A ray from the upper extremity a will be refracted through the focus c to the opposite end of the box BC, and by means of the rays from the lower extremity b, similarly refracted through c, will on that end formi, the image reversed of the external object a b. Suppose now that in c be placed the plane mirror de, inclined at an angle of forty-five degrees to the horizon. In this case the ray from a will be reflected from c in the mirror to the upper part of the box at n, the ray from ó will be reflected to o, and the whole rays from a b will form the image no horizontal, but in its natural position. This image is received on a piece of plain glass or of oiled paper, fastened in the top of the camera, on which the lines may be traced with a pencil, to be afterwards transferred to other paper. The glass E is placed in a moveable tube to be set to the proper focus.

The common Magic Lantern is a machine of the same kind, within which is a lamp, which by its light transmitted through a large planoconvex glass in a tube in the front, strongly illuminates a small transparent painting on glass placed before the lens, in an inverted position. A different sort of magic lantern excited much surprise some time ago, under the name of phantasmagoria (the raising of phantasms or spectres). In the common machine the figures are painted on glass, the remainder being transparent; of course the image on the screen is a circle of light having a figure in the midst. In the phantasmagoria, on the other hand, the whole of the glass is dark except the figure, which alone appears on the screen, which is of thin silk, and placed between the lantern and the spectator. By moving the light near to or farther from the screen, the image seems to retire or approach; and no part of the screen being perceptible, but the figure only, this seems to be formed in the air, and to be enlarged as if it came nearer to the spectator, and be diminished as if it receded from him; although in fact it be always at the same distance. The multiplying glass is cut into a number of faces, through each of which the rays of light proceeding from a single object are refracted in different angles to the eye, which thus instead of one single image seeing a number, is apt to conclude it really sees a number of objects instead of one only.

Explanation of Figures.

Plate II. Fig. 3. represents a compound microscope. AG is the body or internal part, moveable up and down in the outer case CD, supported on three legs as E. The plate or stage F is open in the centre, to admit the light to pass through the glass containing the objects to be viewed; the light is reflected upwards by the inclined mirror H. At G is a magnifying lens, another at B, and the eye-glass at A.

Plate IV. Fig. 1. is a longitudinal section of the Gregorian or reflecting telescope described in the text.

Plate IV. Fig. 2. is a similar section of the camera obscura, also deseribed in the text.

Plate IV. Fig. 3. is a similar section of Parker's very powerful burning lens or glass. The lens itself C is fixed in the circular frame DD, connected with another frame and lens at E. The rays collected by the great lens (as partly shown in the figure,) fall upon the small lens, and are there again converged into the focus at F, which is a stand supporting the object on which the experiments are made. The whole apparatus rests on the fulcrum A.

SECT. VI.

ELECTRICITY.

THIS branch of natural knowledge draws its name from electron, the Greek term for amber; because from observing certain properties of that substance, men have been led to the discovery and arrangement of facts resulting from other bodies, on which the doctrine of electricity is founded. The Greek electron, and the corresponding Latin electrum, were also applied to a very different substance, being a compound of gold and silver, more resplendent, says Pliny, by the light of a lamp or torch, than silver itself. Of this metallic composition, cups, statues, and even columns were formed by the ancients. It is however the natural amber to which we now allude.

Amber, or more properly ambar, according to its origin in the Arabic language, is chiefly found on the shore of the Baltic sea, in the Prussian dominions; sometimes up the country, resting upon wood-coal, or black charred trees, at the depth of 80, or 100 feet under ground: it is almost completely combustible, and possessing many of the properties of resinous substances, is therefore considered to be of vegetable origin. It is brittle, light and hard, usually pretty transparent, and commonly yellow or even deep brown. It is tasteless, and without smell, unless when pounded or heated, when it emits a fragrant odour. On this account amber is carried from Prussia to the eastern parts of the world, particularly to Persia and China, where it is burned on chafing-dishes, during their festivities, to perfume the chambers of the great. When roasted or exposed to a melting heat, it combines with drying linseed oil and turpentine, and then forms amber varnish.

