, and laid on the top of a pillar, inclining one to the other, and so joined together.  And let them be set upright upon someplace perpendicularly, so the glasses fastened, may be moved on the movable side.  It is no doubt but you shall see the image to come in one,and go back in the other glass.  And the more this comes near, the farther will the other go.  And in one will it be seen coming, and in the other going.  Also you may see,

"In plain Glasses those things that are done afar off, and in other places."

So may a man secretly see, and without suspicion, what is done afar off, and in other places, which otherwise cannot be done.  But you must be careful in setting your glasses.  Let there be a place appointed in a house or elsewhere, where you may see anything, and set a glass right over against your window, or hose, that may be toward your face, and let it be set straight up if need were, or fastened to the wall, moving it here and there.  Inclining it till it reflect right against the place.  Which you shall attain by looking on it, and coming toward it.  And if it be difficult, you cannot mistake, if you use a Quadrant or some such instrument.  And let it be se perpendicular upon a line, that cuts the angle of reflection, and incidence of the lines, and you shall clearly see what is done in that place.  So it will happen also in diverse places.  Hence it is, that if one glass will not do it well, you may do the same by more glasses.  Or if the visible object be lost by too great a distance, or taken away by walls or mountains coming between, moreover, you shall fit another glass just against the former, upon a right line, which may divide the right angle, or else it will not be done, and you shall see the place you desire.  For one glass sending the image to the other tenfold, and the image being broken by many things, flies from the eye, and you shall see what you first light upon, until such time as the image is brought to you by right lines, and the visible object is not stopped by the winding of places or walls.  And the placing of it is easy.  So often I use to convey images of things.  But if otherwise you desire to see any high place, or that stands upright, and your eye cannot discern it.  Fit two Looking-glasses together longways, as I said, and fasten one upon the top of a post or wall, that it may stand above it, and the object may stand right against it.  The other to a cord, that you may move it handsomely when you please, and that it may make with the first sometimes a blunt, sometimes a sharp angle, as need requires, until the line of the thing seen may be refracted by the middle of the second glass to your sight, and the angles of reflection and incidence be equal.  And if you seek to see high things, raise it.  If low things, pull it down, till it beat back upon your sight, and shall you behold it.  If you hold one of them in your hand, and look upon that, it will be more easily done.  I show you also,

"How to make a Glass that shall show nothing but what you will."

Also a glass is so framed, that when you look into it, you shall not see your own picture, but some other face, that is not seen anywhere round about.  Fasten a plain glass on a wall upon a plain, set upright perpendicularly, and bow the top of it to the known proportion of the angle.  Right against it cut the wall, according as the proportion of some picture or image may require, and set it by it, according to a fit distance, and cover it, that the beholder may not see it (and the matter will be the more wonderful) nor can come at it.  The glass at a set place will beat back the image, that there will be a mutual glance of the visible object and the sight, by the Looking-glasses.  There place your eye, you shall find that place, as I taught you before.  Wherefore the spectator going thither, shall neither see his own face, nor anything else besides.  When he is opposed to it, and comes to the set place, he shall see the image or the picture, or some such thing, which he can behold nowhere else.  You shall now how,

"How a Glass may be made of plain glasses, whereby you may see an image flying in the air."

Nor is that glass of less importance, or pleasure, that will represent men flying in the air.  If any man would do it, it is easily done thus.  Fit two pieces of wood together like a square or gnomon of a dial, and being well fastened, they may make an angle as of right angled triangle, or isosceles.  Fasten then at each foot one great Looking-glass, equally distant, right one against the other, and equidistant from the angle.  Let one of them lie flat, and let the spectator place himself about the middle of it, being somewhat raised above the ground, that he may the more easily see the form of the heel going and coming.  For presently you shall perceive, if you set yourself in a right line, that cuts that angle, and it be equidistant to the horizon.  So the representing glass will send that image to the other, which the spectator looks into, and it will shake and move the hands and feet, as birds do when they fly.  So shall he see his own image flying in the other, that it will always move, so he depart not from the place of reflection, for that would spoil it.

Chapter III

"A Looking-glass called a Theatrical Glass."

