Book XVII, chapters 18–23: Burning-glasses and wonderful sights

CHAP. XVIII.

To make a Burning-Glass of many Sphaerical Sections.

VItellio describes a certain composition of a Burning-glass, made of divers Sphaeral Sections: but what he writes he proves not, nor doth he understand what he says: whilst I was searching for that, I found this. Propound the distance of combustion, let it be CB, let it be doubled, CA shall be the semidiamiter of the Sphaere, whose Centre B must be extended to D, and the Diameter will be AD. Divide CA into four points, but the more the parts are, the more precise will be the description of the line, and set the numbers to the divisions: so setting the foot of the compass fast in I, and the moveable foot in B, make the semicircle EF, and mark it BI: and setting it in the 2. Centre at the same wideness, and the other moveable foot in the line BD, describe another semicircle and mark it 3. and so to the fourth and mark it 4. Then setting the foot firm in B, at the distance of BC, or B4, make a circle, and the immoveable foot standing on the Centre B, upon the distance B3, describe another: so there is the third B, and the fourth BA, as BI. Then from the point, A, draw a line, and another from the point B; and let them meet in a point where the circled meets, with the semicircle 1. for let them be cut in G; then draw the second line from circle 2. and another from the same A the Centre, and let them meet, where the second circle cuts with the second semicircle in H; then from the third circle, and from B the Centre, and where they meet in, I, by the meeting of the semicircle: so from the fourth, where the fourth begins in K, and from KIHG draw a line, which shall be the Section to be described. The same may be done on the other part of the circle, the reason is this: The beam of the Sun LI falling upon the point I, of the Glass, is reflected to B, because B3. and BI are equal from the same circle: therefore the Angle B3I, is equal to BI3. But B3[…] is equal to 3IL, because it is subalternate, for the ray of the Sun LI is equidistant to the diameter of the circle, wherefore the Angles LI3 and 3IB, are equal, therefore it is reflected upon B. The same is to be said of the beam MH and NG, and this Glass is contrary to a Sphaeral Glass: From divers points of the circumference, the rays are reflected upon different parts of the diameter, and all the diameters are from the Centre: but in this the reflected beams unite, not in one point, and the diameter are various from the fourth of the diameter. But of this more largely in my Opticks. Lastly, I will not omit that the Cane doth kindle fire

[Figure]

circularly, when that as far as this circle it kindles in a point. Divide the Parabolical line by sinus versus, and let them meet upon contrary parts. For example, let the Parabolical Section be CEF, the sinus versus DE: cut this circumference in E, and let CF meet together in the manner they stood before, that it may be EGFE, and about the axis GH turn it round, there will be made a round Cane, make it of Steel, or other Metal; and polish it, and it will kindle fire round about,

CHAP. XIX.

Fire is kindled more forcible by refraction.

I Have spoken of Burning-glasses by reflection: Now I shall speak of those which burn by refraction; for these kindle fire more violently, I shall shew my reason in the Opticks. Wherefore

By a Cylindre of Crystal to kindle fire.

We may do it by setting it against the Sun, but very slowly and by leasure; for all the beams do not meet in one point, but in a line. The same way almost are we wont

To burn with a Pyramidal Crystal Glass.

But this burns about a line, yet both burn more strongly than a pillar Glass of a Pyramidal, in the place of this we may use a Vial full of water. But the most violent of them all, is with

A Crystal Sphaere, or portion of it.

And if a Sphaere be wanting, we may supply it with a Vial full of water, that is round and of Glass, set against the Sun: if you set behind it any combustible matter, that is friendly to the fire, so soon as the rays unite about the superficies, it forthwith kindleth fire, to the wonder of the Spectators: when they see fire raised from water, that is extreme cold, so will the portions of Sphaeres, as spectacles, lenticulars, and such like, which we speak of already.

A Crystal parabolick-Glass will kindle fire most vehemently of all,

we shall see it, because the beams all meeting, it kindles more than a Glass. We may also, as I said of a Glass

By refraction, kindle fire afar off,

And almost to infinite distance, as is demonstrated by Obtick reasons; and the more by how much as refractions work more forcibly than reflections: and I shall perform this many ways, as I said before, not onely by reason, but by experience. Almeon said, That he made the same way parallel lines cut a cross. I have said also, that if they be opposed in place, Crystal Sphaeres are so perfectly opposite by coition, as are Sphaeral and Cylindrical portions. Nor do they cast forth fire so far, that it is hard to believe it, and more than imagination can comprehend. Behold, I shall shew you a more forcible way to kindle fire. It sends forth also unequal, and combust parallels. Let a uniform Section fall in, and it will carry forth oblique beams, you shall see the fire by a hidden and open beam, falling upon a right superficies, and it will come forcibly and uniformly into that place, where the beams unite most in a fit combustible matter: for if that combustible matter that is opposite, be not dry, it is in vain to set a Glass against it, either a Convex Cylindrical, or Concave Sphaerical; for the matter will be found almost pierced through with strong fire, and if it be not truly opposite it will burn, whether it be small or great. But it is considerable, the portion of which it is. It will do also the same thing, if the thing be opposite, and be small or great, if need be.

