Colour Wheel.—Take a piece of white cardboard, and from it cut a circle about 3 inches across. Now from the middle of this cut another circle about 3/4 in. across. This can be done quite easily by putting a sharp-pointed knife blade into the compass in place of a pencil.
Divide the circle into seven equal parts, and paint or crayon the sections with the colours of the rainbow—red, orange, yellow, green, blue, indigo, violet.
When this is dry, make a large loop of string and put it through the round hole of the card; and hold the ends of the loop one in each hand. Now if you turn the string at one end as if you were turning a skipping rope, and then suddenly pull it tight, your card will revolve very rapidly, and you will find that instead of a coloured card you have what appears to be a light grey one. This is really a little piece of science, for it shows that the white light about us is really made up of the different colours of the rainbow (Fig. 4).
Fig. 4.
A simple Counting Top.—Take a piece of cardboard, and on it draw two hexagons having 1-inch sides. To draw a hexagon, first draw a circle with its radius equal to the length of the side of the hexagon. Then without altering the compass measure off the radius six times round the circumference, and join the neighbouring points. Now cut out each of these and from each one cut out one triangular section (Fig. 5). Scratch lightly along the other lines with the back of the knife-point. Now bend these so as to form two five-sided pyramids. Close up the open space by binding the edges together with a strip of gummed paper (Fig. 6). When you have done this, place the two pyramids base to base, and secure them by means of small strips of gummed paper fixed along the edges. Bind all the edges in similar fashion for the sake of uniformity. All that is necessary now is to make a hole at the apex of each pyramid, and push an ordinary safety match through (Fig. 7). Now if you twirl the match in your fingers, and release it suddenly, the top will spin for quite a long time. To use it as a "counting top" or "dice top" paint the numbers 1 to 5 on the five triangular surfaces of the upper pyramid. Then when the top ceases spinning, the uppermost number is the one which counts.
Fig. 5.
