CAMELOPARDALUS
, a new constellation of the northern hemisphere, formed by Hevelius, consisting of 32 stars first observed by him. It is situated between Cepheus, Cassiopeia, Perseus, the Two Bears, and Draco; and it contains 58 stars in the British catalogue.
CAMERA Æolia, a name given by Kircher to a contrivance for blowing the fire, for the fusion of ores, without bellows. This is effected by means of water falling through a sunnel into a close vessel, which sends from it so much air or vapour, as continually blows the fire. See Hook's Philof. Coll. n° 3, pa. 80.
Camera Lucida, a contrivance of Dr. Hook to make the image of any thing appear on a wall in a light room, either by day or night. See Philos. Trans. n° 38, pa. 741.
Camera Obscura, or Dark Chamber, an optical machine or apparatus, representing an artificial eye, by which the images of external objects, received through a double convex glass, are shewn distinctly, and in their native colours, on a white ground placed within the machine, in the focus of the glass. The first invention of the camera obscura is ascribed to John Baptista Porta. See his Magia Naturalis, lib. 17, cap. 6, where he largely describes the effects of it. See also the end of s'Gravesande's Perspective, and other optical writers, for the construction and uses of various sorts of camera obscuras.
This machine serves for many useful and entertaining purposes. For example, it is very useful in explaining the nature of vision, representing a kind of artificial eye: it exhibits very diverting sights or spectacles; shewing images perfectly like their objects, clothed in their natural colours, but more intense and vivid, and at the same time accompanied with all their motions; an advantage which no art can imitate: and by this instrument, a person unacquainted with painting, or drawing, may delineate objects with the greatest accuracy of drawing and colouring.
Theory of the Camera Obscura. The theory and | principle of this instrument may be thus explained. If any object AB radiate through a small aperture L, upon a white ground opposite to it, within a darkened room, or box, &c; the image of the object will be painted on that ground in an inverted situation. For, by the smallness of the aperture, the rays from the object will cross each other there, the image of the point A being at a, and that of B at b; so that the whole object AB will appear inverted, as at ab. And as the corresponding rays make equal angles on both sides of the aperture, if the ground be parallel to the object, their heights will be to each other directly as their distances from the aperture.
Construction of a Camera Obscura, by which the images of external objects shall be represented distinctly, and in their genuine colours. 1st, Darken a chamber that has one of its windows looking towards a place containing various objects to be viewed; leaving only a small aperture open in one shutter. 2d, In this aperture fit a proper lens, either plano-convex, or convex on both sides; the convexity forming a small portion of a large sphere. But note, that if the aperture be made very small, as of the size of a pea, the objects will be represented even without any lens at all. 3d, At a proper distance to be determined by trials, stretch a paper or white cloth, unless there be a white wall at that distance, to receive the images of the objects: or the best way is to have some plaister of Paris cast on a convex mould, so as to form a concave smooth surface, and of a curvature and size adapted to the lens, to be placed occasionally at the proper distance. 4th, If it be rather desired to have the objects appear erect, instead of inverted, this may be done either by placing a concave lens between the centre and the focus of the sirst lens; or by reflecting the image from a plane speculum inclined to the horizon in an angle of 45 degrees; or by having two lenses included in a draw-tube, instead of one.
That the images be clear and distinct, it is necessary that the objects be illuminated by the sun's light shining upon them from the opposite quarter: so that, in a western prospect the images will be best seen in a forenoon, an eastern prospect the afternoon, and a northern prospect about noon; a southern aspect is the least eligible of any. But the best way of any is, if the lens be fixed in a proper frame, on the top of a building, and made to move easily round in all directions, by a handle extended to the person who manages the instrument; the images being then thrown down into a dark room immediately below it, upon a horizontal round plaister of Paris ground: for thus a view of all the objects quite around may easily be taken in the space of a few minutes; as is the case of the excellent camera obscura placed on the top of the Royal Observatory at Greenwich.
The objects will be seen brighter, if the spectator sirst wait a few minutes in the dark. Care should also be taken, that no light escape through any chinks; and that the ground be not too much illuminated. It may further be observed, that the greater distance there is between the aperture and the ground, the larger the images will be; but then at the same time the brightness is weakened more and more with the increase of distance.
To construct a Portable Camera Obscura. 1st, Provide a small box or chest of dry wood, and of about 10 inches broad, and 2 feet long or more, according to the size of the lenses. 2d, In one side of it, as BD, fit a sliding tube EF with two lenses; or, to have the image at a less distance from the tube, with three lenses, convex on both sides; the diameter of the two outer ones to be about 7 inches, but that of the inner to be less, as 4 3/4 or 5 inches. 3d, At a proper distance, within the box, set up perpendicularly an oiled paper GH, so that images thrown upon it may be seen through. 4th, In the opposite side, at I, make a round hole, for a person to look conveniently through with both eyes. Then if the tube be turned towards the objects, and the lenses be placed by trials at the proper distance, by sliding the tube in and out, the objects will be seen delineated on the paper, erect as before.
The machine may be better accommodated for drawing, by placing a mirror to pass from G to C; for this will reflect the image upon a rough glass plane, or an oiled paper, placed horizontally at AB; and a copy of it may there be sketched out with a black-lead pencil.
Another Portable Camera Obscura is thus made. 1st. On the top of a box or chest raise a little turret HI, open towards the object AB. 2d, Behind the aperture, incline a small mirror ab at an angle of 45 degrees, to reflect the rays Aa and Bb upon a lens G convex on both sides, and included in a tube GL. Or the lens may be fixed in the aperture. 3d, At the dis- tance of the focus place a table, or board EF, covered | with a white paper, to receive the image ab. Lastly, In MN make an oblong aperture to look through; and an opening may also be made in the side of the box, for the convenience of drawing.
This sort of camera is easily changed into a show-box, for viewing prints, &c: placing the print at the bottom of the box, with its upper part inwards, where it is enlightened through the front, left open for this purpose, either by day or candle-light; and the print may be viewed through the aperture in HI.—A variety of contrivances for this purpose may be seen described in Harris's Optics, b. ii. sect. 4.—Mr. Storer has also procured a patent for an instrument of this sort, which he calls a delineator; being formed of two double convex lenses and a plane mirror, fitted into a proper box. One lens is placed close to the mirror, making with it an angle of 45 degrees; the other being placed at right augles to the former, and fixed in a moveable tube. If the moveable lens be directed towards the object, which is to be viewed or copied, and moved nearer to or farther from the mirror, till the image is distinctly formed on a greyed glass, laid upon that surface of the upper lens which is next the eye, it will be found more sharp and vivid than those formed in the common instruments; because the image is taken up so near the upper lens. And by increasing the diameter and curvature of the lenses, the effect will be much heightened.
