ultramicroscope(redirected from dark-field microscope)
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optical instrument used to increase the apparent size of an object. Simple Microscopes
A magnifying glass, an ordinary double convex lens having a short focal length, is a simple microscope. The reading lens and hand lens are instruments of this type.
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an optical instrument for the observation of minute particles whose sizes are smaller than the limit of resolution of conventional light microscopes (seeRESOLVING POWER OF OPTICAL INSTRUMENTS). Such minute particles may be observed with the aid of an ultramicroscope because they diffract light.
Under illumination by a convergent pencil of very bright light entering from one side, each particle in an ultramicroscope produces a small diffraction ring system that appears as a minute bright speck on a dark field. Very little light is scattered by the minute particles during diffraction. As a rule, therefore, extremely strong light sources are used with ultramicroscopes. The minimum size of observable particles depends on the intensity of the illumination and can be as low as 2 x 10–9 m. The true size, shape, and structure of the particles cannot be determined from the diffraction specks, since an ultramicroscope does not yield optical images of the objects being investigated. An ultramicroscope may be used, however, to establish the presence and concentration of particles and to study their motion.
The ultramicroscope was invented in 1903 by the German physicist H. Siedentopf and the Austrian chemist R. Zsigmondy. In their slit, or “classical, ” ultramicroscope (Figure 1,a), the system being investigated is stationary. A cell containing the substance of interest is illuminated through a narrow rectangular slit,
the image of which is projected into the area of observation. The bright specks corresponding to the particles located in the plane of the slit image are visible in the eyepiece of the viewing microscope. Particles located above and below the illuminated area are not observed. For the study of colloids, conventional microscopes with dark-field condensers are often used instead of slit ultramicroscopes (seeMICROSCOPE: Methods of illumination and observation [microscopy]).
In the flow ultramicroscope (Figure l,b), which was developed in the 1950’s by the Soviet scientists B. V. Deriagin and G. Ia. Vlasenko, a liquid sol or an aerosol flows along a tube toward the eye of the observer. Particles crossing the illuminated area are detected as bright flashes, either visually or with the aid of a photometric device. Particles whose size exceeds some given limit may be isolated for detection by adjusting the brightness of the light flux by means of a movable photometer wedge. The flow ultramicroscope may be used to determine particle concentrations of up to 1010 particles per cm3 in sols.
Different types of ultramicroscopes and methods of ultrami-croscopy are used for studying various disperse systems; for monitoring the quality of atmospheric air, water used in industry, and drinking water; and for checking the degree of contamination of optically transparent media by impurities.
REFERENCESKouzov, P. A. Osnovy analiza dispersnogo sostava promyshlennykh pylei i izmel’chennykh materialov. Leningrad, 1974.
Voiutskii, S. S. Kurs kolloidnoi khimii. Moscow, 1964.
Deriagin, B. V., and G. Ia. Vlasenko. “Potochnaia ul’tramikroskopiia.” Priroda, 1953, no. 11.
L. A. SHITS