An apparatus for the microscopic measurement of fog particles is described. The particles were collected on a slightly greased glass slide and viewed with the aid of dark‐field illumination. The particle sizes were measured either visually with an eyepiece micrometer or, more usually, by the subsequent measurement of photomicrographs. With the aid of this apparatus a series of fog particle size distribution curves were obtained. These curves are characterized by a single maximum which always occurs at a particle diameter which is an integral multiple of 3.1 microns. The curves approach the axis asymptotically for large particle diameters but have a definite minimum. An explanation for this minimum on the basis of certain properties of the nuclei of condensation is offered.