Prepare a strip of cardboard about three inches wide and fourteen
inches long. Get as many colors of paper as possible, cut them into
strips of unequal width and paste them on the cardboard so that each
color will be from one to three inches wide, according to the number
secured.
Stand across the room holding the back of the strip towards the
children, then turn it over so that they get one clear glance. This
glance should not exceed the length of time it takes you to count
rapidly one-half the number of colors. There should not be less than
six colors on the slip, in which case you count from one to three.
After this first quick glance see who can tell accurately HOW MANY
colors there are on the slip. Let each write down the number his mind
registered without checking up to see if he is correct.