In studying the performance of a system for producing sprays of any kind it would be desirable to arrive at an expression which would relate the drop-size distribution and the spatial distribution of the drops in terms of the variables of the system including pressure, density, surface tension, and geometry. In an earlier study of the performance of hollow-cone swirl-type nozzles, photographs were taken of the region where the fluid sheet breaks up into drops. These photographs showed that before the break-up occurred, waves or ripples appeared on the surface of the liquid sheet. It was believed that the waves were responsible for the break-up and that frequency and wave length of the ripples might be related to the size range of drops produced. Difficulties were encountered in an analytical treatment of the stability of the surface produced by a swirl-type nozzle, the surface being a hyperboloid of revolution. Therefore a system was selected for study in which a flat sheet of fluid is produced by a slender orifice, and which may be subjected to waves of any desired frequency. This plane sheet, exposed on both sides, appears to have the essential characteristics of the sheet produced by a conventional nozzle.