All you need to do is to set up the equipment as shown in Figure 12.  The individual components are shown in Figures 13 and 14.  The carbon arc lamp serves as the continuous light source while the lens turns the beam into parallel light.  The tank provides a continuous mist of water droplets; the tank prevents a spilling water everywhere; and the black board stuck onto the tank serves to eliminate colours produced by the lens.  The angle at which the rainbow can be seen is the angle at which Figure 13  was taken.