IN the Journal of the Franklin Institute Prof. S. W. Robinson has recently described some experiments upon the effect produced upon sound-waves by repeated oblique reflections at membranes forming the boundary of two gaseous media. These experiments, as far as they go, would appear to show that after repeated reflection at oblique surfaces set in vertical planes a sound-wave acquires new properties by virtue of which it is reflected at another such oblique surface with an intensity which is a maximum if this surface be also in a vertical plane, and a minimum if the surface be equally inclined to the direction of propagation of the wave, but turned through a right angle about that direction as an axis. If this be indeed established as an experimental result it is equivalent to a proof that sound-waves can he polarised by reflection. The apparatus consisted of a series of L-shaped tubes of tin plate, one inch in diameter and three inches long, the parts joined at an obtuse angle, but having the outer angle cut away and covered by a thin membrane. This membrane was fixed so that a sound-wave coming in either direction should be incident on it at the angle of supposed maximum polarisation, the angle being calculated by Brewster's Law so that its tangent should represent the ratio between the velocities of wave-propagation in the two media, namely, coal-gas within, air without (14: 11). The series of tubes was so set that at first the membranes were all parallel, and then a “pulse” was sent along the tube in the following manner:—The initial and final openings were closed by membranes stretched across the tubes orthogonally. Against each a small ball of ivory or glass was hung by a thread. The ball at one end was raised to a given height and dropped on to the membrane, and the impulse given to the ball at the other end was noted. Then the second half of the system was turned round so that the membranes of this portion were at right angles to their former position, and the ball was again dropped. The impulses received on the second ball were in general feebler when the second system, or “analyser,” stood at right-angles to the first system, or “polariser,” the diminution varying in different experiments from 16 to 38 per cent. No diminution was observable when the interior of the tube was filled with air instead of coal-gas. It remains to be seen whether the results are capable of being reproduced under other circumstances, or whether they are due to some mechanical peculiarity of Prof. Robinson's apparatus. Whether this be so or not we must absolutely reject the very unwarranted conclusion at which Prof. Robinson arrives, namely, that the vibrations of light as well as of sound are longitudinal until they are polarised, and that they become transversal only in the act of polarisation.