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 either by day or by niulit. It is described in the Phi!. Trans, vol xxxviii. p. 741. The Instrument now known by the name of Camera Lucida is a very ingenious invention of the late Dr. Wolh» ton, for the purpose of enabling any one, without a knowledge of the rules of drawing or perspective, to delineate distant objects, or trace the outlines of landscapes, . with perfect accuracy. It cor. sists of a quadrangular gla prism, a, 6, c. d. by means of which rays of light are bent. by two reflections, into a path at right angles to their pn direction. A ray ol light pro cee ding from O enters 11 M |p jjOf■tinuesthe its prism course at ina, a and straight con line till it meets the adjacent side of ihe prism at h. and making with it a very acute angle, is wholly reflected In Ike b e. At e it again meets the side of Ihe prism, and is in like manner reflected in the direction c E. The eye being placed al E. sees the Image of the object 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 paper. The point of a pencil can also be seen on the paper, and thus the accurate outline of the object may be traced. It is easy to see from this the proper form which the crystal should have. Suppose the two reflect qual, then the ray will be bent 45° from its original path by the first, and also 45° by the second; and the four angles which it makes with the faces of the prism at b and e are each 22J°; hence the faces a and d being perpendicular, the faces ii and b must be inclined in an angle equal to three fourths of a right angle, or 67J°; h and c must make an angle of 135°, or three fourths of two right angle*; 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 the distance of the eye from the pape i bears to the distance of the object. Hence, in order to increase or diminish the size of the picture, the prism imounled in a bra frame, supported by bra lubes capable of being lengthened or shortened al pleas* thin bra plate, a flixed to the upper surface ol 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 met 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 Iucida may be varied in several ways. The following Amici.was A proposed'parallel piece by profe or of plale gla , efg, is connected with a re- •lectins: speculum A. The ray of flighted proceeding from Ihe speculum from Oatisa reflect-to the plato gla at 6, and there reflected again to the eye atE. 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, or DARK CHAM BEU. 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 Iucida; but as it is from its construction le convenient, it is chiefly used as a source of amusement,orfor explaining the nature of vision. The theory of the Camera Through Obs cur a a is convex shortly lens,this.or small circular hole at C, the J6 TjpI lightened is room admitted or box,into soa dark-that __J" /I rays proceeding from an ob- 1r^jject A B, and falling on a white ground within the room, paint an imaze 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 m [s. 199]