=Visibility under the "Ultramicroscope."=--The particles of a sol, in contrast with the molecules of a true solution, are visible as bright scintillating points under the ultramicroscope. This is a modification of the type of dark-field illumination of the ordinary microscope, as applied to microscopic studies, in which the solution to be studied is contained in a small tube or box of clear glass which is mounted on the stage of an ordinary microscope and instead of being illuminated from below by transmitted light is illuminated by focusing upon it the image of the sun, or of some other brilliant source of light such as an electric arc, by passing the rays from the source of light through a series of condensing lenses which are adjusted at the proper distance and angles to bring the image of the illuminating body within the tube containing the substance which is to be examined and in the line of vision of the microscope. Obviously, this results in intense illumination of any particles in the solution which come within this brilliant image of the sun, or arc, and therefore renders visible particles which are of less diameter than the wave-length of ordinary light (450µµ to 760µµ for the visible spectrum) and, hence, are not visible by the ordinary means of illumination in the direct line of vision. It will be apparent that what is seen in the field of the ultramicroscope is not the particles themselves, but rather the image of the sun (or other illuminating body) falling upon the particles which come within the image, just as one does not see the paper but only the image of the sun when the rays from the sun are brought to a focus upon a sheet of paper through any ordinary convex lens, or "burning glass." Hence, the ultramicroscope gives no idea of the shape, color, or size of the particles upon which the image falls; but it does permit the counting of the number of particles within a given area, and a study of their movements, from which it is possible, by mathematical computations, to calculate the relative size of the particles themselves. Repeated studies have shown that particles of the sizes between 5µµ and 250µµ in diameter, which are visible under the ultramicroscope, are sufficiently small to bring about the surface phenomena which are known as properties of colloidal solutions. Further, the ultramicroscope permits the observation of the growth, or disintegration, under various chemical reagents, of the individual colloidal particles, which appear as scintillating points in the field of the microscope; and the study of changes in relationships during gel-formation, peptization, etc.