Smoke appears to consist of particles of all sizes from 10⁻³ cm., which may just be resolved by the unaided eye, to molecular dimensions, 10⁻⁸ cm. The larger particles settle out most rapidly and so do not remain long in suspension. MEASUREMENT Wells and Gerke have developed a form of ultra-microscope which is well adapted to the measurement of the size of smoke particles. The ultra-microscope is a low power microscope using intense dark ground illumination for viewing particles which are too small to be seen by transmitted light. They are rendered visible in this way, since any object, no matter how small, which emits enough light to affect the retina is visible, provided the background is sufficiently dark. Thus stars are visible at night and dust particles are easily seen in a sunbeam in a darkened room. The larger particles, viewed in this way, do not appear larger but brighter. The apparent size of the particles is determined by the diffraction pattern and is thus dependent only on the optical system used to view them. The more intense the incident light, the brighter the particles appear. In the ultra-microscope described, the image of an intense source, such as a concentrated filament lamp, or an arc, is focused upon the particles in the microscopic field, but the axis of the illuminating beam, instead of coinciding with the axis of the microscope, as ordinarily used, is perpendicular to it. The beam itself, therefore, never enters the microscope at all, but passes under the objective into a blackened chamber where it is absorbed. The field of the microscope is made dark by placing underneath the objective another “black hole” or blackened chamber with an opening just a little larger than the field.[34] [Footnote 34: This ultra-microscope is described in _J. Am. Chem. Soc._ =41=, 312 (1919).] The method used for measuring the velocity consisted in causing the particle to describe a definite stroke many times in succession in an electric field. This was accomplished by reversing the direction of the field with a rotating commutator. The convection due to the source of light is perpendicular to this motion so that a zigzag line is obtained (see Fig. 88). The amplitude of this oscillation is an accurate measure of the distance traversed by the particle under the electric force for a definite small interval of time. The speed of the rotating commutator and the electric field are both susceptible of precise measurement, so that the size of a single particle is precisely determined. [Illustration: FIG. 87.—Ultramicroscope for Measuring Size of Smoke Particles.] [Illustration: FIG. 88.—Measurement of Smoke Particles by Use of Ultramicroscope.]