Electron microscope studies of bacterial viruses.

This work demonstrated the absorption of the phages on the host cell, and the lysis of the host cell and the liberation of a hundred or so daughter paritcles from each cell. It also showed that the particles multiply inside the cells rather than at their surfaces, since unti lysis occurs the number of particles visible at the surface remains constant. 
Digital facsimile from PubMedCentral at this link. For an extensive commentary on this paper, including correspondence between Anderson and Delbruck before and after publication see T. F. Anderson, "Electronic microscopy of phages," Phage and the origins of molecular biology. Expanded Edition, edited by John Cairns, Gunther Stent and James D. Watson (1992) 63-78.