THE VISUAL FIELD
THE problem is to measure the field of vision for white and colors, and to determine some characteristic color-changes in the indirect field.
Make the following preparation for the experiment: Lay a piece of cardboard back of Fig. 1. Prick through the page with a pin at each number on the arc, at the free end of the heavy line, and at the principal points in the light line. Trim the card by cutting according to the tracing of the light line. Insert the degree-numbers at the appropriate points. Draw the heavy line. Punch a pinhole at the free end of the heavy line, .which is the center of the arc. Put a thread through the hole and tie a knot at the back of the card. Tie a knot at the other end of the thread, fifty centimeters away. Take the large black square and the small white, red, green, yellow, and blue squares from the book envelope. Lay the white square upon the large black square as a background; then stick a pin through the knot at the free end of the cord, through the two squares near a corner of the small one, and finally through a cork