Prudent antiquity found out a Looking-glass made of plain glasses, wherein if one object might be seen, it would represent more images of the same thing, as we may perceive by some writings, that go in Ptolomy's name.  Lastly, I shall add to this what our age has invented, that is far more admirable and pleasant.

"To make an ancient fashioned Looking-glass, wherein more pictures will represented of the same thing."

The way is this.  Make a half circle on a plain table, or place where you desire such a glass to be set up.  And divide this equally with points according to the number of the images you would see.  Make subtendent lines to them, and cut away the arches.  Then erect plain Looking-glasses, that may be of the same Latitude, and of the same parallel lines, and the same longitude.  Glue them fast together, and fit them so, that they may not be pulled asunder, as they are joined lengthways, and erected upon a plain supersidies.  Lastly, let the spectator place his eye in the center of the circle, that he may have his sight uniform, in respect of them all.  In each of them you shall see several faces, and so quite round, as we see it often when people dance round, or in a theater, and therefore it is called a Theatrical Glass.  For from the center all the perpendicular lines fall upon the supersicies, and they are reflected into themselves, so they reflect the images upon the eye, each of them drawn forth its own. This is the ancients way of making a Theatrical Glass, but it is childish.  I will show you one that is far more pleasant, and wonderful.  For in the former, the images were seen no more then the glasses were in number.  But in our glass, by the manifold and reciprocal dartings of the object and the glass, you may see far more, and almost infinite images.  The way is this,

"How to make an Amphitheatrical Glass."

Make a circle on a table what largeness you desire, and divide it into equal parts.  And in the place where the object or face to be seen must be opposed, leave two void spaces.  Over against the parts, let a right line be made upon the lines that determine the parts.  Let Looking-glasses be raised perpendicularly, for the face that shall be against the Looking-glass, placed in the middle, will fly back to the beholder of it, and so rebounding to another, and from them to another.  By many reflections you shall see almost infinite faces, and the more glasses there are, the more will be the faces.  If you set a candle against it, you shall see innumerable candles.  But if the glasses you erect, shall be of those already described, from so many diverse faces of asses, sows, horses, dogs.  And of course colors, yellow, brown, red, the spectators shall see a far more wonderful and pleasant sight, for by reason of the manifold reflection, and diversity of the forms of the glasses, and colors, an excellent mixture will arise.

But I will now make one that is far more wonderful and beautiful.  For in that the beholder shall not see his own face, but a most wonderful, and pleasant, an orderly form of pillars, and the basis of them, and variety of architecture.  Make therefore a circle as you would have it for magnitude, but I hold the best to be where the diameter is two foot and a half.  Divide the circumference into equal parts, as for example, into fourteen.  The points of the divisions shall be the places, where the pillars must be erected.  Let the place where the spectator must look, contain two parts.  And take one pillar away, so there will be thirteen pillars.  Let one pillar be right against the sight.  Then raise Looking-glasses upon the lines of space between, not exactly, but inclined.  Place then two Looking-glasses at opposition to a right line, but the rest about the beginning, where they join, and that for not other reason, but that the beholders face, being not rightly placed, may not be reflected, as I said before.  For thus the glasses will not represent faces, but pillars, and spaces between, and all ornaments.  Hence, by the reciprocal reflection of the glasses, you shall see so many pillars, bases, and varieties, keeping the right order of architecture, that nothing can be more pleasant, or more wonderful to behold.  Let the perspective be the Dorick and Corinthian, adorned with gold, silver, pearls, jewels, images, pictures, and such like, that it may seem the more magnificent.  The form of it shall be thus.  Let H. G. be the place for the beholder to look.  The pillar against him shall be A, in the glass A B, or A C, the face of the beholder shall not be seen, but A B is reflected into I H, and I H into B D, so by mutual reflections they are so multiplied, that they seem to go very far inwardly, so clearly and apparently, that no spectator that looks into it, unless he know it, but he will thrust his hands in to touch the others.  If you set a candle in the middle, it will seem so to multiply by the images rebounding, that you shall not see so many stars in the sky, that you can never wonder enough at the order, symmetry, and prospect.  I have raised and made this amphitheater diverse ways, and to show other orders, namely two ranks of pillars, so that the one stuck to the glasses, the other stood alone in the middle, bound with the chief arches, and with divers ornaments, that it may seem to be a most beautiful perspective or architecture.  Almost the same way is there made a little chest of many plain glasses, covered round.  This they call the Treasury.  On the ground, arches and walls, were there pearls, jewels, birds, and monies hanging, and these were so multiplied by the reflections of the glasses, that it represented a most rich treasury indeed.  Make therefore a chest of wood, let the bottom be two foot long, and one and half broad.  Let it be open in the middle, that you may well thrust in your head.  On the right and left hand, erect the sideboards a foot long, semicircular above, that it may be arched, but not exactly circular.  Namely, divided into five parts each a hand-breadth.  Cover this all about with glasses.  Where the glasses join, there put pearls, precious stones, specious flowers, diverse colored birds.  Above the bottom set heaps of gold and silver medals.  From the arches, let there hang pearls, fleeces of gold.  For when the coffer is moved gently, they will move also, and the images will moe in the glasses that it will be a pleasant sight.

