Illusion of Convergent Beams of Light

IN a recent letter1 under this title, Dr. A. H. Rosenthal describes an optical illusion produced by the rotating beam from a lighthouse. We studied this phenomenon last year2, and we think that both perspective and photometry are required for its explanation. Diffusion laws show that the brightness of the beam is great near the opposite horizon; this fact, together with the apparent convergence, gives to the observer a strong impression that the light-house is just beneath the horizon.