During some work on Neon Lamps for Stroboscopic Work, it was noticed that, when the flash of an ordinary spectrum tube containing neon was analysed by a rotating mirror, it consisted of two parts, an extremely short flash followed by a flame or arc, and that the latter consisted of bright striations travelling from anode to cathode. The same phenomenon was observed with helium. The velocity of these striations was roughly measured, and found to be about 50,000 cm./sec. in neon and 100,000 cm./sec. in helium. As these figures approximated to the velocities of sound in these gases, it was hoped that investigation of the phenomenon might lead to an explanation of striations generally. This expectation has, unfortunately, not been realised, for these velocities were soon found to be special or limiting cases of far more complex phenomena. The following paper is an account of the preliminary experiments, which have given interesting, but somewhat inconclusive results. The first account of moving striations observed by means of a rotating mirror is given by A. Wüllner.† They were investigated more fully by W. Spottiswoode. Both these observers used several elementary gases then available, but most of Spottiswoode’s results were obtained with compounds, these giving more definite striations. Unfortunately, neither observer recorded any figures for the velocities, but Wüllner mentions that his mirror was rotated at ½ to 2 revolutions per second.