this exhibit a light source shines through a screen with a square
array of small holes in it projecting a series of dots on a movable
screen. The farther back the screen, the more widely spaced the
dots become. The visitor can count the number of dots enclosed by
a square on the screen at a given distance from the light source.
If the distance is doubled, the number of dots within the square
is reduced to 1/4 of its previous value ( the inverse square.)