Model demonstrating the principle of the camera obscura, 1752.
© Science Museum / Science & Society Picture Library
The camera obscura was a popular sketching instrument in the 18th century. A lens in the draw tube and a mirror at 45 degrees to the horizontal focused the image onto a ground glas screen on the top of the box. A piece of paper was put onto the screen and the image was copied directly. This model was used for demonstration purposes by Dr Stephen Demainbray (1710-82), a science lecturer of the time. The light rays, represented by threads, start from the coloured cros on the right, through the lens and are then reflected by the 45 degree mirror onto the horizontal screen above it. The cros on the left demonstrates the path of the light rays had the mirror not been present.