"The principle of the camera obscura can be demonstrated with a rudimentry type, just a box (which may be room-size) with a hole in one wall, (see Pinhole cameras for construction details). Light from only one part of a scene will pass through the hole and strike a specific part of the back wall. The projection is made on paper on which an artist can then copy the image. The advantage of this technique is that the perspective is right, thus greatly increasing the realism of the image (correct perspective in drawing can also be achieved by looking through a wire mesh and copying the view onto a canvas with a corresponding grid on it). With this simple do-it-yourself apparatus, the image is always upside-down. By using mirrors, as in the 18th century overhead version illustrated, it is also possible to project a right-side-up image. Another more portable type, as in the second drawing, is a box with an angled mirror projecting onto tracing paper placed on the glass top, the image upright as viewed from the back." from wikipedia.org

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