Chapter IV

"Diverse operations of Concave-glasses."

But the operations of Concave-glasses are far more curious and admirable, and will afford us more commodities.  But you can do nothing perfectly with it, until you know first the point of inversion.  Therefore that you may do it the better and more easily,

"Know the point of Inversion of Images in a Concave-glass."

Do thus.  Hold your glass against the sun, and where you see the beams unite, know that to be the point of inversion.  If you cannot well perceive that, breathe a thick vapour from your mouth on it, and you shall apparently see where the coincidence is of the reflected beams.  Or set under a vessel of boiling water.  When you have found the point of inversion, if you will,

"That all things shall seem greater."

Set your head below that point, and you shall behold a huge face like  a monstrous Bacchus, and your finger as great as your arm.  So women pull hairs off their eyebrows, for they will show as great as fingers.  Seneca reports that Hostius made such Concave-glasses , that they might make things show greater.  He was a great provoker to lust.  So ordering his glasses, that when he was abused by Sodomy, he might see all the motions of the Sodomite behind him, and delight himself with a false representation of his privy parts that showed so great.

"To kindle fire with a Concave-glass."

This glass is excellent above others, for this, that it unites the beams so strongly, that it will show forth a light pyramis of its beams, as you hold it to the sun.  And if you put any combustible matter in the center of it, it will presently kindle and flame, that with a little stay will melt lead or tin, and will make gold or iron red hot.  And I have heard by some, that gold and silver have been melted by it.  More slowly in winter, but sooner in summer, because the medium is hotter.  At noon rather than in the morning, or evening for the same reason.

"To make an image seem to hang in the air, by a Concave-glass."

This will be more wonderful with the segment of a circle, for it will appear farther from the glass.  If you be without the point of inversion, you shall see your head downwards.  That with fixed eyes, and not winking at all, you may behold the point, until it comes to your very sight.  For where the Cathetus shall cut the line of reflection, there the species reflected will seem almost parted from the glass.  The nearer you are to the center, the greater will it be, that you will think to touch it with your hands.  And if it be a great glass, you cannot but wonder, for if any man run at the glass with a drawn sword, another man will seem to meet him, and to run through his hand.  If you show a candle, you will think a candle is Pendulons lighted in the air.  But if you will

"That the image of a Concave-glass should go out far from the center."

When you have obtained the image of the thing in its point, if you will have it farther distant from the center, and that the picture of a thing shall be farther stretched forth, then you shall decline from the point a little toward the right or left hand, about the supersicies of the glass, and the image will come forth the farther, and will come to your sight.  There, namely where the Cathetus does the farthest off that is possible touch the line of reflection, which few have observed.  From which principle many strange wonders may be done.  When you have this, you may easily,

"Reflect heat, cold, and the voice also, by a Concave-glass."

