Camera Obscura. An instrument for a similar purpose as the foregoing, but constructed upon different principles, the rays from the object being caused to pass through a convex glass, on to a white surface placed opposite to the glass, in a darkened chamber. A camera obscura may be constructed as follows: darken a chamber, and into a small aperture made for the purpose in one of the window shutters, fit a lens, either plano-convex, or convex on both sides; and at a due distance, to be determined by experience, place a paper or white cloth, and the objects opposite the lens will be shown upon the paper in their natural colours and proportions, but in an inverted position; by placing a concave lens between the centre and the focus of the first lens, the images will be shown erect. If the aperture do not exceed the size of a pea, the objects will be represented without a lens.