Ragnar Granit and L. A. Riddell: The electrical responses of light- and dark-adapted frogs' eyes to rhythmic and continuous stimuli. J Physiol. (London) 1934 Mar 29; 81(1): 1–28.
Ragnar Granit and Per-Olof Therman: Excitation and inhibition in the retina and in the optic nerve. J Physiol. (London), 1935 Feb 9;83(3): pp. 359-381.