technique that relies on the property of certain components of the specimen not to take up the dye or negative stain. These then show up light against a dark background. For example, in light microscopy, bacteria can be made visible by mixing them with a dark dye such as nigrosin or Indian ink. The method is used widely in electron microscopy to study viruses and certain large molecules. The material is mixed with negative electron-dense stains, such as potassium molybdate or phosphotungstic acid (PTA). Any proteinaceous material present in the specimen does not take up the stain. In the resulting electron micrograph the electron-transparent proteinaceous material will appear dark against a light background.