IN Philips Technical Review of June, which is edited by the Research Laboratory of Philips Glow Lamp Factory, Eindhoven, Holland, there is an interesting paper on “Synthetic Sound” by J. F. Schouten. A sound of prescribed character of periodic vibrations, in other words, a sound consisting of harmonic components of prescribed amplitude and phase, is obtained synthetically. This method permits a closer study of a number of problems which are connected with the nature of the perception of sound. The principle used is to make a beam of light vary as a function of the time in the desired wave form. The beam is incident on a photo-electric cell and actuates a loud-speaker. A paper stencil is made and is uniformly illuminated by a point source of light from a tungsten are lamp placed at a considerable distance away. Behind the stencil there is a rotating aluminium disk in which nine slits have been made, each with a width of 1 mm. and at distances of 40° from each other. Behind the disk, which is driven by a motor, is a lens which focuses the light source upon the photo-electric cell. The only light which can fall upon the photo-electric cell is that which has passed through the part of the stencil cut away and one of the slits. When the disk is turning, the amount of light transmitted at each moment is proportional to the height of the part cut away at the point behind which there is a slit at that moment.