In this developmental period, profound new knowledge about the anatomical and neurophysiological properties of the retina emerged as a result of advances in sophisticated instrumentation and research techniques. This knowledge greatly influenced the development of the duplicity theory: it provided an insight into the rod and cone processes, and also created and paved the way for new ideas of rod and cone functioning. Outstanding contributions to the development were provided by Polyak, Hartline, Kuffler and Granit. Using the Golgi impregnation method, Polyak's investigation of the primate retina helped to elucidate the extremely complex structure of the many types of retinal cells and the character of their connections. Hartline, Kuffler and Granit, using microelectrodes for registration of the action potential from individual nerve fibres in response to illumination, increased our knowledge about the relationship between light stimuli and nerve impulses in the retina.