CHAP. XX.

In a hollowed Glass how the Image may hang without.

BEfore I depart from a plain Glass, it is performed by the later Artists industry, that in the same Glass many faces may be seen, or likenesses of the same Image, without any hindrance to the first: for behind it they make the Glass hollow, and make a little Concave, whence a foil being laid on, as I shall shew, and fitted well, it will hold another forth without. Hence comes it to pass by this excellent invention, that a man looking in a Glass, may see the upright Image of some other thing, and wonders at it, for catching at it, he can catch nothing but Air. I remember that I have often seen it, and the matter is thus. A Glass being made of Crystal, they make a hollow place on the backside like an Image, as curiously as they can; then they foil it over, and set it in its place, now as deep as the hollow is with in, so much will it shew it self without the superficies; and you cannot satisfie your self, unless you touch it with your hands, whether it truly stick without the Glass or not. So Letters are truly read, that they will seem to be made in Silver upon the Crystal; nor is the eye so quick, but it may be deceived when it looks on. Nor will I omit the Artifice,

To see in a plain Glass that which appears no where.

I have often much delighted my friends, and made them admire with this Glass. Provide thirty or forty little Tables ready, of a foot and half long, and two fingers broad, and a third part of a finger thick; so artificially hewed, that the thickness may be upon the one side, and the thinness on the other side, like the edge of a knife. Place all these boards together, that the solid parts may stand altogether, as to make a perfect plain: Then paint your own Picture, or of some other thing upon it: yet by this artifice and great observation, that if the Image be neer the Glass, it must be drawn as it were afar off. If you would have it far distant, let the forehead be unmeasurably long, the nose somewhat longer, and the mouth, and the chin, likewise. The manner how to draw this Form exactly in Tables, I said in my Opticks. When the Image is now described, fasten the little boards upon a plain Table, that the head may be set downwards, and the chin upwards; and place the first Table after the second, and the second after the third, till they be all fastned. Hang the Table above a mans height, that no man may see into it, above the degrees of the Tables: and place a Glass over this, distant two foot from the Table, so long lifting it up, and putting it down till you see the perfect Image. Now when any man comes neer the Glass to see his own Image, he shall see the Image of some other thing that appears no where. In the breadth of the Tables you may draw some Picture, lest they should give some occasion to suspect.

CHAP. XXI.

How Spectacles are made.

VVE see that Spectacles were very necessary for the operations already spoken of, or else lenticular Crystals, and without these no wonders can be done. It remains now to teach you how Spectacles and Looking-glasses are made, that every man may provide them for his use. In Germany there are made Glass-balls, whose diameter is a foot long, or there abouts. The Ball is marked with the Emrilstone round, and is so cut into many small circles, and they are brought to Venice. Here with a handle of Wood are they glewed on, by Colophonia melted: And if you will make Convex Spectacles, you must have a hollow irondish, that is a portion of a great Sphaere, as you will have your Spectacles more or less Convex; and the dish must be perfectly polished. But if we seek for Concave Spectacles; let there be an Iron-ball, like to those we shoot with Gun-powder from the great Brass Canon: the superficies whereof is two, or three foot about: Upon the Dish, or Ball there is strewed white-sand, that comes from Vincentia, commonly called Saldame, and with water it is forcibly rubbed between our hands, and that so long until the superficies of that circle shall receive the Form of the Dish, namely, a Convex supreficies, or else a Concave superficies upon the superficies of the Ball, that it may fit the superficies of it exactly. When that is done, heat the handle at a soft fire, and take off the Spectacle from it, and joyn the other side of it to the same handle with Colophonia, and work as you did before, that on both sides it may receive a Concave or Convex superficies: then rubbing it over again with the powder of Tripolis, that it may be exactly polished; when it is perfectly polished, you shall make it perspicuous thus. They fasten a woollen-cloth upon wood; and upon this they sprinkle water of Depart, and powder of Tripolis; and by rubbing it diligently, you shall see it take a perfect Glass. Thus are your great Lenticulars, and Spectacles made at Venice.

CHAP. XXII.

How upon plain Concave and Convex Glasses, the foils are laid on and they are b[…]ed.

NOw it remains that I speak of some few things, not to be overpassed of the banding of Convex Glasses, and of foiling plain Glasses, and Convex Glasses, that so I may set down the perfect Science of Looking-glasses. First, for the terminating of Looking glasses, that are made of Crystal and Glass, then of other mixtures, and polishings, that a knowing Artificer may know, and know how to make them: For though amongst many things, that shew the Images of things, as water, some Jewels, and polished Metal do it; yet nothing doth so plainly represent Images, as Lead foil'd upon Glass. Plain Looking-glasses are prepared of Crystal, and of Glass: those of Crystal are polished by wheels, and require another Artifice. But at Venice

How Glass Looking-glasses are made,

I have seen it. They take the melted Glass out with an Iron; with their blast they frame an empty Pillar; they open it on one side with their tongs, and whilst it is red hot they lay it upon a plain plate of Iron, that is equally made; and they put it into the furnace again, to make it softer; and that it may get the perfect plainness of the iron plate, they leave it over the furnace to cool by degrees: When it is cool, they do thus

Polish plain Glasses.

