Research Foundation, Palo Alto, California, 9430 1 S U M M A R YIt has been suggested in the literature that the core of the ferritin molecule contains a tetrad or other substructure resolvable in electron micrographs. The work described in this paper indicates :1. At a potential resolution of 5 A, near-focus electron micrographs of ferritin molecules show no substructure in the core.2. By defocusing the microscope a computed amount, electron micrographs of the same ferritin molecule can be produced which show apparent core substructure similar to that shown in several patterns reported in the literature.3. Apparent ferritin molecules with or without substructure, depending upon the phase granularity in the negative, can be added in printing by over-exposing an area of the background equal in size to a ferritin molecule.4. Underfocus contrast enhancement, frequently used by electron microscopists at lower magnifications without loss of resolution, does result in a loss of resolution at maximum useful magnifications.5. The established physical principles of electron-optical image formation which should be applied to all high-resolution electron microscopy to distinguish substructure from artifact, are reviewed.This work is compatible with recent X-ray diffraction studies of ferritin molecules which indicate that iron-containing micelles in the core of ferritin are smaller than the practical resolution of the best electron microscopes and therefore neither they nor their substructure could be imaged.
SUMMARY
Optical shadowing offers a valuable technique for the study of many transmission electron microscope specimens. Simply blocking half of the illumination with the objective aperture produces an image with a striking shadowed effect which gives a distinctly three‐dimensional appearance to the specimen's surface topography. Theoretical analysis shows that this is due primarily to a discrimination between electrons refracted in opposite directions, and that the characteristic features of the effect are successfully explained by a refraction model. The capability of visualizing surface topography is applicable up to the full resolving power of the instrument and accordingly opens many new avenues of investigation.
Heavy-metal stain aggregates on the surface of thin sections of biological material have higher contrast than those embedded within the sections and both have greater contrast than can be accounted for by the amplitude image. Disturbances of the incident illumination by a specimen in both light-and electronoptical systems and their possible contribution to image contrast are considered. The hypothesis is proposed that a lens effect produced by the stain aggregates may account for their contrast in the electron microscope in a similar manner to the contrast of glass beads in the light microscope with a low numerical aperture.
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