Dark-field microscope

Microelectrophoresis is the most common technique for electrokinetic measurements in colloidal systems. Here individual particles can be observed, in their normal environment, under the microscope. Very dilute dispersions can be studied and very small particles, down to about 0.1 pm diameter, can be observed using the dark-field microscope (ultramicroscope). High magnifications allow minimization of observation times, and in polydisperse systems a given size range of particles can be studied to the exclusion of others. 

Another optical version of a microscope is called the darkfield microscope. This microscope has proven to be particularly useful for biological studies. The dark-field microscope uses a specialized illumination technique that capitalizes on indirect illumination to enhance contrast in specimens. An opaque disk is set in a condenser under the stage of the microscope (the solid support that the object being studied rests on). The disc is also known as the stop. The stop prevents light from shining directly on the specimen. Instead, light passes around the stop and is reflected off the condenser s walls. 

The figure shows dark field microscopic photographs of a zoospore suspension containing Chromosorb W AW particles (ca 150 x 200 pm) with and without the attractant, cochliophilin A (6). [A] a particle soaked in 1.0 x 10-8 M cochliophilin A, [B] a control particle treated with solvent alone. The zoospore density around the particle [A] containing the attractant is clearly higher than around the control particle [B],... 

Dark-Field Microscope See Dark-Field Illumination, Ultramicroscope. 

Ultramicroscope An optical microscope that uses dark-field illumination to make visible extremely small (submicrometer-sized) particles or droplets. Also termed Dark-Field Microscope. See also Dark-Field Illumination. 

Dark field microscope Oblique illumination of the specimen based on scattering of light by the specimen background appears dark and features as bright spots Specimens with small particulates ... 

The chylomicra enter the systemic blood circulation by way of the jugular vene. They can be seen in the blood, following a fatty meal, by a dark field microscope (Gage and Fish 1924). They gradually disappear from the circulation at a rate varying with the species. In the human, the rate of entry into the blood after a fat rich meal may exceed the rate of removal and the blood becomes turbid. In the rat removal is much more rapid and turbidity of plasma is rarely encountered. 

Gage, S. H., and P. A. Fish Fat digestion, absorption and assimilation in man and animals as determined by the dark-field microscope and a fat soluble dye. Amer. J. Anat. 34,1 (1924). Gitlin, D., and D. Cornwell Plasma lipoprotein metabolism in normal individuals and nephrotic children. J. din. Invest. 35, 706 (1956). 

Using a dark-field microscope, Perrin was able in 1908 to measure repeatedly the displacements of colloidal particles and verified Eq. (10.4-3) experimentally. For many skeptics this was considered to be the definitive verification of the existence of atoms and molecules, since Einstein s derivation of Eq. (10.4-3) depended on the assumption that the colloidal particle was bombarded randomly by solvent molecules. Perrin was able to obtain an approximate value of Boltzmann s constant from Eq. (10.4-3), and thus calculated a value of Avogadro s constant using the known value of the ideal gas constant. 

Bright-field, phase contrast or dark-field microscopic examination of stained or unstained tissue smears, whole-mounts, wet-mounts, etc. of diseased or abnormal specimens. 

---