Camera Obscura is the latin for 'dark chamber'. The camera works best in bright light, with a black wool blanket covering the artist and the glass screen.  The image comes in through the lens, which is fixed to a sliding drawer to enable some focussing.  The image hits a first surface mirror and is projected onto the glass plate, onto which a piece of semi transparent paper is placed.  The image is reversed.  By using soft pencil, this image can then be placed onto art paper and rubbed over to transfer the image and finish with pen and ink, as can be seen in the Sandby picture below.  Below left is the image of the soldier projected onto the paper, with the drawing just moved to one side to check the outline.