Dark field illumination

Figure Bl.18.5. Dark-field illumination the aperture of the objeetive is smaller than the aperture of the beams allowed by the annular diaphragm.

Figure 4.11 Electron micrographs of polyethylene crystals, (a) Dark-field illumination shows crystals to have a hollow pyramid structure. (Reprinted with permission from P. H. Geil, Polymer Single Crystals, Interscience, New York, 1963.) (b) Transmission micrograph in which contrast is enhanced by shadow casting [Reprinted with permission from D. H. Reneker and P. H. Geil, /. Appl. Phys. 31 1916 (I960).]...

Dark-field illumination is classified into three types. The first one is for a microscope equipped with low numerical aperture (NA) objective lenses (see Fig. 1). To cast a shadow at the objective lens, a ring-slit as shown in Fig. IB is inserted into the light path. The second is for highNA (>0.5) objective lenses. Special, ready-made dark-field condensers or lenses are used for dark-field illumination. The third is independent... 

As described before, special built-in equipment is not always necessary for dark-field microscopy, and one can readily convert bright-field illumination to dark-field illumination. An inverted microscope equipped with long-distance (low NA) lenses is suitable for setup of a dark-fieldmicroscope. A handmade ring-slit is available when objective lenses such as x4 lens withNA 0.13, xlO lens withNA 0.30, and x20 lens... 

In order to adjust the dark-field illumination, project the illumination light onto a piece of paper placed on the microscopic stage or on the obj ective lens. Adjust the position of the slit and the height of condenser so that the transmission light illuminates the sample uniformly and brightly, and that the diameter of the shaded area produced on the surface of the objective lens is larger than that of the objective lens (see Fig 1C). 

Plates containing varying numbers of silver atoms per unit area were immersed for 5 minutes in a hydroquinone-silver nitrate physical developer which gave no spontaneous deposition of silver within 30 minutes. The number of developed centers per unit area wras counted under dark field illumination at a magnification of 500 X. 

The stability of most colloidal solutions depends critically on the magnitude of the electrostatic potential ( /o) at the surface of the colloidal particle. One of the most important tasks in colloid science is therefore to obtain an estimate of /o under a wide range of electrolyte conditions. In practice, the most convenient method of obtaining /q uses the fact that a charged particle will move at some constant, limiting velocity under the influence of an applied electric field. Even quite small particles (i.e. <1 xm) can be observed using a dark-field illumination microscope and their (average) velocity directly measured. The technique is called microelectrophoresis . 

Figure 6.16 Diagram of the dark-field illumination system used to visualise colloidal particles in water.

Crystals of Potassium Protactinium Fluoride—KsPaFT. Left Dark field illumination X 60. 

Microelectrophoresis depends on the visibility of the migrating particles under the microscope. As such, it is inapplicable to molecular colloids such as proteins. By adsorbing the protein molecules on suitable carrier particles, however, the range of utility for microelectrophoresis can be extended. Dark-field illumination (see Section 1.6a. lc) can sometimes be used to advantage to extend microelectrophoresis observations to small, high-contrast particles. 

Dark-field illumination is a particularly useful technique for detecting the presence of, counting and investigating the motion of suspended colloidal particles. It is obtained by arranging the illumination system of an ordinary microscope so that light does not enter the objective unless scattered by the sample under investigation. 

Cylindrical cells are easier to construct and thermostat than flat cells and dark-field illumination can be obtained by the ultramicro-scopic method of illuminating the sample perpendicular to the direction of observation (see page 52 and Figure 7.6). The volume of dispersion required is usually less for cylindrical cells than for flat cells and, owing to the relatively small cross-section, it is more often possible to use platinum black rather than reversible electrodes with cylindrical cells. However, unless the capillary wall is extremely thin, an optical correction must be made with cylindrical cells to allow for the focusing action of the tube, and optical distortion may prevent measurements from being made at the far stationary level. Cylindrical cells are unsatisfactory if any sedimentation takes place during the... 

Fig. 2 (b) Profile of germanium surface lapped with No. 305 alumina. Profiles obtained by cleaving perpendicular to lapped surface. Cleaved surface in lower half of picture. The radial lines are "tear marks" (31) which originated at a deep flaw to the right. They are steps, not cracks. Dark field illumination. 

Figure 1.28 (a) Bright-field illumination and (b) dark-field illumination in transmitted mode. Shaded... 

Figure 1.29 Dark-field illumination in a reflected light microscope.

Which parts of a specimen will be highlighted under dark-field illumination in an optical microscopic study if the specimen is a polycrystalline metal with a few ceramic particles ... 

Fig.78. A. x28 and B. x42 power photomicrographs of autoradiographs of iodocyanopindol ( ICYP) binding to adrenergic beta receptors in rat cerebellar cortex illustrating the high density of silver grains in the molecular layer and patches of increased receptor density over the Purkinje cell layer, m, molecular layer g, granule cell layer. (C) shows the patches at x 40 magnification with dark-field illumination. Sutin and Minneman (1985).

Incomplete spreading sometimes occurs due to poor selection of spreading conditions. This can be detected by simple visual observation using suitable lamps to illuminate the surface, such as dark-field illumination. The best test for completeness of spreading is the reproducibility of the experimental results. When we are satisfied with the homogeneous monolayer formation, we can start to examine its properties. When the available area for the monolayer is large, the distance between adjacent solute molecules in the monolayer is... 

Bright-Field Illumination A kind of illumination for microscopy in which the illumination of a specimen is arranged so that transmitted light remains in the optical path of the microscope and is used to form the magnified image. This is different from the arrangement in Dark-Field Illumination. 

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

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