Abstract
Optical diffraction studies of electron micrographs of potato virus T particles have been made to try to understand the variation in appearance of the particles under different staining conditions. These studies show that the particles are constructed from a helical arrangement of protein subunits lying on a primary helix of pitch 3.4 nm. They also suggest that the different surface patterns observed on the virus particles are due to stain lying in secondary helical grooves which are formed by aggregation of the structural protein subunits. The optical diffraction patterns indicate a variation in the periodicity of the secondary helix, which implies a variation in the number of structural protein subunits per turn of the primary helix.