Camera Obscura
an optical instrument used to view or sketch objects at a short distance. It consists of a rectangular box, formed of two parts sliding in each other, like the joints of a telescope, so as to adjust the focus to CAMERON bodies more or less distant. A tube with a lens is fixed in one side of it, and is turned to the object to be represented.
The rays entering fall on a mirror sloped at an angle of 45°, which reflect them upward to the observer's eye. It is con venient that they may be made to pass through a horizontal plate of glass, on which tracing paper may be placed so as to enable one to draw the figure if he be so disposed, but now this is generally done by photography. A lid to the box is of use in ridding the observer of superfluous light. It is supposed to have been invented by Baptista Porta in the 16th century, although it is said that Roger Bacon wrote a description of it 300 years before.