Phi Phenomenon Apparatus PSAW An instrument to determine optimal distance and time interval for occurrence of phi-phenomenon. It consists of a wooden box having arrangement of a pair of lights in shape of dots and arrows. The distance between a pair of light can be adjusted and recorded with the help of an arrangement provided on front panel. There is a window at the rear to observe the lights. When a subject is made to observe two lights at a particular frequency , the distance is so varied that at particular distance the two lights appear a continuous light. The experiment is repeated at different frequencies and with different shape of light.