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CAMERA OBSCURA.—An instrument in which, as its nam eimplies, the essential feature is a dark chamber. It is said to have been invented by John Baptista Porta, a Neapol itan philosopher. But although a description of the instru ment is given in his " Seventeenth Book of Natural Magick, wherein are propounded burning glasses and the wonderful sights to be seen by them," published in x658, yet it is generally believed that the invention dates much farther back, and was known to Friar Bacon in the thirteenth century. It we take a room and exclude all light except that which enters at a small hole made in the window shutter, we shall obtain, when the sun is shining, an in verted image of the objects outside thrown upon the wall opposite the aperture. The delineation is, however, very imperfect, but by placing a double convex lens in the aperture and a sheet of white paper in its focus, we obtain a perfectly sharp image. By making a

miniature room out of a box formed of two parts sliding into each other to adjust the focus for objects more or less distinct, we have a mod ern camera obscura (Fig. 91). A lens is fitted into one side, and the rays entering it fall on the mirror arranged at an angle of 45 deg., which will reflect them upwards and reverse the image, which would otherwise appear upside down at the end opposite to the lens. The image is reflected on to a piece of ground glass or a piece of plain glass and a sheet of white paper laid over it, enabling one to trace the outlines of the picture.

Another form of camera obscura is that often seen at fairs. In this a triangular prism, acting as a condensing lens and mirror combined, throws down upon a white table a representa tion of the surrounding scenery.