to compare three various methods in the diagnosis of dentinal cracks formed in the apical third after root resection.one hundred extracted human maxillary central incisors were selected. the root canals were prepared with step-back technique. then 3mm from the apical end of all roots was cut perpendicular to the long axis of the tooth. the apical end of each root was evaluated to make sure ther...

The cores of a small proportion of the particles in purified preparations of type C viruses are characterized by two concentric shells that have a "double-ring" appearance in specimens prepared for electron microscopy by negative staining or thin sectioning techniques. This distinctive feature allows such particles to be recognized as viral in impure preparations. Detectability of double-ring p...