WHEN travelling by tram, I have often seen the light of street lamps drawn out into beams perpendicular to the direction of streaks left upon the window glass. Effective fibres of the cleaners' cloth are sensibly parallel, and though I had mentally noted that the beams were due to diffraction, it had not hitherto seemed worth while publishing the observation. I would suggest, however, that it provides a simple analogue for the atmospheric light columns mentioned by Dr. Currie in NATURE of April 5.