Electron microscopy: a single field of particles. The IC fraction from Figure 2b was prepared for electron microscopy by negative staining with 1% sodium phosphotungstate, pH 7.6 . A single field of particles is shown that has several potential IC's. The length of the bar is 100 nm. The variability of the thickness of the negative stain is larger than sometimes observed, because of the extent of both the size variability and the diluteness of the particles of the specimen.