CAMERA LUCIDA

The name given by Dr. Hooke to an instrument contrived by him for making the image of any object appear on a wall in a light room cither by day or by night. It is described in the Phil. Trans, vol. xxxviii. p. 741. The instrument now known by the name of Camera Lucida is a very ingenious invention of the late Dr. Wollaston, for the purpose of enabling any one, without a knowledge of the rules of drawing or perspective, to delineate distent objects, or trace the outlines of landscapes ^, ., with perfect acculi: racy. It consists of a quadrangular gla prism, rt, b,c, d, by means of which rays of light are bent,by two reflecO -^ "4 ■.:ji^ i tions, into a path at right angles to their previous direction. A ray of light M jp X proceeding from O enters CAME RILL A.: the adjacent side of the prism at b, and making with it a very acute angle, is wholly reflected in the direction be. At c it again meets the side of the prism, and is in like manner reflected in the direction c E. The eye being placed at E, sees the image of the object j on the surface of the prism at c, and refers it to P, on a; plane M N, which may be covered with a sheet of white j paper. The point of a pencil can also be seen on the paper, and thus the accurate outline of the object may be I traced. It is easy to sec from this the proper form which the crystal should have. Suppose the two reflections to be equal, then the ray will be bent 4.5° from its original path by the first, and also 4o° by the second; and the four angles which it makes with the faces of the prism at b and c are each 22|0; hence the faces a and d being perpendicular, the faces a and b must be inclined in an angle equal to three fourths of a right angle, or G7|°; b and c must make an angle of 135°, or three fourths of two right angles; and c and d make an angle of 67J°. By the laws of optics, the size of the picture will bear the same relation to the object delineated, that tlie distance of the eye from the paper bears to the distance of the object. Hence, in order to increase or diminish the size of the picture, the prism is mounted in a bra frame, supported by bra tubes capable of being lengthened or shortened at pleasure. A thin bra plate, aflBxed to the upper surface of the prism, and having a small hole in it for the observer to look through, keeps the eye in its proper place, and procures steadine of vision. A convex lens may be placed over the hole in the bra plate, for the purpose of magnifying the image; or a concave lens placed before the prism at a will adapt it to short-sighted persons. The instrument is extremely convenient, on account of its portability. The construction of the camera lucid a may be varied in several ways. The following was proposed by profe or Amici. A parallel piece of plate gla , cfg, is connected with a reflecting speculum A. The ray of light proceeding from O is reflected from the speculum at a to the plate gla at b, and there reflected again to the eye at E. The frame in which the instrument is placed has a rectangular opening at the top, through which the eye receives the image, and is prevented from receiving the rays coming directly from the metallic mirror. The image is referred to a plane below at P, where the pencil is seen through the gla . CAMERA OBSCURA,orDARK CHAMBER, is an optical apparatus, by which the images of external objects are thrown on a white surface, and represented in a vivid manner in their proper colours, shapes, . Hence the apparatus may be used for the purposes of delineation, as well as the camera lucid a; but as it is from its construction le convenient, it is chiefly used as a source of amusement, or for explaining the nature of vision. The theory of the Camera Obscura is shortly this. Through a convex lens, or small circular hole at C, the light is admitted into a darkened room or box, so that rays proceeding from an object A B, and falling on a white ground within the room, paint an image of the object on it in an inverted position, a b. But the image is easily restored to its natural position by causing the rays to pa through two convex lenses inserted in a draw tube placed in the opening C. Sometimes a mirror D is placed in the interior of the box, making an angle of 45° with its sides, whereby the image is thrown down on the bottom of the box at a b, and by means of the reflection restored to its natural position. The best ground for receiving the image is plaster of Paris, formed somewhat concave. The image is viewed through an oblong aperture cut in the box. The most perfect camera obscura is formed by placing an inclined mirror in a revolving frame at the top of a building, by which the rays are thrown down on a convex lens in the roof, and the images of all the surrounding objects received on a table. The images being accompanied by the motions belonging to the objects, a very pleasing picture is formed.' The invention of the camera obscura is usually ascribed to Baptist a Porta; it was, however, known to Friar Bacon, in the 13th century. [s. 197]