They fasten it upon a plain Table with Gyp; underneath lyeth a most polite plain plate of iron; they cast upon it the foresaid sand; they rub it with water by a stick, leaning thereon, until it be perfectly plain; they take it from the Table, and glew it on the other side, to polish them both: then they make them perspicuous, as I said they did. Now will I shew

To terminate plain Glass Looking-glasses.

Glass or Crystal Looking-glasses, when they are made plain and equal, the Artist makes a foil of the same bigness of Tin, that is level and thin, as perfectly as he can. For if Crystal or Glass had no foil of Lead behind it, by its strength and thickness it could never terminate our sight, nor stay the Image Printed upon it, but it would let it slip away; for Glass is pure and transparent, and so would not contain it, by reason of its brightness; and so the Image would vanish in it, as light in the Sun. Wherefore upon this foil you shall wipe over with Quick-silver, by the means of a Hares foot, that it may appear all as Silver: and when you see it fast on the superficies, you shall put it upon a fair white paper, and so upon the Glass; but first made clean with a linen clout, and polished: for if you handle it with your hands, the foil will not stick to it: with your left hand press down the Glass, and with the right take away the Paper, that the foil may cleave every where, and they bind fast together; laying a weight upon it for some hours, and so let it stand and stir it not. Now I will shew

How a foil is put upon a Concave Glass.

But it is more laborious to lay a foil on a Concave-Glass: Prepare then a foil of the bigness of your Glass, that you shall lay upon the Convex superficies; and holding it fast with a finger of your left hand upon the Centre, with your right hand you shall fit the foil round about, and shall extend it on the said superficies, until it become of the same form with that convex superficies, and stick every where even unto it. Then of moist Gyp shall you prepare a form of the Glass, namely, by pouring Gyp upon the Convex superficies; and when the Gyp is dry, you have the form. Upon the form extend a foil of Tin, and let it agree perfectly with the form every where, because the form and the foil are made after the same superficies: strew quick-silver upon the foil, and as I said, make it stick by means of a Hares foot. The Artists call this Av[…]vare: put paper upon it, and pressing this upon the Glass, take away the paper; when you know it sticks fast, take away your hand, and lay on a weight, and after […]ke it away, but with a careful balancing of your hand, lest it take wind, and that the quick silver may all stick fast every where. Now remains how

To terminate Convex-Glasses.

Make Glass Balls, but of pure Glass, and without bladders as much as you can, as the receivers for distillations; and from the hollow iron that it is blown in by, let this liquid moisture be projected, namely, of Antimony and Lead; but the Antimony must be melted twice or thrice, and purged, and cast Colophonia in. So stir the mixture in the hollow vessel, and what remains cast forth: and so in Germany they make Convex-Glasses.

CHAP. XXIII.

How Metal Looking-Glasses are made.

BUt Metal-Glasses are made another way. Wherefore if a Parabolical-Glass be to be made, draw a Parabolical line upon a brass or wooden Table; what is without it, must be filed away, that it may be equal, smooth, and polished: fasten it upon an Axis in the middle, and fit it with Instruments, that may be fitly turned about, let there be clay with straw under it, made up with dung, that the Table being turned about, it may receive a Concave form exactly; then let it dry, strew ashes upon it, and plaister clay above that, of a convenient thickness; let it dry by the fire, or if you will, by heat of the Sun, take it off, for it will easily part from the ashes: unite them together, that as much space may be between both forms, as you think fit, for the thickness of the Glass: when it is dry, cover it with this, leaving an open orifice on the top, and some breathing places, that the Air may breathe forth at it. Then make such a mixture; let them be put into a new pot that will endure the fire, and lute it well within, that it may hold the faster; let it dry well, and do this twice or thrice over: set it to the fire, and melt in it two pounds of Tartar, and as many of white Arsenick; when you see them fume, pour in fifty pounds of old brass, often used, and let it melt six or seven times, that it may be pure and cleansed; then adde twenty five pounds of English Pewter, and let them melt together: draw forth some little of the mixture with some Iron, and try it, whether it be brittle or hard; if it be brittle, put in more Brass; if too hard, put in Pewter: or else let it boil, that some part of the Pewter may evaporate: when it is come to the temper it should be, cast upon it two ounces of Borax, and let it alone till it dissolve into smoke; then cast it into your Mold, and let it cool: When it is cool, rub it with a Pumice-stone, then with powder of Emril. When you see that the superficies is perfectly polished and equal, rub it over with Tripolis. Lastly, make it bright and shining with burnt Tin; most adde a third part of Pewter to the Brass, that the mass may be the harder, and become more perspicuous.