an aid to computerization of heymans afterloading system the authors describe a new type of dummy implant for use in the heyman afterloading system inactive dummies with the same dimensions as the radioactive sources are loaded into the capsules before obtaining the orthogonal radiographs the actual sources are not inserted until computer verification of satisfactory dose distribution has been obtained modularity and stability in ecological communities networks composed of distinct densely connected subsystems are called modular in ecology it has been posited that a modular organization of species interactions would benefit the dynamical stability of communities even though evidence supporting this hypothesis is mixed here we study the effect of modularity on the local stability of ecological dynamical systems by presenting new results in random matrix theory which are obtained using a quaternionic parameterization of the cavity method results show that modularity can have moderate stabilizing effects for particular parameter choices while antimodularity can greatly destabilize ecological networks