CAMERA LUCIDA, a contrivance of Dr Hook for making the image of any thing appear on the wall of a room during sunshine. Opposite to the wall on which the image is to be received there is an aperture in the window shutter of at least a foot in diameter. The object is placed outside the aperture. The object must be strongly enlightened by a mirror which throws the sun's rays upon it. Between the object and the wall which is to receive the image place a large convex lens, whose focal distance is sufficiently great to give an image on the wall. In proportion as the lens is placed nearer to the object, the image on the wall will be larger; and when the lens is moved farther from the object, the object will be less. (Phil. Trans. No. xxxviii. p. 741 et seq.) With a good lens, magnified images of objects, such as small gems in bas-relief, may be formed on a wall or screen so perfect as to be mistaken for real bas-reliefs.