uch 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 (450 mu mu to 760 mu mu 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 5 mu mu and 250 mu mu 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.

=The "Tyndall Phenomenon."=--Colloidal solutions exhibit this phenomenon;
that is, if a bright beam of light be passed through a sol which is
contained in a clear glass vessel having parallel vertical sides, and the
solution be viewed from the side, it appears turbid and often has a more or
less bluish sheen. This effect is due to the small particles in the sol, of
polarizing the light which is reflected from them, the blue ray