CAM′ERA OBSCU′′RA

[L. and Eng.] An optical instrument for producing upon a screen the image of a field of view more or le extensive. It was invented by Baptist a Porta in the 16th century. The principles and construction of the camera obscura may be thus described:—A convex lens ( B ) is placed in a hole admitting the light into a darkened box or chamber ( A ), which, falling on a white ground ( D ), produces an inverted picture of every object within its range. The image thus formed may be restored to its natural position, by allowing the rays of light to pa through two lenses instead of one, or by receiving the rays on a mirror placed at an angle of 45°, when the image will be thrown on the floor in its original position. The picture may be viewed through an oblong aperture cut in the box, or the experiment may be performed in a darkened room, by placing the lens in a hole in the shutter, and allowing the image to fall on the wall, or on a sheet of white paper stretched to receive it. IMG:596405606043057898_i403-2.png: In the simplest form, when intended for taking views or portraits, the image is thrown upon a mirror placed at an angle of 45°, and resting on the bottom of the box, by which means it is thrown upwards against a plate of gla , also placed at a similar angle. On this is laid a piece of semi-transparent tracing paper, on which the object is distinctly seen painted, and may be traced out with a pencil. When the camera is used in photography, slides are provided to retain the sensitive paper in the proper position in the box or dark chamber to receive the image, and the whole apparatus is adjusted with screws, and slides of the most delicate description. Achromatic gla es are also employed. See Photography .