If a man puts a candle in a place, where the visible object is to be set, the candle will come to your very eyes, and will offend them with its heat and light.  But this is more wonderful, that as heat, so cold, should be reflected.  If you put snow in that place, if it come to the eye, because it is sensible, it will presently feel the cold.  But there is a greater wonder yet in it.  For it will not only reverberate heat and cold, but the voice also, and make an echo.  For the voice is more rightly reflected by a polite and smooth superficies of the glass, and more completely than by any wall.  I prove this, because, if a man turns his face to the glass, and his friend stands far behind his back, when he beholds his face, he shall decline his face from the point of inversion.  But on the right hand, about the supersicies of the glass, his face will come far from the glass, and will seem very great about the face of his friend.  Whatsoever he shall speak with a low voice against the glass, he shall hear the same words and motions of his mouth, and all motion from the mouth of the reflected image.  And they that stand in the middle between them, shall perceive nothing at all.  But he that would send his won image to his friend, must observe till his head shall come to the glass.  It is profitable also,

"By a Concave-glass to see in the night what is done afar off."

By this very glass, we may in a tempestuous night, in the middle of the streets, cast the light a great way, even into other men's chambers.  Take the glass in your hand, and set a candle to the point of inversion, for the parallel beams will be reflected to the place desired, and the place will be enlightened above sixty paces, and whatsoever falls between the parallels, will be clearly seen.  The reason is, because the beams from the center to the circumference, are reflected parallel, when the parallels come to a point.  And in the place thus illuminated, letters may be read, and things done conveniently, that require great light.  By the same art we may,

"With a few small lights give light to a great hall."

In temples, watches, and nightly feasts, any man may thus with a few lights make a great light.  At two or more places of the chamber set Concave-glass above, and let them be so ordered.  That the place of concurrent parallels may be coincident in the place required.  And in the point of inversion of them, the light will be so multiplied, that it will be as light a noon-day.  Lamps are best for this purpose, because the light varies not from the place.  Candles are naught, because they alter the places of reflection.  More commodiously then by a plain glass, to signify by a Concave-glass  secretly some notes to your friend.  Thus, do as I said, make the marks upon your glass superficies with wax or some dark substance, and setting it against the light, it will cast the light upon the walls of the chamber, and there it will be dark where the letters are made.  One that knows the craft, may easily read them. But this is more admirable for one that knows not the cause,

"To read letters in a dark night."

A Concave-glass is of great use for this, and it may be this may be good in time of necessity.  Set your Concave-glass against the starts of the first magnitude, or against Venus or Mercury, or against a fire or light that is far off.  For the light reflected will meet in the point of burning, and reflects a most bright light, whereby you may easily read the smallest of letters, for putting the point of reflection to every word, you shall see all clearly.  But this is more necessary and profitable,

"At any hour of the day with a Concave-glass, to set a house or fort on fire."

You may so burn the enemy ships, gates, bridges, and the like, without danger or suspicion, at a set hour of the day, appointed the day before.  Set your glass against the sun, and order it so, that the coincidence of the beams may fall upon the point.  Lay fuel there, and things that will take fire, as I have shown you.  And if you would blow up towers, make heaps of Gunpowder.  At night, set your glass, and hide it, that it may not be seen, for the next day the sun will fall upon the same point, where you set fuel for the fire.

Chapter V

"Of the mixed operations of the plain Concave-glasses."

I shall set down the mixed operations and benefits of both these glasses, that what one cannot do alone, it may do by the help of another.  If we would,

"Kindle fire afar off with a plain and a Concave-glass ."

It falls out sometimes that one shut up in prison needs fire, and sun beams do not shine in.  Or else I will show how we may kindle Gunpowder without fire, or make mines and fill them with Gunpowder, to blow up castles or rocks far off without danger, setting them on fire by a plain glass.  A plain glass as it receives the parallel beams of the sun, it so reflects them, and therefore will cast the beams that are equidistant, a great way.  But if a Concave-glass receive them, it so unites them, that it sets things on fire.  Wherefore, first proving where the Concave-glass must be placed, that it may fire the fuel cast in.  the next day, at the hour appointed, let the glass, cast in the beams upon the Concave-glass , that will unite them.  So without danger, or any suspicion of the enemy, we may kindle fire for out use.  Nor is it useless,

"That by a plain and Concave-glass , the smallest letters shall appear very great."

When letters are so small that they can only be seen.  For I have seen St. John's Gospel, "In the beginning,..." written so small, in so little place, that it was no bigger than a small pimple, or the sight of a Cock's eye.  By this artifice we may make them seem greater, and read them with ease.  But a Concave-glass, with the back of it to your breast, over against it in the point of burning, set the writing.  Behind set a plain glass, that you may see it.  Then