The thesis of this communication is that the 'secret method' of Leeuwenhoek was the augmentation of the magnification of his single glass lens by the refraction at the quasi-hemispherical surface of a column of liquid containing the microorganisms. The linear increase in magnification thus secured could be as much as three-fold, depending on the degree of curvature of the liquid surface, the diameter of the column of liquid, and the index of refraction necessary for observation of details beyond the resolution of his best glass lenses. The hypothesis also accounts for the second part of the secret method, the high density of microorganisms in a single field of view, by successive recostitutions of the curved surface of the medium.