ceptible interval must be left between the two spaces seen through two contiguous facets: it will be better on the whole if they rather encroach a little on each other. Care must also be taken to mark each space with the same number as that assigned to each facet, in order that they may be again known. This however will be easy, by observing that the space! corresponding to each facet is always transferred parallel to itself from top to bottom, or from right to left, on the other side of the centre.

The next thing is to delineate the regular picture' intended to be seen, and to transpose it into the spaces where it appears distorted. According to mathematical accuracy, it would be necessary for this purpose to form a projection of the glass cut into facets, supposing the eye at the distance at which it is really placed; but as we suppose it a little more remote, we may without any sensible error assume, as the field of the regular picture, the vertical projection, as seen fig. 44 N° 1, where it is represented such as it would appear to the eye placed perpendicularly above its centre, and at a very considerable distance.

Delineate in the field, which in this case will be hexagonal, and composed of 6 rhomboids and 6. triangles, any figure whatever, as a portrait for example, and then, considering that the space a b c d is that where the portion of the picture marked 1 ought to appear, it must be transferred thither with as much care as possible; do the same thing in regard to the rest; and by these means the principal part of the picture will be completed. But as it is intended to shew something else beside what ought to be seen, it must be disguised by means of some other objects painted in the remaining part of the

field, making them to harmonize with what is already painted, in such a manner, that the whole shall appear to form one regular and connected subject. All this however must depend on the taste and genius of the artist.

In the Perspective curieuse of father Niceron, a much more minute explanation of the whole process may be found. Those to whom what is here said does not seem sufficient, must consult that work. Niceron tells us that he executed, at Paris, and deposited in the library of the Minimes, of the Place Royale, a picture of this kind, which when seen with the naked eye represented fifteen portraits of Turkish Sultans; but when viewed through the glass was the portrait of Louis XIII.

A picture by Amadeus Vanloo, much more ingenious, was shewn in the year 1759, in the exhibition room of the Royal Academy of Painting. To the naked eye, it was an allegorical picture, which represented the Virtues, with their attributes, properly grouped; but when seen through the glass, it exhibited the portrait of Louis XV.

REMARKS.

1st. It is necessary to observe that the place of the glass, when once fixed, must be invariable; for as glasses perfectly, regular cannot be obtained, if they are moved it will be almost impossible to replace them in the proper point; hence it will be necessary to be assured that the glass is of a good quality; for if it be too alkaline, and happen to lose its polish by the contact of the air, another capable of producing the same effect cannot be substituted in its stead. This is an accident which, according

to what we have heard, happened to the glass of Vanloo's picture.

2d. Instead of a glass, like that employed in the above example, or of one more compounded, a plain pyramidal glass might be employed, by which the problem would be greatly simplified.

3d. A glass, the portion of a prism, cut into a great number of planes parallel to its axis, might also be employed; in this case the painting to be viewed through the glass ought to be delineated on parallel bands.

4th. A glass might be formed of several concentric conical surfaces, or of several spherical surfaces of different diameters, likewise concentric: in this case the picture to be viewed through the glass ought to be distributed in different concentric rings.

5th. A magic picture might be formed by reflection. For this purpose, provide a metal mirror with facets well polished, and having very sharp edges; place before it, in a direction parallel to its axis, a piece of white paper or card, and by means of the principles above explained delineate a picture, which when viewed in front by the naked eye shall represent a certain subject; if you then make a hole in the middle of the picture, and look through this hole at the image of it formed by the mirror, it will appear to be entirely different.

PROBLEM L.

To construct a Lantern, by means of which a book can be read at a great distance, at night.

CONSTRUCT a lantern of a cylindric form, or shaped

like a small cask placed lengthwise, so that its axis shall be horizontal; and in one end of it fix a parabolic mirror, or merely a spherical one, the focus of which falls about the middle of the length of the cylinder if a taper or lamp be then placed in this focus, the light will be reflected through the open end, and will be so strong that very small print may be read by it at a great distance, if looked at through a telescope. Those who see this light at a distance, if standing in the axis of the lantern continued, will imagine that they see a large fire.

PROBLEM LI.

To construct a Magic Lantern.

THE name of magic lantern, as is well known, is given to an optical instrument, by means of which figures greatly magnified may be represented on a white wall or cloth. This instrument, invented, we believe, by Father Kircher, a jesuit, has become a useful resource to a great number of people, who gain their livelihood by exhibiting this spectacle to the populace. But though it has fallen into vulgar hands, it is nevertheless ingenious, and deserves a place in this work. We shall therefore describe the method of constructing it, and add a few observations, which may tend to improve it, and to render it more interesting.

First, provide a box about a foot square (fig. 45 pl. 13) of tin-plate, or copper or wood, and make a hole towards the middle of the fore-part of it, about three inches in diameter: into this hole let there be soldered a tube, the interior aperture of which must be furnished with a very transparent lens, having its

focus within the box, and at the distance of two thirds or three fourths of the breadth of the box. In this focus place a lamp with a large wick, in order that it may produce a strong light; and that the machine may be more perfect, the lamp ought to be moveable, so that it can be placed exactly in the focus of the lens. To avoid the aberration of sphericity, the lens in question may be formed of two lenses, each of a double focus. This, in our opinion, would greatly contribute to the distinctness of the picture.

At a small distance from the aperture of the box, let there be a slit in the tube, for which purpose this part of it must be square, capable of receiving a slip of glass surrounded by a frame, four inches in breadth, and of any length at pleasure. Various objects according to fancy are painted on this slip of glass, with transparent colours; but in general the subjects chosen are of the comic and grotesque kind (fig. 46 pl. 13).

Another tube, furnished with a lens of about 3 inches focal distance, must be fitted into the former one, and in such a manner, that it can be drawn out or pushed in as may be found necessary.

Having thus given a description of the machine, we shall now explain its effect. The lamp being lighted, and the machine placed on the table opposite to a white wall, if it be exhibited in the day time, shut the windows of the apartment, and introduce into the slit above mentioned one of the painted slips of glass, but in such a manner that the figures may be inverted: if the moveable tube be then pushed in or drawn out, till the proper focus is obtained, the figures on the glass will be seen painted

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