The earliest account we have of the peculiar properties of amber mount up about 600 years before the Christian era, when Thales the Greek philosopher of Miletus in Asia Minor, observed that, when heated by rubbing, it attracted straws, feathers, and other light bodies. About 300 years later Theophrastus noticed light bodies to be attracted by another substance, when exposed to heat without friction. This he called lyncurium, probably what is now called tourmaline, a species of shorl consisting of the basis of alum, flint and iron. This stone was first brought from Ceylon in the East Indies about 60 years ago: but is now found to enter into the composition of mountains in various parts, generally crystallized in prisms of 3, 6, or 9 sides. It is commonly green, and partially transparent: when heated to 200° of Fahrenheit's thermometer, (a little lower than boiling water, which rises to 212°) it becomes electric without friction. The zeolite possesses the same property.

Down to the end of the 16th century, no farther notice of the properties of these substances occurs in the history of science. In 1590, a treatise on the magnet was published by an English physician, D. William Gilbert, in which were introduced a number of experiments illustrative of electricity, as well as of magnetism, and containing a list of various substances possessing attractive properties, similar to those of amber. Since that period many ingenious persons have devoted

themselves to the study of electricity, particularly the late Dr. Joseph Priestley of Birmingham, and Dr. Benjamin Franklin of Philadelphia, by whose labours the science was placed in the way to be afterwards carried on to perfection.

The doctrine and practice of electricity are founded on this supposition, that this earth, and all bodies with which we are acquainted, are endowed with a fluid substance, extremely subtile and elastic. Of this fluid each body possesses a certain share adapted to its nature; and so long as it contains neither more nor less than its natural share, the electric fluid produces no sensible effect, and is therefore imperceptible. If, however, the natural share be increased or diminished, very striking and important effects are produced: the body is said to be electrified, and the electricity becomes apparent. This increase or diminution of the electric fluid, in any body, would immediately be brought to the proper quantity, by communication with other bodies, provided all substances were equally fitted for the transmission of the fluid, which we know is not the fact. Those bodies which permit the fluid to pass through them freely, without retaining it, are called conductors and non-electrics: while those which retain the fluid in accumulation are termed electrics and non-conductors. Of this last class of bodies in which the electric fluid is accumulated, the most perfect among solid substances are glass, resins, bees' and sealing wax, sulphur, wood baked to perfect dryness in an oven; and among fluids air and oils. The best conductors of electricity, or the substances through which it passes with the greatest freedom, are all the metals and water: but all substances become conductors when made very hot. A body possessing more than its natural share of electricity, is positively electrified; and one possessing less than its due share is negatively electrified. A body so surrounded by nonconductors, that the fluid cannot pass between it and the earth, is said to be insulated, as an island is separated from all other land by the surrounding sea. Thus a piece of metal supported on a pillar of glass or wax is insulated: but if a communication be made between it and the earth, by an iron chain or other conductor, the equilibrium between the body and the earth will be immediately restored, whether the piece of metal contain more or less than its due share of electricity.

The equilibrium of the electric fluid in bodies, is disturbed, or it is made to pass from one to another, chiefly by gentle friction or rubbing of the one against the other; if this be done by a conductor against a non-conductor the electricity is the most powerfully excited. The fluid will leave the less perfect electric, and pass upon the more perfect. Thus if a smooth glass tube be drawn through the hand, the friction causes the electric matter to leave the hand, and to pass upon the glass, where it is accumulated, in addition to the natural share already in the glass. Neither the glass nor the air being conductors, the superabundant fluid has no way to escape: but if a finger, a piece of metal, or any other conducting substance be presented to the glass thus overcharged, the fluid will immediately pass from it to them, and the equilibrium will be restored as before the excitement. In this case the glass is said to be excited.

The quantity of electric fluid which can be assembled in this way being very small, machines have been contrived for producing more powerful friction between electric and non-electric substances, and