• Ask your friend to stand at a distance facing you.
• Look through the narrow slit cut on the front plate of  the exhabits
• Rotate the extended arm slowly till your friend appears to just completely fill up one of the openings in the shape of human figures.
• The number below this particular opening gives you the distance of your friend from you.
• The distance is related to the size of the opening by the laws of similar triangles. The  further is your friends , the smaller. is the opening.